STUDIES IN THE CAUSES AND CONSEQUENCES OF INTRASPECIFIC

VARIATION IN SIZE OF APHIDS.

G. Murdie, B.Sc.

Being a thesis submitted for the Ph.D. degree,

University of London.

Imperial College of Science and Technology, Field Station, Silwooe Park, Sunninghill, Ascot, Berks. August, 1965.

1.

2.

ABSTRACT.

The work described was done mainly with larvae and adults of apterous

virgnioparae of the pea aphid, Acvrthosiphon pisum (Harris). The aphid

was reared under different conditions of crowding, starvation, temperature

and photoperiod. Larval and adult development and weights and morphometrics

of larvae and adults reared under these conditions are described and compared.

Biologies of different sized individuals reared under various conditions

of crowding and temperature are compared in terms of their fecundity,

longevity, reproductive rate and size of the new-born progeny they produce.

The differing abilities of individuals to recover from size decrease due to

crowding and to high temperature and the ways in which recovery is accomplish-

ed are described and discussed.

Preliminary experiments are described in which the ability of different

sized individuals to survive certain natural and artificial stresses is

compared. Stresses included crowding, starvation in low and high humidities

at low temperature, high temperature, and an insecticide - DDT.

The possible causes of size decrease induced by crowding and by high

temperature are discussed and the possible ecological importance of size is

considered both in terms of the individual and of the population.

3

TABLE OF CONTENTS.

SECTION.I. GENERAL INTRODUCTION, REVIEW OF LITERATURE, MATERIALS

AND METHODS.

A. INTRODUCTION. S.

B. REVIEW OF LITERATURE. 94,

C. MATERIALS AND METHODS. 27.

27• a. General methods.

b. MG§suring methods. 34.

SECTION.2. CAUSES OF SIZE VARIATION IN A. PISUM.

A. THE EFFECT OF LARVAL CROWDING ON ADULT SIZE. 50.

a. The effect on size of adults of crowding three 50.

generations of Acvrthosiphon Disum larvae.

b. The effect of larval rearing density on size. 55.

c. The effect of larval starvation on development

and size of adults. 60'

Conclusions. 64

B. THE EFFECT OF T&PERATURE ON SIZE OF APTEROUS

VIRGINOPARAE ACYRTHOSIPHON PISUM 65.

a. The F1 generation. 66.

b. The F2 and F3 generations at 25°C. 69.

Ccnolusions. 73.

C. THE EFFECT OF PHOTOPERIOD ON SIZE. 81.7

4

SECTION.3. THE EFFECTS OF SIZE ON THE BIOLOGY OF ADULT

ACYRTHOSIPHON PISUM.

A. THE EFFECT OF SIZE VARIATION CAUSED BY CROWDING

87.

a. The pro-reproductive period 87.

b. Effect of adult weight on fecundity, longevity

and reproductive period. 93.

c. Reproductive characteristics of four adult

weight groups. 98.

d. The effect of first instar size on larval

growth and adult size 106.

e. The effects of crowding stress during the adult

stage on fecundity of different sized adults. 116.

Conclusions 118.

B. THE EFFECT OF SIZE VARIATION CAUSED BY TEMPERATURE 120.

The effect of temperature on adult life and

pattern of reproduction. 120.

Conclusions 129.

SECTION 4. THE EFFECT OF BODY SIZE ON THE ABILITY OF

ADULT APTEROUS VIRGINAPARAE OF A. PISUM

TO SURVIVE STRESSES. 131.

A. THE EFFECT OF BODY SIZE ON THE RESISTANCE OF

A. PISUM.TO TOPICAL APPLICATION OF DDT. 132.

a. treated aphids held at 25°C. 134.

b. Treated aphids held at 20°C. 145.

5.

B. SURVIVAL TIMES UNDER STARVATION CONDITIONS AT HIGH

AND LOW HUMIDITIES AT LOW TEMPERATURE 150.

a. Survival at low humidity. 150.

b. Survival at high humidity. 150.

C. THE ABILITY OF DIFFERENT SIZED ADULTS VIRGINOPARAE

OF A. PISUM TO SURVIVE AT HIGH TEMPERATURE. 156.

Conclusions. 161.

SECTIONS. AN EXAMINATION OF THE DIFFERENCES IN

GROWTH RATES OF INDIVIDUALS REARED UNDER

STRESS CONDITIONS. 162.

a. Initial experiment to determine which characters

should be measured. 162.

b. The effect of crowding on growth. 166.

c. The effects of starvation on growth. 169.

d. The effect of high rearing temperature on larval

growth. 170.

e. Comparisons of the growth rates of antennal

segments of crowded, starved and high

temperature-reared A. pisum. 172.

Conclusions. 173.

SECTION.6. GENERAL DISCUSSION. 183.

a. Crowding: its meaning with reference to

A. pisum and its mode of action. 184.

b. Temperature as a stress factor. 189.

c. The possible ecological significance of

size variation in A. pisum. 193.

SUMMARY. 197.

ACKNOWLEDGEMENTS. 202.

BIBLIOGRAPHY. 203.

6.

SECTION 1,

General introduction, review of literature, and materials

and methods.

7.

A. INTRODUCTION

8.

The growth processes of any particular species are canalised to

produce individuals of a size which varies between certain limits. Such

processes maintain specific size within an error not impairing adequate

efficiency (Bidder, 1925; Comfort, 1964) which probably resulted from

natural selection and ensures that the average size and extent of size

variation of any particular species is such as to fit it to its environment.

The importance of form (which includes size) was emphasised by

Wigglesworth (1945) when he stated that- "The essence of an organism

is its form. The infinite variety of chemical changes that proceed within

it are directed solely to produce and conserve that form and to provide

for its eventual reproduction"; but situations exist where environmental

conditions do not permit the realisation of ideal size, for instance where

animals are seriously undernourished or overcrowded. The amount of "error"

permissable, whilst still ensuringsurvival and reproduction, no doubt varies

with the species but has been investigated for very few of them.

This study was done to examine some of the causes of size variation,

the limits of size variation and its consequences for an aphid species,

Acyrthosiphon pisum (Harris). The causes of size variation studied were

crowding, starvation, temperature variation and photoperiod, and studies

were made not only of size but also of changes in shape. The consequences

studied were primarily the effects of size variation on fecundity and long-

evity and also on ability to recover normal size and to survive certain

natural and artificial stresses.

on the growth and form of higher

on which much work has been done

9, B. REVIEW OF LITERATURE

Some aspects of the causes and consequences of size variation in

the animal kingdom are outside the scope of this work, and this review

will deal primarily with intraspecific size variation in the Insecia,

particularly the effects of crowding and temperature, apart from some

introductory remarks on animal size generally.

Generalisaticns can be made about the world distribution of size in

animals which is influenced by the existing climatic zones. Thus, on

average, species of homoiothermic animals tend to be larger in cold than

in hot regions and tend to have smaller extremities (Bergmann's and Allen's

rules, respectively, Allee et al, 1949). Those differences can be explained

in terms of heat conservation. Conversely, the larger species of

poikilotherms occur in warmer regions where temperatures are sufficiently

high throughout the year to warm larger bodies and allow metabolic processes

to proceed at a normal rate (Alice et al. loc. cit.). The largest mammals

are found in the sea whence buoyancy overcomes the problems of movement and of

weight/skeleton ratios faced by large terrestrial mammals.

Although such criteria may sometimes determine or explain inter-

specific size variation and the average size of

may not help our understanding of the sometimes

variations which occur within one species.

Probably the most exhaustive studies

animals have been with domesticated species

a particular species, they

small, but often important,

on the effects of breeding and nutrition. For example, notable differences

can be seen between the smaller native Jersey breed of cattle and the

genetically and nutritionally improved mainland stock,and between the smaller

10.

Southdown sheep reared on their native Downs and on Romney harsh (Cooper,

1957); also betweenferal horses and their domesticated relatives (Wynne-

Edwards, 1962). The effects of competition are well illustrated by the

"social hierarchy" and "pecking order" in cattle (JacClusky, 1957) and

domesticated fowls (Weekes, 1957) where members of the lower ranks thrive

less well and are smaller than their superiors. Animals with continuous

growth, like fowls and cattle, are, however, able to make compensatory growth

once stresses are relieved (Wilson andOsbourn, 1960), but the amount of

recovery depends on the stage of growth when the stresses are removed. Rats

exemplify the recuperative ability of higher animals (McCay, 1952) since

after being kept on a low calorific diet for up to 1,000 days they were able

to accelerate growth on a normal diet, even though they had passed the mean

life-span of the strain, and they lived for about twice as long as the un-

starved controls.

Such a deferment of growth has not been reported for insects and is

probably less likely to occur because of the physical limitations imposed

by the hard exoskeleton, although some flexibility is allowed by the

relatively elastic intersegmental membranes. Insects which moult as adults

(e.g. Thysanura) (Imms, 1957) may, however, be able to complete compensatory

growth.

Population density has been shown to affect the growth of several

animal species but the levels at which densities begin to harm the individual

depends on the species concerned. Overcrowding is generally harmful

(Alice et al, 1949) but there is no standard density which can be termed as

a crowd and applied equally to all species. This question will be considered

further in the discussion but the term "crowding" will be retained in the

11.

context of the species and conditions described by the authors concerned.

Crowding has been shown to retard growth of toad tadpoles (Bilski,

1921) axolotl larvae and frog tadpoles (Goetsch, 1924) and tadpoles of Rana

pipiens (Richards, 1958) (examples cited by Wynne-Edwards, loc= cit). The

inhibition of R. pigigns larvae appeared to be caused by polluting organic

particles. Rose and Rose(1961) showed that larger tadpoles produce more

inhibitant, and were less susceptible than small ones.

However, there is some evidence to show that apparent crowding is not

always harmful. Allee et al (loc,cit) quoted work by Alice (1938) which showed

that goldfish grew faster when crowded than when isolated and that conditioning

of the water appeared to be responsible for the observed stimulation of growth.

They also cited examples where grouping increased developmental rates of

ciliate protozoans (Robertson, 1921); of Oxytridae (Yocum, 1928); and of

Paramecium (Paterson, 1929).

The short resume given above indicates that size variation is prevalent

throughout the animal kingdom and is caused by such factors as crowding which

may decrease growth rates and if sustained can result in permanent stunting;

but higher animals have a capacity for continuous growth and is particularly

striking in some fish which have an indefinite capacity for growth and probably

can recover at any time during development (Bidder, 1925). In contrast, most

Arthropods, and in particular insects, pass through well defined periods of

discontinuous growth demarcated by the exuvial moult. Thus,insects appear

to present special problems of growth and size.

12.

Size variation in insects.

1. The causes of size variation.

The effects of crowding and starvation on size.

Crowding and starvation have been considered under a single heading

since starvation may occur in crowds either because less food may be available

to each individual or the individual may be allowed less time in which to feed

as a result of disturbance by others in the crowd. Thus some crowding

factors (e.g. lack of space) are inseperable from starvation effects.

In common with some other animals discussed above (p.10 ) apparent

crowding can be beneficial to some insects. Thus growth of Acheta domestica

(Chauvin, 1958) and Disdercus fasciatus (Hodjat, 1963) larvae was stimulated

by moderate degrees of crowding. Hoejat showed that the adult weight of

s . D. fapatus increased as the number reared per jar was increased from 1 to 4,

but adult weight decreased to below that of 1 per jar when the number was

raised to 9 (Giles, 1958).

Crowding causes size increases associated with phase change in locusts

(Uvarov, 1921). Many authors have since made morphometric studies on phases

of locust species to evaluate a better yardstick for differentiating the two

extreme types solitaria and crecaria and the intermediate forms now known as

conorecans and dissocians (Gunn and Hunter-Jones, 1952; Dirsh, 1953; and

Stower, Davies and Jones, 1960). It has been shown that full expression of

phase characteristics does not occur within a single generation since crowding

over two or more generations is required to produce a fully gregarious type,

and, conversely, release from crowding for at least two generations is

required to obtain characteristic solitaria (Albrecht, Verdier and Blackith,

1958). Albrecht and Blackith (1957) had shown that crowded females produced

13.

larger sized larvae than solitary females demonstrating amaternal influence

on size of the next generation and illustrating that a build-up of phase

expression occurred. The size of locusts is also influenced by temperature

and is discussed later.

Long (1953) noted that crowding increased the developmental rates of

Plusia gamma and Pieris brassicae larvae but it also decreased larval, pupal

and adult weights lower adult weights were accompanied by decreases in

forewing and femur lengths (Long and Zaher, 1958; and Zaher and Long, 1959).

Long (loc cit) reported that the crowded larvae were more active and fed more

than uncrowded ones, so the effect did not appear to be one of food shortage.

The early work on stored product insects contributes much to the under-

standing of effects of crowding on natality, mortality and dispersion of

such insects as Sitophilus oryzae (MacLagan and Dunn, 1936); Tribolium species

(Park, 1941)5 and Rhyzopertha dominica (Crombie, 1947), but only Park (loc cit)

showed that size of adults (Tribolium sp.) was decreased by crowding.

Gunn and Knight (1945) found that increasing the numbers of Ptinus tectus

larvae in a standard container increased the larval developmental time and

decreased adult weight. The observed adult weights were more variable under

crowded conditions and appeared to be caused neither by conditioning of the

food, which was renewed regularly, nor by food shortage since observable

responses were obtained even where food was 100 times in excess of require-

ments for larval development. Similar low density thresholds were demonstrated

by Anderson (1956) for Enerosis sarcitrella where only lo% of the available

barley was eaten and 15% of the kernels attacked. Andersen also showed that

trebling the density of larvae decreased the weight of adult males by 1.09

times and of females by a factor of 1.15.

14.

Crowding has been shown to affect another stored product moth, Cadre

cautella (Takahashi, 1956). The adult head width varied inversely with the

degree of crowding and the author attributed the size reduction to food

shortage.

Reductionsin size, resulting from crowding, have also been shown to

occur in Lucilia cuprina (Webber, 1955); Mamestra brassicae (Ishikura and Ozaki,

1958); Apanteles alomeratus (Matsuzan►a and Okamato, 1957); and Arvtaina spartii

and A. ccnisiaq (Watmough, 1963).

Field studies of Psychoda species L. alternate and P. severini)

breeding in sewage beds showed that both intraspecific and interspecific

competition were important factors determining size of the emerged flies

(Golightly and Lloyd, 1939; and Golightly, 1940). Competition was most

intense in the summer when development was rapid and populations dense, and

size reduction was explained in terms of competition as well as temperature,

the principal factor (Golightly, loc cit).

Way and Banks (unpublished data, 1959) showed in laboratory experiments

that the size of Aphis fabae adults decreased as the population aged and

crowding increased, and that the rate of size decrease varied with the number

of aphids placed initially on the culture plants. The weight of adult

apterous virginoparae decreased from 1.3 mg to 0.55 mg in 30 days in populations

established with 2 aphids per plant, but decreased from 1.0 mg. to 0.3 mg in

only 11 days when they were established with 32 adults per plant. In a

further experiment they found that populations established with 2, 4, 8, 16

and 32 aphids per plant produced aphids whose weight was inversely proportional

to initial numbers but as numbers increased the weights of adult progeny

decreased at similar rates in all treatments.

15.

Some of the effects reviewed above have been explained by food

shortage due to crowding but may also result from other effects of crowding

on the individual. However, the separate effects of food shortage have

been studied notably on species of Drosophila. Smirnov and Zhelochovysev

(1957) underfed larvae of D. funebris and found that the lengths of some

wing veins were decreased while the variability of other measurements

increased. Bridges and Gabritschevsky (1928) attempted to explain the

differences in size of the normal and giant mutant of D. melanociaster in

terms of semi-starvation since larvae of the mutant fed for longer periods

than the normal. Alpatov (1930) was able to reduce the size of emerging

D. melanopaster adults by removing them, as larvae, from the culture medium

before they were full grown, and Gause (1931) demonstrated that underfeeding

larvae of the same species and D. funebris led to decreased wing and egg

lengths of the emerged adults.

Other workers have shown that either insufficient or inferior larval

food caused decreases in size of adult Lymantria eispar (Kopec, 1924);

Lucille species (M3cKerrs 1931); and Oscinella frit (Hillyer, 1965).

Host suitability is relevant to an analysis of the effect of food

on size since resistant host plants of Aphids for example (Auclair and

Cartier, 1960) or unsuitable prey of Anthocorids (Anderson, 1962), can

affect the rate of growth and ultimately adult size. Auclair and Cartier

(loc cit) showed that A. nisum reared continuously on resistant varieties

of pea were decreased in size to an extent similar to that of aphids fed

on non-resistant peas but starved 10-12 hours daily. The reactions of

A. fabae to a resistant variety of broad bean (Rastatt) were studied by

Willer (1961) who showed that the body lengths of daughters produced by

16.

virginoparae were proportional to the leaf area of the host plant and

that so-called resistance could be explained by the plant's growth

activity compared with that of a susceptible variety (Schlanstadt)9 since

plants of similar size supported A. fabae equally well. These three

authors attributed the observed effects to semi-starvation and not to toxic

inhibitors or to shortage of "token" feeding stimuli which could have

produced similar results. Further evidence of the starvation effect of

host unsuitability is shown by the selectivity of alate yzus persicae

for leaves of specific age (Kennedy and Booth, 1951), and the greatly

reduced size of Megoura viciae reared on flowering broad bean plants compared

with aphids reared on pre-flowering beans (Lees, 1959)

Cartier and Painter (1956) showed two strains of Rhopalosiphon maidis

varied in their reactions to resistant sorghums. Although weights of adults

were decreased on resistant varieties, the amount of decrease depended on

the strain Host-correlated variations have also been demonstrated with

Bemesia tabaci (Mound, 1963) the size of which decreased in the host plant

order tobacco>dolichos :!*cassava.

Quantitative assessments have been made of the nutrient requirements of

several insects, among which are the attempts of Auclair and Cartier (1963)

and kittler and Dadd (1963) to design a synthetic diet for the aphids, Mvzus

persicae and A. pisum. However, the complexity of diet and difficulties

associated with artificial feeding substance have so far produced only

incomplete generations and undersized adults.

Temperature

In the introductory remarks to this review it was pointed out that broad

regions of climate occur which, to some extent, influence the basic size

17.

of species within them. Seasonal and local variations of factors, such

as temperature and humidity, between and within habitats of each region may

cause variations in size of individuals of a particular species. There is

much evidence from laboratory experiments and also from limited field studies

to support this conclusion.

In his review of "Insects and Climate., Uvarov (1931) mentioned, without

quoting references, some early work which showed that size was reduced at

low temperatures, but he also quotes Titschak (1925, and 1927) who showed

that adults of Tineola bisselliella were larger ane heavier when reared at

low temperatures (comparing 30, 25, 20 and 15°C). Titschak criticised earlier

work for faulty techniques, but it is possible that low temperatures near the

threshold for development could reduce size, e.g. as in Tronoderma

anthrenoides (Surges and Cammell, 1964) where adult weights were decreased

at temperatures near the lower and upper developmental thresholds and

increased at temperatures near the optimum. As early as 1896 Stanefuss

had shown that Lasiecarm mercifolia reared at high temperature were small,

which Uvarov suggested was due to semi-starvation since larval development

was rapid but food intake was not correspondingly increased.

Several workers on Drosophila sp. studied the effect of temperature on

phenotypic expression. The general conclusion was that adult size was

inversely related to the rearing temperature (Alpatov and Pearl, 1929;

Alpatov, 1930; Eigenbroet, 1930;Hersh and 1iard, 1932; and Imai, 1933) but

size decreases did not operate at the same rate for all parts of the body,

nor equally for both sexes. For example, Imai (loc cit) showed that the

femur was more stable than the wing, the variability of which increased with

increases in rearing temperature. Imai concluded that at "low temperature....

18.

.... growth occurs slowly and more completely.- while... at high temperatures

growth proceeds more rapidly and with less integrated completeness."

This conclusion is somewhat similar to that of Uvarov (1931).

The effect of differential growth has been well investigated for

locusts both in relation to crowding, reviewed above, and also temperature

(Husain, Lahore and Mathur, 1944; and Dudley, 1964). Husaini et al showed that

raising the temperature from 27 to 40°C. reduced adult weight of Schistocerca

areaaria but affected the relative proportions of body parts suggesting that

hich temperatures induced development of gregariform individuals. Dudley

(loc cit)confirmed the results of Husain et al and also demonstrated that

humidity had a considerable modifying effect on size and ratios of body parts.

It was interesting that the size of the adult wing was much more variable

than that of the femur (cf. Drosophilai Imai, 1933).

The effects of temperature on size of stored products insects have

also been examined, although many observations were by-products of population

studies. Menusan (1936) showed that size of adult Acanthoscelides obtectus

decreased with increase in rearing temperature, but size of Tribolium confusum

was unaffected by increasing the rearing temperature from 25 to 31°C. That

high rearing temperatures decrease adult size has been shown also for:-

Pseudorostus hilleri and Tricionadenius gl2jaulus (Howe and Burges, 1943);

Ephestia elutella (Waloff, Norris and Broadhead, 1948); Sitophilus oryzae

(Reddy, 1952); and for some species of Callosobruchus (Howe and Currie, 1964)

Three Lepic'optera, Peridroma maraaritosa (Snyder, 1954), Plutella

maculipennis (Atwal, 1955) and Mamestra brassicao (Ishikura and Ozaki, 1958)

were lighter and more variable in weight as pupae when reared at high

temperatures.

19.

The size of some Diptera is also affected by rearing temperature.

In laboratory studies Golightly (1940) showed that size of Psychoda species

was inversely related to rearing temperature. Seasonal size variation of

the flies was observed in the field and, by partial regression analysis,

was found to be affected by larval crowding and by temperature, with

temperature the most important factor determining size (Golightly and Lloyd,

1939). The size of trapped tsetse flies was similarly related to temperature,

but was found to be inversely related to the mean temperature occurring two

months previous to adult emergence, i.e. acting on the adults of the the

previous generation (Jackson, 1933). Temperature changes during/long pupal

period did not affect adult size.

Hosoi (1954) and van den Heuvel (1963) working with two species of

mosquito, Culex 212iens and Aeries aenypti respectively, showed that long

periods of high temperature during larval development decreased adult body

weight and also wing length. However, decrease in breeding temperature

caused a disproportionate increase in wing length relative to body weight;

the weight of A. aeovpti was only affected by exposure to high

temperatures during early larval life whereas wing length was affected by

temperature change at any time up to the pupal stage.

The size of aphids may also be decreased by rearing at high tempera-

ture but no quantitative studies have been made on size, 0.1e most authors

simply stated that "minute" or "undersized" adults were produced at high

temperatures. Thus, high temperatures are stated to decrease size of

adults of; Toxoptera aurentii (Rivnay, 1938); Aphis chloris (ililson, 1938);

Acvrthosiphon pisum (Kenton, 1955); and fv,ecioura viciae (Lees, 1959).

Boc'enheimer and Swirski. (1957) referred to observations in Israel where

20.

the largest apterous virginoparae occurred in the cool season and the

smaller in the hot, which they suggested was due to relatively unsuitable

hosts in the hot season.

To summarise:-

1. Apparent crowding stimulates growth of some insects but generally

it decreases size;

2. food shortage, caused directly or indirectly, may slow growth

rates and decrease size of the individual;

3. high temperatures increase developmental rates of insects but

size is decreased; there is some evidence that critical low temperatures may

also decrease size.

2. The effects of size variation on biotic potential of the individual and

on its reactions to stress factors.

The effect of size on biotic potential.

Each species has a particular capacity for increase (innate capacity

of increase, rm; Andrewartha and Birch, 1957) of which fecundity and re-

productive rate are important parameters. An individual would be considered

successful if it realised its full potential, producing the maximum number

of eggs in the minimum time.

Fecundity has been assessed either directly by allowing the adult

female to produce its progeny or indirectly by dissecting the ovaries from

which some estimate of potential fecundity is obtained. Thus, ovariole number

has been shown to be size dependent in: Lpriliri_ cuprina (Webber, 1955);

Aedes aeavpti (van den Heuvel, 1963); and Oscinella frit (Hillyer, 1965), but

ovarioles may vary in productivity (Webber, loc cit and Hillyer, loc cit).

Thus examination of ovarioles may not give a good estimate of fecundity except

21.

where related to actual numbers of eggs laid, e.g. Waloff, Norris and Broad-

head (1948) showed that the number of ovariole rudiments increased with

weight of the adult female Ephestia elutella and that numbers of eggs laid

was also a function of female weight. MacKerras- (1933) showed that the

number of mature ova increased with the weight of the newly emerged female

Lucilia sp.

Albrecht, Verdierand Blackith (1958) demonstrated that the number of

ovarioles in a newly hatched female Locusta miciratoria mioratoroides larva

was inversely related to its weight. This appears to be a contradiction of

the observations above but can be explained by phase differences where larger

larvae produced by gregarious females have a special survival value in

maintaining fat body rather than ovarioles.

Increases in true fecundity (i.e. number of eggs produced per female)

associated with increase in weight have been established for.EaychQdil species

(Golightly, 1940); Hofmannophila oseudospretella (Woodroffe, 1951A);

Enerosis sarcitrella (Woodroffe, 1951B); Plutella maculipennis (Atwal

1955); Cadra cautella (Takahashi, 1956); Phytodecta olivacea and Phaedon

cochleariae (Donia, 1958). Boch and Jamieson (1960) found that brood area

was significantly correlated with weight of the honeybee queen. Durinc his

studies of physiological strains of the pea aphid, Harrington (1943) found

that average fecundity increased with size of the aphid strains although

the different sized individuals presumably represented genetically distinct

types.

Although it might be expected that its larger food reserves would en-

hance a large individual's chance of surviving, and hennce of producing its number

maximum/of eggs, there is little information on the effect of size bn longevity

Somewhat anomalous results were published by Kopec (1924)

220

who found that small adult Lymantria dispar, produced by starving the larvae,

often lived for longer, and seldom for a shorter time than large "control"

adults. Alpatov (1930) detected no differences between longevity of small

and large adult D. melanoaster. However, weight at emergence was positively

correlated with longevity of adult E. elutella (Waloff et al, loc cit);

C. cautella (Takrilfashi, loc cit); H. pseudospretella and E. sarcitrella

(woodroffe, loc cit).

The effect of body size on the ability of an insect to survive stress.

Stress factors can be divided into two broad groups, namely; natural,

e.g. crowding, temperature; and second, artificial, e.g. insecticide.

Natural stress factors. Albrecht and

Size may have an important effect on survival. Thus,Blackith (1960)

and Albrecht (1962) have shown that large hatchling locusts are able to

survive longer periods without food than small ones, which was attributed

byBlackith 0.961) to the higher proportion of moisture present in larger

hatchlings.

Cockbain (1961) demonstrated large alate A. fabae could sustain

longer periods of tethered flight than small ones, inferring that larger

alatae would have a better chance of successful dispersal and subsequent

reproduction. The differences in performance were explained in terms of

the greater fat reserves in large alatae which provided fuel for longer

flights.

The importance of insect size in relation to heat resistance was

indicated by Lellanby (1932). He suggested that a small individual would

be at a disadvantage at high temperatures relative to a large one because

of its (the small insect) greater surface area relative to body mass and

23.

thus would receive more heat by conduction. The smaller insect would also

less be/able to withstand loss of water. There appears to be no published

evidence on the effects of intraspecific size variation on heat resistancew but

Broadbent and Hollings (1951) showed that heat resistance of live aphid

species decreased in the order; Brevicoryne brassicae>Myzus persicae =

Macrosiphum euphorbiae >Acyrthosiphon pisum = Aulacorthum solani, indicating

that the smaller individuals are relatively more resistant.

Insecticides as an artificial stress.

It has been appreciated for some time that larger individuals are same

more resistant to drugs than arc smaller ones of the/species (Bliss, 1936;

Busvino, 1957). Moore (1909) suggested that dose should be related to body

surface, since drugs are often surface-acting, but other workers have used

weight as a criterion of body size.

Campbell (1926) treated silkworm larvae with arsenic and found that,

given the same dose of insecticide, larger larvae died more slowly than did

smaller ones. Bliss (1936) reanalysed Campbell's data and suggested a 'size

factor' which adjusted dose according to body weight to give equivalent

survival times for larvae of various sizes.

Way (1954) showed that large last-instar larvae of Diataraxia oleracea

were notably more resistant to DDT and e/ BHC than small ones even when doses

were calculated per unit of body weight, but resistance to parathion was

linearly related to weight. Similarly, Gast et al (1956) and Gast (1959)

showed that IV,LD's of DDT and "Phoserin" adjusted for body weight were

practically constant for Proeenia eridania larvae, whereas Heliothis zea

24.

larvae needed more than a thousand-fold increase in dose of DDT for a

doubling of weight. Guthrie (1954) and MacPherson et al (1956) also

demonstrated weight dependent resistance for larvae of Photoparce sexta

and H. zea respectively, as did Ishikura and Ozaki (1958) for Mamestra

brassicae larvae.

MacCuaig used three insecticides, DNC (1956) diazinon and % BHC

(1961) against adult Locusta miglat2112 miqratorioidesand Schistocerca

gregaria. Schistocerca resistance was directly related to weight for all to

three insecticides, but results with Locusta showed that resistance/(BHC

and DNC, but not diazinon, was related to weight.

Two studies on aphids are relevant to the effects of weight on

resistance to insecticides. Potter and Gillham (1957) sprayed apterous

virginoparae of A. pisum with rotenone in a Potter tower and found that smaller

clover-reared aphids (3.1 - 3.6 mg) were significantly more resistant than

larger bean-reared aphids (5.2 - 6.6 mg) in five out of ten experiments.

The authors suggested that the differences might be explained by the larger

aphids retaining more insecticide than smaller ones. They also attached

possible importance to nutritional effects as indicated by the clover-reared

individuals having a greater percentage of dry weight. They suggested that

the differences in resistance might have been greater if equal amounts of

poison per unit weight had been applied. If their hypothesis was proved

correct, then the effects of size would be the reverse of those observed

for other insect species where the MLD's per unit of body weight, were

either equal for various weicht groups or greater for larger insects.

The second insecticide study involving different sized aphids was

on resistance of Phorodon humili to demeton-methyl compounds (Dicker and

25.

Muir, 1964). Differences in resistance between three strains of the

aphid were related to size of the strains since the largest was the most,

and the smallest the least, resistant. Adjustment for weight reduced

differences, but did not eliminate them. The observed resistances could

have been due to genetic selection for resistance but appear very similar

to the increased tolerances of larger insects quoted by Way (1954) and

Cast (1959).

It is apparent from the literature that there is no constant

factor which can be used to define the relationship between body size and

resistance to insecticides since species differ in their responses to

various insecticides and the same insecticide possesses different potencies

for unequal sized individuals of the same species. There seems, however,

to be general agreement that within a single species larger individuals

are relatively more resistant.

To summarise:

In comparisons between large and small individuals of the same species

the evidence indicates that;-

(1) Larger insects are relatively more fecund, with the

notable exception of locusts;

(2) Longevity and presumably reproductive life of some

insects increase with body size;

(3) Larger insects have a greater capacity for survival

when subjected to:-

(a) natural stresses, e.g. starvation;

(b) artificial stress, e.g. an insecticide;

(4) Little or no quantitative evidences appears to be available

on the ability of eifferent sized individuals of the

same species to survive crowding or extremes of

temperature anc' humidity.

26.

27.

C. MATERIALS AND IvITHODS

a. General methods - aphid clone and rearing methosia.

All the laboratory experiments were done with a clone of .qyrthosA.phcml

pisum established in October 1960 from the larvae of a single virginoparous

apterae The line was reared continuously on potted seedlings of Dwarf

Sutton broad bean which were replaced weekly by fresh 4 inch high seedlings,

some of the aphids being shaken off the old plants onto the new. Crowding

was generally avoided. Six 5 inch pots of seedlings were kept in a rearing

cage 24 inches high by 18 inches square with nylon gauze on the base and

sides and a glass roof.

For about the first nine months the rearing and the experiments were

done in a glasshouse within a conservatory. The temperature in the glasshouse

was controlled at 15.5 ± 0.6°C. between 0000 and 0600 hours al.., and 21.7

0.6°C. for the remainder of the day. Natural daylight was supplemented by

fluorescent lighting to give a regulated 16 hour day. Subsequently, both the

rearing and the experiments were done in constant temperature growth rooms at

20°C. and with 16 hours artificial light per 24 hours, excepting those on the

effects of temperature and photoperiod, details of which are given in the

relevant sections.

Experimental cages.

Three types of cage were used to confine aphids on whole plants or

parts of plants, namely cellulose acetate cylinders (Fig. 1A); clip-on leaf

cages (Fig. 113) and leaf-disc cages (Fig. 2).

The cellulose acetate cylinders were made to fit inside 5 inch plant

pots and had circular gauze-covered ventilation holes and a removable gauze

lid. They were used to confine aphids on whole plants for experiments on

28.

the effects of starvation (p. 60) and photoperiod (p.81 ) on size.

The clip-on leaf cages were similar to those described by Noble (1958).

They differed in having perspex sides and a removeable slip-on muslin lid

and were 1 inch diameter. This type of cage was used for determining

fecundities 93), crowding at different densities (p. 50) and fecundities

under crowded conditions (p.116).

The third type of cage was designed to hold insects for assessment of

mortalities after insecticide (p.132) and high temperature treatments (p.156).

The apparatus comprising 40 cage units is shown in Figs. 2A and 2B. The

apparatus was made as follows. Four rows of ten circles, 74 inch diameter,

were cut out of a 12 x 6 x k inch perspex sheet. A 1/16 inch flange was

milled out of the wall of each circle to a depth of i inch to provide a shelf

upon which rested a 24 inch high, 7/8 inch internal diameter, perspex ring,

which was closed at the upper end by muslin. A sheet of perspex was placed

below the perforated sheet to form a floor for the cages and to stop

evaporation from the filter paper; the two sheets of perspex were bolted

together at each corner. Before this was done, sheets of moist filter paper

were placed between the perspex sheets. They were kept moist by two filter

paper wicks which dipped into two reservoirs of water, one at each end of

the apparatus (Fig. 2A). lk inch diameter leaf discs were cut from fresh

mature leaves of broad bean plants and placed on the filter papers centrally

below each hole in the top perspex sheet. The leaf discs remained fresh and

turgid for at least 72 hours so they were therefore satisfactory for assessing

24 hour mortalities of aphids.

Handling of aphids.

First, second and third instar larvae were removed from the host-plant

FIG. 1A. Cellulose acetate cage used to confine

aphids on potted plants.

FIG. 1B. Clip-on leap cage.

FIG. 2A. Apparatus used to cage aphids on individual

leaf discs.

FIG. 2B. Cross-section or single leaf disc cage.

FIG. 3A. ',later bath used for tests of aphid survival

at high temperature.

FIG. 33. tubes and perspex boxes placed in

water Lath.

PERSPEX RING SPRING CLIP

FOAM PADS

PERSPEX RING

2 U

FIG.I. A

30.

12 C M

< NE-CELLULOID CYLINDER

I7 CM

MUSLIN COVERING AIR VENTS

F-2 CM-1

MUSLIN tr• OVERLAPPING LID

FIG. 2. A.

12"

31.

FILTER PAPER WICK

WATER RESERVOIR

0000000000 0000000000 0000000000 0000000000

MUSLIN COVERING

PERSPEX RING

B.

PERSPEX

FILTER PAPER

Ar---2 5 CM-04

F I '

ST I PP ,

1

LAGGED

GLASS TANK

FIG 3

16 MERCURY COLUMN

THERMOSTAT

33.

with a fine camel hair paint brush, but this method was found to be impractic-

able for the later instars which were larger and more active and tended to

fall from the paint brush.

Older aphids were handled by a suction-operated holder similar to those

of Wallis, Kerr and Hewlett (as illustrated by Busvine, 1957). A short

piece of t inch glass tubing was drawn out to a fine capillary which was

apposed to the thorax of the insect which was held in place by suction. The

most useful aspirator was a simple straight-through tube with mouthpiece and

capillary connected by rubber tubing, and with suction provided orally by the

operator. This method was very convenient for handling single adults,

especially for placing them on the pan of a torsion balance.

The feeding A. oisum usually reacts immediately to any external

stimulus by removing its stylets and dropping from the plant. Thus "tickling'

with a paint brush or the slightest touch with the capillary aspirator was

sufficient to make aphids withdraw their stylets. As shown in Section 2

(p.63 ) starved individuals required somewhat longer periods of stimulation.

Experimental aphids removed from the host plants by these methods

appeared to suffer no damage from handling.

Insecticide microapplicator.

Measured doses of DDT in acetone (cf p.132) were applied with an "Agla"

syringe fitted with a bent canula and a micrometer head. A 6 inch diameter

notched wheel was clamped to the micrometer head and was fitted with a spring-

loaded steel ball-bearing which engaged the notches: two notches represented

a standard delivery of 0.5 Fl of solution. Adult aphids were first

an-l esthetised with a small quantity of carbon dioxide and then a 0.5 Tal

droplet of acetone containing the insecticide was placed on the dorsum of the

abdomen.

34.

Apparatus for experiments on survival at high and low temperatures and high and low humidities.

A water bath was used for controlling high temperature (Fig. 3A).

A glass fish tank 8 x 8 x 12 inches was lagged with polystyrene foam.

Water in the bath was brought to the required temperature by a 1 kilowatt

heater and then reaulated by a 100 watt heater connected by a relay switch

to a mercury column thermostat (sensitivity - 0.01°C.).

Insects to be subjected to heat treatment were placed in a series of

4 inch long glass tubes closed at their lower ends by gauze and at their

upper ends by corks (Fig. 3B). The tubes were suspended in a 5 high x 9 long

x 1 inch wide perspex box, the lower 4 inches of which were immersed in the

water bath. Humidity in the box was regulated by saturated solutions of

appropriate salts placed in the bottom below the level of the holding tubes;

water was used to give a saturated atmosphere. After the required periods

of exposure single tubes were withdrawn from the boxes and the aphids were

put in leaf disc cages (cf p. 28) and examined at intervals to determine

mortalities.

Similar tubes were used for survival experiments at low temperature

(p. Insects in the glass tubes were suspended through holes in

the tops of glass preserving jars which were kept in a constant temperature

room at 10°C. Water or phosphorus pentoxide was placed in the bottom of

the jar to provide a saturated or dry atmosphere respectively.

JD. Measuring methods

Live adults were weighed on a 10 mg torsion balance (sensitivity 0.01 mg.)

and live first instar larvae on a Cahn-Gramme Electrobalance (sensitivity 0.1 pig)

IV,easurements of fourth instar larvae, adults and embryos were made with a x 10

35.

Baker micrometer eyepiece in a binocular dissecting microscope fitted with

a x 2 objective, while measurements of first, second and third instar larvae

were done using a monocular microscope with a x 5 objective and the micrometer

eyepiece. Measurement units were calibrated with a graduated millimetre scale

slide.

Body length and abdominal and thoracic widths of adults, and embryo

lengths were measured on aphids killed in glycerine and alcohol and examined

within the next few minutes. Lengths of other parts, i.e. appendages and

cornicles, were measured by detaching them from the aphid's body and mounting

them in polyvinyl lactophenol on glass slides. Antennal measurements of

first to fourth instar larvae and exuviae were also done on slide-mounted

specimens. Antennal segments were measured separately to avoid errors due

to curvature of the antennae.

Choice of characters to determine size variation.

Weight is theoretically the simplest to obtain and, probably, the

most important measurement of size because it can be obtained without killing

the insect and because it is likely to be sensitive to the conditions under

which the insect was reared, and thus gives a good measure of the stage and

extent of its development. However, weight is seldom static being affected

by feeding, food reserves, embryo formation and parturition. Thus when

making comparisons between different individuals, it is important to measure

weight at the same time in the life cycle, ideally at a time when feeding

does not take place and before reproduction begins. The period immediately

following the last larval moult is the only time when these conditions are

satisfied in the adult which begins feeding after only a few hours. Aphids

36.

were therefore weighod within four hours of moulting to the adult.

Unfortunately, the aphids moulted at all times though moulting was at a peak

in the early morning which meant that many aphids were wasted because the

experimentor could not maintain continuous observation to ensure that all

were weighed within the maximum 4 hour time limit. Body length and abdominal

width are useful complements to weight, but are subject to the same dis

advantages because the abdomen becomes distended after feeding and with

embryo development.

It was decided, therefore, to use some other characters either in

addition to, or instead of, weight as measurements of size. Ideally, the

chosen characters should be correlated with weight, be constant in size

throughout the instar, and be reasonably durable so that they can be measured

at any time after the moult either as preserved or freshly-killed specimens

or in the exuvium. The characters most likely to be of use arc the

appendages, the head and thorax, all of which are strongly chitinised. How-

ever, the head and thorax are difficult to orientate accurately causing

errors in angle of view and hence of measurement. In contrast, the appendages

can be removed from the body and mounted flat on a glass slide. An additional

advantage of using appendages are that they can be determined from the

6xuviac ; so that intermediate instars can be measured without killing the

insect. Appendage lengths can be calibrated with weight but they might also

reveal different allometric growth patterns under various experimental

conditions, e.g. crowding or temperature:

The value of different measurements was determined in two preliminary

studies ofadult apterous virginoparae and first instar larvae.

37.

The adults

Characters chosen as of possible use in the determination of

adult size were: lengths of antennal segments 3, 4, 5 and 6; lengths of

tibiae 1, 2 and 3; length of femur 3; cornicle length; and fresh body

weight. The boundaries used to define measurements are shown in Fig. 4.

The data obtained were analysed to compare measurements made on the

left and right sides of the aphid body and to determine the correlations

between different parts of the body and of these with weight. The differences

between dimensions of the right and left side were analysed by Student 't'

test, and the relationships between the various measurements by correlation

coefficient.

It was found that right and left sides wennot isometric (P <z0.001)2

but the differences had random sign (P > 0.10). Thus, reliance could not be

placed on single measurements from one side of the insect2 and for subsequent

analyses the means of the summed values of each side were used.

Table 1 gives the mean lengths of the appendages and the mean weight

and mean cube root of weight for 84 adults of A. pisum. The cube root of

weight has been used because weight increase is proportional to the cube of

a linear dimension. Table 2 gives the correlation matrix for all characters

measured. Figures 5 and 6, are plots of the third antennal segment and the

third tibia r-,spectively2 against cube root of weight. Fig. 7 is a plot of

the length of the third tibia against the third antennal segment and Fig. 8

shows the length of the sixth antennal segment plotted against antennal

segment 3.

The very close correlation between all the characters used (P 4: 0.001)

indicates that any one would give a good estimate of individual size.

38.

TABLE 1. Ranges means and standard deviations of morphometrico for 84 adult apterous virginoparae of A.4 p-sum.

Antennal segment (mm) Range Mean Standard deviation

3 0.770 - 1.322 1.031 0.138

4 0.617 - 1.117 1.956 0.107

5 0.548 - 1.000 0.775 0.110

6 1.052 - 1.652 1.337 0.179

Total antennal length (3 + 4 + 5 + 6) (mm) 2.987 - 4.957 4.267 0.517

Tibiae (mm)

1 1.404 - 2.196 1.814 0.210

2 1.430 - 2.209 1.813 0.198

3 2.048 - 3.226 2.623 0.303

Femur 3 (mm) 1.130 - 1.852 1.483 0.177

Cornicle (mm) 0.752 - 1.252 1.003 0.141

Weight (mg) 0.54 - 3.19 1.639 0.669

Cube root of weight (mg) 0.814 - 1.472 1.157 0.162

TABLE 2 Correlation coefficient matrix of eleven measurements of A. pisurri. Antennal segments Tibiae Femur

3 4 5 6 1 2 31 . 3 Corlicle Wt. 3N4TE: Total antennal length 3+ 4+ 5+ 6 0.9593 0.9633 0.9634 0.9511 C.9739 0.8903 0.8991

3r--- 0.8289 0.8884 0.8708 0.9049 0.9041 0.8935 0.9087 C.8978 1 0.8843

Wt. 0.8781 0.8102 0.8814 0.8586 0.9006 0.9013 0.8892 0.9017 C.8909 1

Cormic 130.9150 0.8525 0.8889 0.8885 0.9383 0.9413 0.9415 0.9413 1

Femur 3 0.9249 0.9026 0.9248 0.9737 0.9788 0.9846 0.9841 1

Tibiae 3 0.9385 0.9253 0.9372 0.9564 0.9839 0.9500 1

2 0.9607 0.9 79 0.9346 0.9290 0.9949 1

1 0.9239 0.9476 0.9323 0.9266 1

Antennal segments 6 0.8866 0.8750 0.7836 1

5 0.9478 0.9376 1

4 0.9361 1

FIG. 4. Drawings of A. pisum body parts and the limits of

measurements for morphometric studies.

Adult apterous virginopara

A. Adult body showing limits used for body

length, abdomen width and thorax width.

B. Antenna with limits of 3rd, 4th, 5th

and 6th segments.

C. Metathoracic leg - limits of tibia and

femur.

D. Corniclo

Embryo

E. Length of embryo.

First instar

F. Antenna - limits of 3rd, 4th and 5th

segments.

F i & • +.

r 1 c----- E

A r B

C

The effect of various levels of crowding on individual size

of apterous virginoparae A. oisum.

FIG.5. Relationship between length of third antennal

segLent and weight.

FIG.6. Relationship between length of third tibia and

weight.

FIG.7. Relationship between lengths of third antennal

segment and length of third tibia.

FIG.6. Relationship between length of sixth antennal

segment and length of third thitennal segment.

42. FIG.5,

• 1.30

• I •

1.40

1.20 Y = 0.1641 + 0.7497x

1.10

1.00

0.90

0.70

0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

FIG. 6 .

AN

TEN

NA

3 (m

m).

0.80

• •

•

•

I •

• •

Y= 0.2341 +0.9497x

S ••• •

• •• •4°

• • •

• • • •

• • •

• •

1 •

• • • • • • • •

•

•• • • e• •

•

3.20

3.00

TIB

IA 3

(m

m).

2.80

2.60

2.40

2.20

2.00

I I I I I I I 1 0.9 0.9 1.0 1.1 1.2 1.3 1.4 1,5 CUBE ROOT OF WEIGHT.

(mg).

•

• MM.

•

•

1.6 Y -= 0.1415 +1.1531x

F10.7.

Y= -0.0853 + 0.4257x

II I II I I I i II 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3

TIBIA 3 . (mm)

FIG .8.

1.7 O.

••

1.5

1.4 to

1.3 Z E z E H v 1.2 4

1.1

1.0

1 I J. I 0.5 0.6 0.7 0.8

I I I I I 0.9 1.0 1.1 1.2 1.3 1.4 ANTENNA 3.

( m m )

•

•

43.

1.4

1.3

1.2

z 4 0.9

0.8

0.7

• •

•

• • •

• op. V* •

44.

Errors due to chance variation of a single character in a small sample

would be minimised by using two or more characters, including the total

length of antennal segments 3 + 4 + 5 + 6.

A complete analysis of size was made in the work on temperature

effects on size. This was done partly because the above correlations are

based on size variation due to croweina, not temperature, and it is possible

that temperature could have different effects on the morphometrics of the

aphid.

In experiments where the effect of adult size on the biology of the

aphid and adult resistance to stress factors were considered (Sections 3

and 4) weight alone was used as a measure of size. This was done because

the aphids became desiccated during and/or after death and body parts were

brittle and distorted. '3eight and antennal length were used as size

measurements of adults reared, as larvae, in crowded conditions (Section 2,

p. 50), while antennal length alone was used in the photoperiod experiments

(p. 81) whore there were long intervals between inspections, and weight

would have been subject to too great an error.

The close correlations between characters used make it possible to

calibrate weight with any of the measurements used. However, weights were

determined for most of the insects used in experiments so that it was not

necessary to estimate adult weight in the present work.

The first instar larvae.

The impracticability of weighing large numbers of newly born first

instar larvae made it necessary to determine whether there was an association

between any metric measurement and weight at birth. For this purpose the

antennal lengths and birth weights of 136 first instar larvae were measured.

45.

First instar larvae were collected within one hour of birth and

weighed on a Cahn Gramme Electrobalance. The larvae were then mounted in

polyvinyl lactophenal on glass slides and the 3rd, 4th and 5th antennal

segments of both sides were measured. The measurements of each antennae

were summed separately and the mean of the two lengths was used as the

'antennal length'. The antennal length was tested for correlation with

weight at birth and was used to estimate weight using a regression equation.

A scatter diagram of the data and analysis by correlation coefficient

(Fig. 9) showed a significant relationship between weight and antennal length

(r= 0 37, P 4:0.001) so that it was reasonable to predict weight by

regression where

Y = - 95.83 + 185.50x,

being the birth weight in microgrammes and x the length of the first instar

antenna in mm. Table 3 contains the test of significance of departures from

the linear.

TABLE 3. Test of significance of departure from the linear of data on weight against antennal length of first instar larvae.

Degrees of freedom

Sums of squares

Mean square

Linear regression 1 11,577 11,577.0

Departures from linear 134 12,262 91.5*

Total 135 23,839

F = 126.51 P <0.001

The standard error of b was - 16.4930 which gave confidence limits

for b of 153.1778 to 217.8299.

Thus it was possible to determine the weight of first instar larvae

produced by apterous virginoparae and to use this as a measure of the biomass

FIG. 9. Regrassion of weight on length of antennae

(segments 3 + 4 + 5) of first instar larvae.

110

FIG I. 47.

• Y = -95913+ 18540x

•

e• Os

• • • • 4°9 • I.

• • • il 0 •

• • 0.4P• I. • • • • I

• see_ I •. ... 60 • • • • ••• •

Is • N • . • • • • .

ei • • •

•

60 •

0.90 1.00

ANTENNAL LENGTH. (mm)

1.10 1.20

48e

of larvae (cf. Section 3, p. 10.0. Such antennal measurements of the larvae

can be made at any time between birth and the moult to second instar, and

could no doubt be done on the first instar larval exurvium.

SECTION 2.

Causes of size variation in Acyrthosiphon pisum.

49.

50.

A.THE EFFECT OF LARVAL CRGIDING ON ADULT SIZE.

Aphid species aggregate differently, thus A.fabae Scop. (Ibbotson &

Kennedy 1951) and Brevicorvne brassica L. form very compact aggregates,

while Mvzus persicae Sulz tends to spread more evenly over a leaf.

Acyrthosiphon pisum, observed in the field at Silwood Park, forms loose

aggregates on beans, Vicia faba L., vetch, Vicea sp,-, and on alfalfa,

Medicaqo sativa. Exceptionally many adults were clustered on the stems

and leaves of these plants but spaces were still maintained between

individuals. It might be expected that A. pisum, would suffer from

crowding at densities intermediate between those of say B. baassicae and

M. persicae. This question is dealt with in detail in the Discussion.

Crowding might decrease size of aphids in one or more of the

following ways:

(1) by damaging the host plant (including direct feeding damage,

contamination by excrete and reduction in photosynthesis), and thereby

restricting food intake or decreasin:., quclity :f the foce;,

(2) by mutual disturbance which interferes with feeding,

(3) by stress resulting from restriction on space Inhich .might upset

neuro-homeostatic mechanisms without necessarily decreasing food intake.

The following experiments were done to establish the levels of

crowding which were deleterious to A. pisum and to indicate which factors

were responsible for the observed harmful effects.

a. The effect on size of adults of crowding three generations of Issrthosiphon

aim larvae.

60 first instar larvae from adult apterousvirginoparae were put in

2 Weight

Antennal 3rd Embryo length

3 Weight Antennal 3rd Embryo length

(10)

2.391 ± 0.166

1.161 ▪ 0.012 0.779 - - ,,,

2.264 1.152 0.760

(24)

1.403 1:

1.063 0.678 -

0.050 1.758

0.014 1.097 0.015 0.740

(10) _7 ._ 0.130 0.015

± 0.017

(141 1.192 - + 1.072 T 0.700 -

0.101

1.278 0.017

1.053 0.012

0.726

51.

each of two leaf cages and kept until they became adult. A sample of 10 newly-

formed adults from each cage was then weighed and placed in each of two further

leaf cages and allowed to reproduce, producing an F2 generation which was

crowded at the same density, i.e. 60 first instar per cage. The procedure was

repeated with the F3 but crowding was increased to 100 larvae per cage. Adults

not used to produce following generations were weighed, some were dissected and

the lengths of the six largest embryos in each were determined. The third

antennal segments of each adult were measured. It was not possible to measure

all the antennae because some were damaged, probably while moulting in crowded

cultures. Other aphids were reared individually in leaf 'ages for three

generations. The experiment was conducted in the glasshouse (See Section 1 ).

Results

Mean values for weight, antennal lengths and embryo lengths are shown

in Table 4 .

TABLE 4 Mean morphometric data for three generations of adults reared simultaneously under crowded and isolated conditions. (Number of insects measured in parenthesis).

Generation Measurement Isolated Crowded

1

(10)

A

(18)

Weight (mg) 2.639 ± 0.208 1.561 ±0.075

Antennal 3rd (mm) 1.167- 0.020 1.048- 0.018

Embryo length (mm) 0.797 ± 0.029 0.690 ± 0.019

± standard error.

1.875

1.092

0.688

B

(21)

- 0.097

- 0.016

± 0.013

(19)

± 0.120

41; 0.025 = 0.022

+(9) T 0.120 0.020 0.040

52.

Mortalities in the crowded cages were as follows:-

Generation Replicate A B

1 40% 45%

2 30% 35%

3 74% 71%

By the time larvae became adults the leaves showed extreme necrosis and

desiccation. It is reasonable to conclude that sap flow had largely ceased or

was much diminished.

Variance analysis confirmed differences between crowded and uncrowded

treatments ( P 4:0.001) fox all measurements but only the weights of crowded

individuals decreased significantly (P-‹ 0.01), in successive generations.

Analysis,by 't' test, of the two crowded replicates showed, however, that weight

decreased significantly only between the 1st and 3rd generations (P -40.05).

The lack of decrease in antennal and embryo lengths in successive

generations requires explanation. In order to produce enough 2nd and 3rd

generation larvae for crowding, it was necessary to leave F2 and F3 parents in

the cages for up to 4 days. This period included a pre-reproductive period

of at least 24 hours, during which time adults could feed and partly recover

from deleterious crowding effects (See Section 3 on pre-reproduction period).

The required number of larvae was produced over the next 2 to 3 days, after

which adults were removed. It is probable that some larvae were not subjected

to overcrowding until the 4th or 5th day of development. A proportion would

have had time to recover partially (See Section 3 on 1st instar development).

This would apply even to the third generation where crowding was increased,

thus crowding was not as intensive as final numbers suggest. Larvae developing

early would be the most likely to survive and would be larger than late

developers. Development might have been normal until midway through the larval

period, by which time antennal growth might have been determined and the

n

53. largest embryos well formed, but weight of the adults will not have been

determined and development of younger embryos may still have been affected.

These conditions do not simulate natural conditions when generations

overlap and crowding may be continuous with stress uninterrupted between

generations. The effects of different amounts of crowding of the larvae, when

stress would begin to operate at different stages of larval development, were

investigated in the next set of experiments.

b. The effect of larval rearing density on size.

4-day old adult apterous virginoparae were kept overnight on broad bean

seedlings and then the newly born larvae were transferred to leaf cages on

fresh seedlings. The following densities of first instar larvae per leaf cage

were set up: 1, 4, 16 and 32, and the larvae were reared at a constant tempera-

ture of 20oC.

Experiment 1. Each leaf cage was kept on a single leaf throughout larval

development. Mean weights and lengths of third antennal segments of adults from

larvae reared at each density are shown in Table 5 and Figures 10 and 11.

Although the parents of the larvae used in this experiment were reared

in uncrowded conditions on young broad bean plants, which would be expected to

prevent alate production, many larvae developed into alatae (Table 5 ). Larval

density did not affect the proportions of alatae formed (F., 3 D.F., 3.532,

p 0.10), but this unexpected development meant that, in the treatment where

the aphids were reared singly, only 5 apterous adults were produced. This

limited the value of the treatment.

Mortalities did not increase with crowding (Table 5 ) and were

relatively low. This indicates that crowding stress was not excessive, even

at densities of 64 per cage, and this is reflected in the small changes in

TABLE 5.. Mean weights and third antennal segment lengths of A. pi sum reared at various cage densities (Experiment 1). (Number of insects measured in parenth.3sis).

No.per cage

Weight (mg)

No. of individuals measured

Third antennal No. of Weight segment (mm) individuals Cam0

measured

Percentage Percentage mortality elate

Third antennal during segment (mm) development

Apterae

Alatae

1 5 2.230 ± 0.185 1.170 ± 0.023 9 1.577 ± 0.C47 1.053± 0.018 12.5 64.3

4 19 2.568 +-0.119 1.123 ± 0.012 11 1.421 ± 0.052 1.024 ± 0.013 6.3 36.7

16 2T 2.084 ± 0.073 1.101 - + 0.010 20 1.355 ± 0.039 1.037 - 0.017 3.1 53.2 *

64 44 1.402 ± 0.041 1.053 ± 0.008 24 1.096 ± 0.0213 0.992 ± 0.012 5.5 33.9 *

* Values for percentages of alatae refer to aphids which developed at each density and not to those

measured, since only those with undamaged antennae were used.

FIG. 10. The effect of larval rearing density on weight

of apterous (0 ) and elate ) virginoparae

A°212MM•

FIG. 11. The effect of larval rearing density on length

of third antennal segment of apterous (0 )

and elate ( O) virginoparae A. nisum.

FIG.10.

2,50

2.00

E

O • 1.50

1.00

1 4 16 64

E E 1.20

z I— ;

41:9 i° 1.00

2 —I 0 el /". z

z 0.60 z

FIG.11„

I 'a la 1 I— z st

4 REARING

16 DENSITY.

64

(LARVAE/LEAF CAGE)

56.

57.

antennal length. The non-significant greater mean weight of individuals

reared 4 per cage, over those reared one per cage is not reflected by antennal

length. Decreases in weights of apterae at the higher densities (4 - 16 and

16 - 64, P 4:0.001) seem to occur independently of antennal size between

4 and 16 per cage (difference not significant) but not between 16 and 64 per

cage (P <0.001). Again this might be a reflection-of stress occurring late

in larval development. Thus, with moderate crowding the available food might

development allow normal exoskeletal/but not the development of food reserves and embryos

which would contribute to much of the weight of the newly emerged adult.

Alatae reared under the same conditions responded somewhat differently

to increasing density. Decreases in weight did not occur between 1 and 4

and 4 and 16 pex cage but were significant between 16 and 64 (P <0.001) and

Jess significant decreases in antennal lengths were also observed (16 - 64,

P‹ 0.05). Greater stress is probably required to induce decreases in size

of alatae comparable to those caused in apterae. This might be important in

the field where the species could benefit from the ability of the 2%;pulation

to produce fit elate emigrants under conditions where apterae were unable to

develop normally.

Experiment 2. Two series of larvae reared at densities of 4, 16 and 64 per

leaf cage, were treated concurrently. The larvae of series A were confined

to the same leaves throughout development, as in Experiment 1 above, while

larvae of series B were moved to fresh leaves on the 3rd, 5th, 7th, 8th and

9th days. The aim was to maintain similar levels of crowding with and without

adequate food. This would distinguish between size decrease due to mutual

disturbance and decrease caused by absolute shortage of food.

The results are in Table 6.

TABLE 6. Mean weights and lengths of third antennal segments (Experiment 2). (Number of insects measured

of A. ydsum reared at various densities in parenthesis).

No/cage

4

16

64

(8)

(17)

(34)

Apterae Alatae Weight (mg) Third antennal segment (mm) Weight (mg) % Mortality

A. one host plant through larval development

1.990 ± 0.094 1.1113 ± 0.0273 (6) 1.389 1: 0.101 12.5

2.086 ± 0.099 1.0774 - 0.0097 (12) 1.325 t 0.055 6.3

1.430 t 0.041 1.1069 ± 0.0075 (1I) 1.031 ± 0.063 28.1

% Alate *

37.5

43.3

23.9

B. Host plant changed on 3rd, 5th, 7th, 8th and 9th day.

4 (12) 1.937 t. 0.070 1.1026 - 0.0128 (4) 1.275 ± 0.056 0 25.0

16 (18) 1.819 ± 0.075 1.0808 ± 0.0105 (4) 1.188 t 0.068 31.3 18.2

64 (35) 1.263 ± 0.038 1.0457 ± 0.0094 (14) 1.056 t 0.C34 21.9 28,0

* Proportion of total aphids which developed to adult.

Mean ± 1 standard error.

59.

Again, proportions of alatae were not related to density, but

mortality was slightly increased among individuals reared 64 per cage. A

mortality of 31.3% at 16 per cage of the B series, was partly due to a

fungal infection late in development.

Comparisons between A and B apterae reared at equivalent densities

revealed weight differences at 16 per cage (P <-'0.05) and 64 per cage

(P <0.001); that is, aphids were heavier when not transferred to fresh

leaves. In terms of antennal length the adults did not differ in size,

except with individuals reared 64 per cage when the A treatment produced

larger adults (P <0.001). On the basis of these size differences in apterae,

it would appear that long-term host plant deterioration is not the governing

factor and that crowding, with associated effects of disturbance and

restrictions on space is a critical influence, especially in determining

weight.

Within each series there were no differences between individuals

reared 4 and 16 per cage. In both series apterae reared 64 per cage were

lighter (P <0.001), but only where the host plant was changed (treatment B)

had significantly smaller antennae (D< 0.025).

The weights of alatae reared at equivalent densities did not differ

significantly. Within treatments, differences between those reared at 4 and

16 per cage were not significant, nor between 16 and 64 in B, while in A,

individuals reared 16 per cage were significantly heavier than those reared

64 per cage (P <0.005). In both series individuals reared 4 per cage were

significantly heavier than those reared 64 per cage (P <0.01).

60.

c. The effect of larval starvation on development and size of adults.

The previous experiment suggested that in crowded conditions absolute

food shortage may have less effect on size of aphids than mutual disturbance.

This experiment was done to determine the effects of food shortage on size

and developmental mortality in conditions where possible mutual interference

factors could not operate.

Second instar larvae (36 - 3 hours after birth), produced by apterous

virginoparae during the 4th day of reproduction, were starved singly in corked

2 x 1 inch glass specimen tubes for periods of 0 hours (A), 6 hours (B),

6 + 6 hours (C) and 12 hours (D) daily at 200C constant temperature. Each

tube contained discs of moistened filter paper to maintain high humidity and

prevent desiccation of the aphids. Group C (6 + 6 hours), was starved for 6

hours, replaced on the host plant for 3 hours, and starved for another period

of 6 hours daily. After each period of starvation larvae were replaced on

young broad bean seedlings. These treatments were continued until the larvae

became adult. Adults were weighed within 2 hours of moulting and the four

distal antennal segments were measured.

Two other treatments were also set up using larvae starved in groups of

5 and 10 per tube.

Ten aphids were used per treatment and because of time and space needed,

the experiment was done in two parts (I and II in table 7 ), separated by an

interval of two weeks. Two replicates of each treatment were included in

each part. The data on effects of starvation period and number per tube were

analysed separately by Students 't' test.

Results

Mean values obtained for starved groups are shown in table 7 , and

61. the results of analysis in Table 8 .

Percentage mortalities increased with length of starvation and wore

similar for the two 12 hour periods (C and D).

TABLE 7 . Mean values of weight, antennal length (segments 3-6), development time and mortalities obtained from four levels of starvation of larvae of A. pisum.

Hours starvation/24bours.

Time Replicate 0 (A) 6 (B) 6 + 6 (C) Antepnal Lengths (mom)

+

1 1.432 - 0.282 4.206 - 0.258 3.959 - 0.255 I + ± ±

2 4.312 - 0.290 4.076 0.182 3.827 0.179

1 3.881 ± + +

0.414 4.224 - 0.127 3.965 - 0.196 II ± ±

+

2 4.275 0.146 4.272 0.119 3.941 - 0.156

Poole': mean 4.303 +- 0.036 +

- 0.036 4.184 - 0.024 3.912 - 0.022

+ (Weight (mg) 4. ++ 1 2.386 - 0.240 1.968 - 0.140 1.27E - 0.201

I + ± ±

2 2.301 - 0.314 1.872 0.287 1.015 0.2C4

1 1.563 ± 0.335 1.821 - + 0.859 1.053 ± - 0.121 II

2 2.053 ± 0.230 1.799 ± 0.364 1.0O2 t 0.200

Pooled mean 2.188 ± 0.048 1.870 ± - 0.028 1.097 - + 0.020

Development time (Hours)

1 149.6 - + 10.23 161.5 ± - 11.42 168.1 + - 6.39 I ± ± ±

2 152.1 5.18 157.2 9.03 166.8 8.89

1 164.7 ± ± +

10.75 166.9 7.58 175.5 - 9.58 II ± + ±

2 164.9 6.38 163.8 - 4.27 172.4 7.64

Pooled mean 156.9 t 1.34 162.1 ± 1.08 170.2 ± 1.60

Total number insects surviving per treatment 49 85 74

Mortality (Percentage)

1 0 3.3 20 I

2 0 0 16.6

1 0 6.6 13.3 II

2 0 3.3 46.6

Mean mortality 0 3.3 24.1

Replicate means ± - 1 standard deviation

12 (D) + 3.930 - 0.154 + 3.953 - 0.164 + 3.828 - 0.173 + 3.851 - 0.269 ± 3.903 0.019

1.139 - 0.137 +

1.186 - 0.261

1.005 ± - 0.162

0.943 t 0.262

1.084 ± - C4033

164.5 + - 6.90 +

164.3 - 9.34 +

176.1 - 6.11 ± 186.6 14.14

172.0 t 1.80

59

30

30

66.6

26.6

38.3

+ Pooled means - 1 standard error

62.

TABLE 8. Summary of comparisons between treatment means and probabiity based on students 't' test. levels

Weight (mg)

Antennae (mm)

Development (Hours)

D

C

B

D

C

B

C

B

A

1.1034 *

<0.001 +

1.0907

.c.0.001

0.3174

0.4003

0.3917

0.001

0.1192

•t 0.301

15.11

0.001

13.30

0.001

5.21

0.01

B

0.7860

.‹. 0.001

0.7733

0.001

0.2811

0.001

0.2725

c:0.001

9.90

0.001

8.09

0.001

C

0.0127

ns

0.0086

ns

1.81

ns

* mean diffetende + probability level

ns not significant

Analysis by 't1 test (Table 8 ), showed that increase in the starvation

period decreased weight and antennal length, while time from birth to adult was

increased. The method of starving for 12 hours, whether in one period of 12

hours or in two periods of 6 hours, did not significantly affect size or

development time; although those starved for 12 hours continuously were

consistently smaller in weight and antennal length, and took longer to develop.

63.

Analyses of variance, using mean values for replicates por treatment

(Table 7 ), showed that the division of the experiment into two time

replicates was a major source of variation. This was particularly true of

weight (F = 7.831, P.c. 0.05 and development time (F = 27.855, P< 0.001),

but did not affect antennal length(F c 1). Differences could have arisen as

a result of variations of factors such as; host plant status, slight differences

in handling (although standardised as much as possible) and initial food

reserves of the larvae used.

A change of behaviour was noted in groups starved for 12 hours. Normal

reactions of A. pisum induce it to drop from plants when touched or when

breathed on (Lowe and Taylor, 1964). This occurred when unstarved larvae or

larvae starved for 6 hours were touched with a paint brush when transferred to

specimen tubes. Aphids starved for 12 hours did not react in this way and needed

to be stimulated for a longer period before they would withdraw their stylets

from the plant. When stimulated with a paint brush larvae sidled sideways

pivoting on their stylets. This response was continued for two or three minutes

in some cases, as opposed to normal withdrawal in seconds. Dr. Kennedy (personal

communication), has observed similar behaviour in Tuberolachnus salionus, and

observations have confirmed long withdrawal times in Aphis fabae and Brevicorvnt

brassicae. The habit may be associated with deep feeding, as in T. salionus

where a thickness of bark has to be penetrated before the internal feeding site

is reached, or to some structural feature of the mouthparts which prevents quick

withdrawal. In view of the normal time required for A. pisum to withdraw

stylets, these.observations indicate a change of behaviour in respect of

disturbance response and/Or of actual feeding site within the host tissues. It

appears that starved larvae may seek to make good deficiences by staying on the

plant for longer periods and possibly use feeding time to greater advantage

64.

by tapping deeper sources of sap.

Conclusions.

It can be concluded that crowding affects size and that weight is affected

more than body dimensions, such as antennal length. Within the range of densities

investigated, levels of crowding greater than 5 larvae per square centimetre of

leaf surface decreased size, especially weight. Decreases in weight of larvae

reared at densities of 64 per leaf cage (20 1Prvae per square centimetre of leaf

area), were equivalent to those l?rvae starved for 12 hours per day, but third

antennal segments of the former were relatively longer. It is suggested that,

whereas the starvation treatments imposed stress from an early stage of develop-

ment, crowding stress did not operate until the larvae became relatively larger,

perhaps not until the last larval instar.

Crowding may operate harmfully in two ways, by creating a situation

whereby there is not enough plant food for all individuals to develop satisfact-

orily (absolute shortage of food), and also by causing interference between

individuals in restricted space similar to that described by Klomp (1984) for

larvae of aapalLs piniarius. Mutual interference might stop individuals feeding

at the optimum rate and therefore harm them through starvation, or it might also

prevent the full utilisation of food by the individual without decrease in the

amount ingested.

65.

B. The effect of temperature on size of apterous virginoparae of Acyrthosiphon pisum

That temperature is important in determining size of many insect species

is shown by the literature reviewed in Section 1, but more specifically, it is

known that aphid size is also affected by temperature (Wilson, 1938; Kenten, 195;

Lees, 1959). In view of the results obtained by these authors and the lack of

quantitative evidence showing directions and magnitudes of such changes in aphids,

it was decided to study size of A. pisum reared at different temperatures and

also to compare the effects of temperature with those of crowding.

The aphids were reared at five constant temperatures, 100, 15o, 20

o, 25

o

and 28°C., and at a controlled photoperiod of 16 hours per day. All parents

of the aphid lines were apterous virginoparae reared under uncrowded conditions

at 20°C. Parents were placed on young broad bean seedlings overnight at the

temperatures at which subsequent rearing was to take place. Numbers of larvae

were controlled such that crowding was prevented.

Aphids reared at 25°C and 28°C died out after three generations and one

generation respectively. Similarly, at 10°C the aphids were less fecund and

only one generation was obtained. At 15°C no size changes were observed over

three generations, when the line was terminated, and only one set of data has

been used for analysis. Since 20°C. was the normal rearing temperature of the

stock culture, it was assumed that there would be no observable temperature

effects, only one set of data was collected. Two more treatments were set up.

These were with the progeny of second generation adults reared at 25°C. which

were transferred to 20 C. and kept through two generations. Long term effects

of damage caused by high temperature were assessed on the basis of recovery

over these two generations.

66.

Criteria used to determine size were weight and also lengths of:

antennal segments 3 and 6, tibiae 1 and 3, femora 3, body (from cauda to frons)

and width of the thorax. Interrelationships between these characters were

examined by correlation coefficient, regression analysis and ratios between

sizes of some selected characters.

Analysis of the complete data yielded the correlation coefficients

in Table 9 . Clearly there are high positive correlations between characters.

This is especially true of the three leg measurements (r= 0.99), and probably

one leg measurement would be sufficient to represent changes in leg size.

There is also an expected, close correlation between body length and weight

(r= 0.94) since changes in body length are mainly reflections of abdominal

growth and increases in volume which are closely associated with weight

increases within, and between, instars. By contrast correlations of the sixth

antennal segment with other characters are more variable (r = 0.67 for thorax

width, to r = 0.88 for third antennal segment).

Mean metric values of character sizes for each temperature group are

presented in Table 10. For convenience these are considered in two parts,

namely: effects on first generation adult size, and then effects on later

generations.

a. The F1

Generation

....1••••••••••.

Although correlation coefficients indicate close association between

body parts, discrepancies arise when these characters are used as size indices

to compute rankings of decreasing group size; similarly, variations are

found in the magnitudes of differences between touching groups. For instabce,

the weights show the following order of decreasing size in relation to

temperature; 100 > 150 > 200 > 25o >

280C0while lengths of third antennal

TABLE 9 . Correlation coefficient matrix for 8 characters from pooled morphometrics of

Antenna 3

A.pisum adults reared at various temperatures.

Tibia 3 Femur g Body length Thorax width

n = 136

Weight Antenna 6 Tibia 1

Weight 0,8904 0.7815 0.9031 0.8720 0.8719 0.9392 0.8854 1

n = 148

Thorax width 0.8604 0.6696 0.8769 0.8830 0.894C 0.9220 1

Body length 0.9184 0.7263 0.9345 0,9336 0.9322 1

Femur 3 0.9074 0.7844 0.9374 0.9943 1

Tibia 3 0,9575 0.7998 0.9929 1

Tibia 1 0.9527 0.6932 1

Antenna.. 6 0.8780 1

Antenna 3 1

TABLE 10 Morphometrics for A. pisum adults reared at various temperatures (mean values ± standard error; weight in mg; length in mm.)

No. insects

(1) 10°C.F1 9

(2) 15°C.F1 20

, o (3) 20 C.F1 10

(4) 25°C.F1 16

(5) 25°C.F2 20

20 (6) 25/-YU0C.F3

(7) 25400C.F4 20

- (8) ...25.

oC.F3 20

(9) 28°C.F1 '13

Weight (mg)

2.394 - 0.140

2.295 - + 0.078

+ 1.993 .. 0.107

1.889 - + 0.070

1.265 - + 0.048

± 0.666 0.034

2.243i3=-11-

0.116

0.534 ± 0.028

1.194 ± 0.095

+i-

3rd antennal segment (mm)

1.054 - + 0.012

1.120 + - 0.007

+ 1.070 - 0.015

+ 1.023 - 0.011

0.925 t 0.010

+ 0.815 - 0.014

1.088 - 0.013

0.671 ± 0.1013

0.829 ± 0.019

6th antennal segment (mm)

1.560.-+ 0.011

1.607 -0.009

1.431 ± 0.011

1.316 0.020

1.156 ± 0.016

1,261 ± 0.019

1.500'- 0.016

1.021 ± 0,015

1.034 ± 0.019

1st tibia (mm)

1.903 ± 0.030

2.074.± 0.017

1.910 .!. + 0.020

1.940 t 0.011

1.764 t 0.016

1.494 + - 0.

1.905 - + 0,022

± 1.289 0.019

1.647 ±- 0.019

3rd tibia (mm)

±

2.658 0.037

+ 2.936 - 0.022

2.696 0.031

2.757 ± 0.017

2.472 t 0.019

+ 2.065 - 0.029

2,678 - + 0.

±

1.764 0.019 +

2.262 - 0.031

+019

±+

Irdd fetal'Body (mm}

±

1.522 0.020

+ 1.674 - 0.011

1.566 ± 0.020

1.584 ± 0.011

1.394 ± 0.013

1.145 - 0.016

±036 1.525 C.025

0.983 t C.011

±

1.291 0.024

Length (mm) Thorax

width (mml

+- 0.730 - 0.013

±

0.690 0.006

±

0.713 0.009

0.682 ± 0.005

0.634 ± 0.004

0.545 ± 0.005

0.694 ± 0.010

t

0.500 0.004

+t0.063 0.656 - 0.011

3.762 ± 0.060

+ 3.997 - 0.045

3.733 - 0.081

3.698 0.045

3.311i-10.047

2.583 -0,050

3..950 - 0.067

+ 2.275 - 0.043

3.148,

* only 8 insects weighed.

69.

segments give; 15°•> 20°> 10° 25°> 28°C., and leg mesurements yield:

15> 25°'> 20o > 10

o > 28°C. Clearly, there are alterations in the relative

proportions which are not reflected when the data are considered in terms of

correlation coefficients. Changes in shape of a single group would not

necessarily be shown by considering pairs of characters irrespective of group

origin, especially when sizes range over the wide limits discussed here.

However, character size can be considered in terms of a second

character. Thus ratios are determined, whereby disproportions can be more

readily examined. Plots of several ratios are illustrated in Figures 12 & 13.

In Fig. 12 cube root of weight has been used as the standard measure-

ment and ratios have been plotted against temperature. All ratios of 10°C

are smaller than the corresponding values for 15°C., even for body length.

The tibiae are relatively longer at 15° and 25°C, while the third antennal

segment increases in relative length from 10°C to 20°C and thereafter

decreases. Antennal segment 6 steadily decreases in relative size at

temperatures above 15oC. This is also shown in Fig.13 where ratios of body

length, lengths of tibiae 1 and 3, and antennal segment 3 to antennal segment

6, all increase, showing relative decrease in length of this segment. Antennal

segment 6 appears to be the part most affected by temperature. Ratios of

tibiae 1 and 2 to this segment might be partly explained by the increase in

absolute length of leg segments at 25°C. (Table 10), but does not account

for other larger values.

.The F2 and F

3 generations at 25°C, and recovery at 20°C.

Adults Of the F2 and F3 generations reared at 25°C. were smaller than

F1 adults, F

3 adults being the smallest. As stated above, they did not

reproduce and the line died out.

FIG. 12. Ratios of lengths of body parts to cube root of

weight for adult apterous virginoparae A. pisum

reared at five different temperatures.

FIG. 13. Ratios between lengths of various body parts

for adult apterous virginoparae A. eisum reared

at five different temperatures.

Key to FIGS. 12 an-.4. 13.

BL - body length A3 - third antennal segment;

A6 - sixth antennal segment; T1 -• first tibia;

T3 - third tibia.

3

O_I 4 2

BL /AI)

T346

Ty A

Ti,A3

O 17 4 0:

3

2

1

0

a.=

,...

FIG.12.

BL

T3

T1

A6 A3

i I i

'''".....

I I 10 15 20 25 28

FIG.13.

71.

1

0

Ai k6

I I I I I

10 15 20 25 28

REARING TEMPERATURE.

( °C )

,—,

1.-

72.

Trends of size change were similar to those outlined above for F1

generations. However, the sixth antennal segment apparently approached its

minimum length in the F2 generation at 25°C, and, relative to other body

measurements, did not exhibit the same rate of decrease in the F3

(Tablell ).

TABLE 11. Ratios of characters' sizes to cube root of weight.

Temperature & Generation

Character

25°C.F2

25°C.F3 25o/200c.F3 25°/20oc F4

Third Antennal segment 0.856 0.827 0.933 0.826

Sixth antennal segment 1.069 1.258 1.444 1.146

First tibial segment 1.631 1.589 1.711 1.455

Third tibial segment 2.286 2.175 2.365 2.046

Third femoral segment 1.289 1.212 1.311 1.165

Body length 3.061 2.805 2.958 3.018

Thorax width 0.586 0.617 0.624 0.530

When larvae of F2

adults reared at 25°C. were reared at 20°C size

rec)very was not immediate, but in the next generation size approached the

norm for apterae reared at 20°C. Although recovery was not completed in one

generation, ratios of body parts to cube ro't of weights (Table 11) increased.

Ratios were greater for the 25°/20°C.F3

groups than for any other generation

examined at the various temperatures, that is, weight did not increase in the

same proportions as other characters considered here. Reference to Section 3

will show that adult apterae of this generation were not very fecund (mean of

5.7 larvae,per adult) and thus, lack of embryos in the young adults could

account for their lower weights. Antennal segment 6 showed the greatest

proportional increase in size and seems to be the most sensitive to temperature

stress. Ratios for 25°/20°F4 adults were normal.

73.

Antennal segments 3 and 6 and tibia 3.

Effects of high temperature on antennal segments 3 and 6, and tibia 3,

were further examined by regression analysis of each character in terms of the

others, giving three sets of lines in all. The regressions of antennal 6 on

antennal 3, and tibia 3 on antennals 3 and 6, are plotted in Figures 14) 15 & 16

It is obvious that the plots can be divided into two quite distinct groups,

0 namely 1, 15°C., 25 C.F1, 25 C.F2, 25°AC °C.F4 and 28°C, and 2 25°C.F3 and

25o,/20C.F2. Regression lines based on this division have been calculated,

and all fits accounted for significant amounts of variation (P er0.001).

The changes in relative proportions noted above are re—emphasised when

temperature and generation effects are combined. However, the tibial lengths

of apterae of two groups, 25°C.F1 and 25°/20°C.F4, are not well fitted to the

calculated line. They occur mostly above the line in the former and mostly

below in the latter group. These discrepancies are particularly noticeable

in Figure 16, where greatest separation of groups has occurred. If might have

been more expedient to consider second degree polynomials, where group

o o 25 AO C.F4 would form an extension of the curve through 25°C,F3, and

25°/20°C.F4, • certainly tibia 3 and antennal 6 do not behave linearly throughout

the whole range of temperatures used, but the analyses used illustrate sharp

distinctions between long and short term effects.

Conclusions

Size of apterous virginoparae of A. pisum was inversely proportional

to the temperature at which they were reared (confirming Kenton, 1955), and to

the time of exposure. A weight decrease of about 75% occurred between those

reared at 15oC. and those reared at 28

oC. for three generations compared with

a 90% decrease in weight recorded for c1. viciae by Lees (1959) The

74.

temperature below and above which viability and size are reducild is in the

region of 15o and 20

oC. In view of the results obtained at 10

oC a more

exhaustive study would probably be useful to clarify some points, for example,

loss in viability and greater weight relative to body measurements.

Variations in proportional sizes of separate characters are such that

the taxonomic value of ratios is doubtful at times when homeostatic regulatory

mechanisms are upset under stress. However, ratios are probably more useful

than measurements of absolute size taken in isolation without reference to

several other characters. Whether re-establishment of normal ratios, which

occurred in the group reared for three generations at 25°C. would be maintained

over further generations at 25C could not be determined because the F3 was

inviable. However, it seemed that homeostatic equilibrium had been re-

established.

FIG. 14. Regressions of length of sixth antennal segment

on length of third antennal segment for apterous

virginoparae A. pisum reared:

for one generation at 15°C. (ID);

one generation at 25°C. ( 0

two generations at 25°C. (X);

three generations at 25°C. ( );

one generation at 23°C. (III);

two generations at 25°C.

plus one generation at 20°C. ( • );

two generations at 25°C.

plus two generations at 20°C. (Q ).

FIG.14 . 76.

Y.= 0.0604 + 1.4541 x

0

LEN

GT

H S

IXT

H A

NTE

NN

AL

SE

GM

EN

T (

mm

).

Y= - 0.4767 + 1.8091 x

I I I I I i I I 0.50 0.60 0.70 0.50 0.90 1.00 1.10 1.20

LENGTH THIRD ANTENNAL SEGMENT (mm ).

FIG. 15. Regressionsof length of thire tibial segment on

length of thire antennal segment for apterous

virginoparae A. pisum rearec1;

for one generation at 15°C. (ID );

one generation at 25°C. ( C));

two generations at 25°C. ( X );

three generations at 25°C. ( 11, );

one generation at 28°C. (III);

two generations at 25°C

plus one generation at 20°C. ( 41 );

two generations at 25°C

plus two generations at 20°C. ( )

• • •

••

A A

A A A •

F10.15.

71,

•

3.20 i-•

3.00

2.60

2.60

.-. E 2.40P- E

10 F 2.20e. 0 Z — r I- i 2.00--1- z .. w -a

1.80 .-.

m••

Y=0.6753+1.9491x

•

es

o . d#2' o (a 66 a 0 0 00 A

o A A A

xx ok

x ° a as x x x 1.4x x

- x x I °II P A

1.60 Y =0.4537 + 1.9641 x

1.40 ...

1 I 1 I I 1 1 0.60 0.70 040 0.90 1.00 1.10 1.20

LENGTH THIRD ANTENNAL SEGMENT (mm).

1 0.50

FIG. 16. Regressiorrsof length of third tibial segment on

length of sixth antennal segment for apterous

virginoparae A. pisum rearee:

for one generation at 15°C. ( al);

one generation at 25°C. (C));

two generations at 25°C. ( X.);

three generations at 25°C. ( A );

one generation at 28°C. (III);

two generations at 25°C

plus one generation at 20°C.

two generations at 25°C.

plus two generations at 20°C. ( ).

F1G.16.

80.

•

3.20

3.00

2.80

2.60 •

•••••

E E

2.40 4

N

2.20

2.00

Imo

1.60

Y = 1.3402 + 0.9657x

a P o o 80 a a

(9 X00 A A

I x MI

X X 0 X X as a

x

• x x

■ •

Y =0.5191 +1.2229x 1.60

1.40

A

1 0.90 1.00 1.10 1.20 1.30 1.40 1.50 1.60 1.70

x

ANTENNA 6 (film 1.

81.

C.The effect of photoperiod on size

Several authors have established that photoperiod can affect determina-

tion of form in aphids. Most recently Bonnemaison (1951) working with Brevicoryne

brassicae, Kenton (1955) with Acyrthosiphon pisum and Lees (1959, 1960 and 1962)

with Megoura viciae, showed that photoperiod determined parthenogenetic and

sexual forms. Lees (1961) showed that in M. viciae photoperiod directly

affected the form of the progeny through the parent and he showed that the

light sensitive area was located in the head of the parent.

Photoperiod may also affect the aphid through the host plant and it

seemed probable that size might be affected most by photoperiod acting through

the plants so an experiment was undertaken to investigate this. Potted broad

bean plants were pre-conditioned for various periods to short day-length to

establish the "short-day" physiological condition in the plants. 2 hours/24

hours was chosen as an extreme short day and 16 hours/24 hours as a standard

long day.

The plants were reared in ventilated, lightproof, 2 feet cube cabinets

lit by 2 x 20 watt fluorescent tubes, at a constant temperature of 20°C. 16

pots containing seedlings (4 plants per pot) in the hypicotyl stage of growth,

were kept under the 16 hour photoperiod and 4 were transferred to the 2 hour

cabinet at intervals to give short-day pre-treatments of 12, 4 and 0 days,

leaving 4 control pots in the 16 hour cabinet. On day 12, adult apterous

virginoparae (reared uncrowded), were placed on each of the plants and allowed

to reproduce overnight. A maximum of 15 1st instar larvae were left on each

plant. The aphids were collected when adult and the 4 distal antenna' segments

were measured.

The data were analysed by Students' 't' test to allow direct comparison

of treatment means.

82.

Results

The replicate means are shown in Table12 and the treatment means

and differences with 't' test probabilities are shown in Table 13. Figure 17

shows proportions of insects occurring in different size groups.

The data show that aphids reared on plants which were given short photo-

periods before and during aphid development were smaller than those reared on

plants kept continuously at a long photiperiod (P<0.001). However, compari-

sons of short• day treatments show that when the plants were given 12 hours

prd-conditioning, aphid size was significantly increased compared with plants

either not conditioned (P 0.01), or conditioned for only 4 days (P < 0.02).

Thus 12 days pre-conditioning of the plant was less harmful to aphids reared

under short photoperiod than none or 4 days preconditioning.

Plants pre-conditioned for 12 days were etiolated and their lower

leaves were abscissing during the experimental period. This leaf condition

appeared similar to normal senescence of old leaves which are shed after food

reserves have been extracted. If this were true, then mobilisation and

translocation the food reserves to the growing points could make more

soluble nutrients, of the type needed by aphids (i.e. amino acids and sugars),

available to the feeding aphids (Kennedy and Booth, 1950). This would explain

the benefit of the 12 day pre-treatment. Similarly larger aphids were produced

on plants pretreated for 4 days, than on plants which were not pretreated but

the difference in mean sizes was not significant. The aphids may benefit from

pre-treatments longer than 12 days but there must be a limit beyond which

exhaustion of the plants would lead to a decrease of aphid size.

Lengths of last fourantvnal segments of antennae of adults reared at various photoperiods. (Lengths in mm - one standard error. Numbers of insects in parenthesis).

1 2 3 4

(hours)

16/8

2/22

2/22

2/22

TABLE 12

Replicate

Photoperiod

Control

0 days

4 days

12 days

-4.453 ± 0.025 (51)

4.244 ± 0,020 (29)

4.244 - 0.036 (30)

4.295 ± 0.046 (20)

4.483 +- 0.030 (56) 4.280 t ±

0.033 (44) 4.528 0.033 (43)

4.250 ± 0.029 (56) 4.275 - + 0.028 (55) 4.331 ± 0.024 (35)

4.306 ± 0.042 (28) 4.270 ±

± 0.027 (26) 4.301 0.050 (29)

4.376 ± 0.036 (26) 4.331 - 0.016 (68) 4.363 ± 0.027 (36)

Treatment

Mean antennal lengths

Control 16/24 hour

4.462 + . - 0.016 (194)

Control

0 days

4 days

(mm) and test of significance of mean differences. (Number of insects in parenthesis)

2/24

hour

Pre-treatment 4 days

4.279' ± 0.019 (113)

4

0.182 < 0.001

0.007 AS

TABLE 13 .

0 days

4.272 ± 0.014 (175)

0

0.189 * <0.001+

12 days

4.341 ± G.013 (150)

12

0.120 4:: 0.001

0.069 <70.01

0.062 • 0.02

* Mean difference

+ Probability based on 't' test.

FIG. 17. The effect of photoperiod on size of apterous

virginoparae A. piaq.

Aphids reared in long-day - 16/24 hours (0)

and short-day - 2/24 hours with plants preconditioned

to short day for 0 days (0); 4 days (Ea);

12 days (II).

3 3

PR

OP

OR

TIO

NS

IN

EA

CH

SIZ

E—

GR

OU

P (

oi °

) .

0

O

0

Gs)

401

✓I 0

P3

OS

GI O

4.7 O

•••

alb

414

• S 8

SECTION 3.

86.

The effects of size on the biology of adult Acyythosiphon

87. A.THE EFFECT OF SIZE VARIATION CAUSED BY CROWDING.

a. The pre-re, uductive period,

Campbell (1926) recorded periods of 1-6 days between final moult and

parturition of the pea aphid, while Harrington (1941) quoted values varying

from 19.8 to 22.4 hours at 20oC., but Cartier (1960), investigating one known

biotype of the pea aphid found the pre-reproductive period to be about 43

hours at 21oC. When assessing fecundities of small and large aphids in this

work, it was noted that the pre-reproductive period was about 20 hours at

20oC but varied with adult moulting weight. A more precise study was done

to elucidate this variation and to investigate its significance.

Duration of the pre-reproductive period.

Adult apterous virginoparae of A. pisum of different weights (crowded

v not crowded), during larval development, were collected within 2 hours of

moulting to adult. Each of 92 adults was weighed and then placed in a leaf-

cage on young, mature broad bean leaves. Thereafter adults were examined

every two hours and the time when the first larvae progeny was born was recorded

Pre-reproduction periods were calculated as the time elapsed, (in hours),between

midpoints of the 2 two-hour periods within which, (1) the adult moulted and

(2) the adult reproduced.

The data were analysed by correlation coefficient analysis for associa-

tion, and regression analysis for dependence. Where applicable a quadratic

expression was calculated to summarise inter-dependence. The quadratic form

used was: y = a + b1x1 +

where b1

and b2

are first and second degree coefficients, and x1

and x2 are

x and x2 respectively.

Results

The range of weights of adults studied was 0.60 - 2.99 mg, with mean

8Eso

1.738 ± 0.058 mg; their mean pre-productive period was 26.737 ± 0.751 hot 's.

A scatter diagram (Fig. 18) shows the pro-reproductive period

plotted against weight within 2 hours of ecdysis. Analysis of regression

variance is shown in Table 14 which shows that fitting the line, where

Y= 60.290 - 33.522 X + 7.427X2,

reduced deviations by a significant amount (P w„.:.0.001), proving a curvilinear

relationship.

TABLE 14 . Test of significance of departure from linear regression.

Items Degrees of Sums of Mean Freedom Squares squares

Deviation from linear 90 3,474.30

Deviation from quadratic 89 2,864.86 32.19

Deviations removed by fitting second degree 1 609.44 609.44 ***

609.44 F - = 18.933. P <0.001.

32.19

For individuals weighing less than 2.2 mg the pre-reproductive period

varied considerably with size, suggesting that reproductive maturity of the

adults was not completed and that a longer period was necessary to prepare

the adult for reproduction, for example to develop undernourished embryos

(see Section 2 ) or to correct deficiencies of hormones necessary for

maturation, the production of which was retarded by crowding stress during

the larval stage. The lower limit of the pre-reproductive period seemed to

be about 16 hours, which is slightly less than that recorded by Harrington

(1941).

The data cannot be extrapolated beyond limits set by the range of

insects used in this experiment because at the lower, weight limit, in this

particular cl?ne, insects below 0.60 mg weight are unlikely to live and

reproduce, whereas at the upper limit of the weight range there appears to

FIG. 18. Regression of length of adult pre-productive

period on adult weight.

FIGS 19* Regression of rate of weight increase during

the pre-productive period on adult weight.

FIG.18. SO.

60

PRE—

REP

RODU

CTIV

E P

ER

IOD

(hrs

. ).

20

30

40

50

• Y= 60.260 - 33.522 X + 7.427X2

•

• • •

• •11 s •

•

• •

S ••• ••• • • • • • • • • •• l_..........1.1................ • • % • • • • •Rio • • 8 •• di

5 • _ P • owe • 61• •• ••• imp • •

• •

10

0

FIG.19. 90--

I 1 0.50 1.00 1.50 2.00 2.50 3.99

• 80

• Y = -42.779 + 83.598X - 17.758 X2 • •

• • • • • • •

• • 50

w▪ 4 0

Z 30

z 20 -- 0 1- • •

• 10

0 1 0 0.50 1.00 1.50 ' 2.00 2.50 3.00

WEIGHT (mg),

• 70

a 60 • • •

• •

91.

be a limiting maturation period which has already reached a minimum in

individuals weighing about 2.2 mg. Although the calculated curve indicates

an increase in the pre-reproductive period of individuals weighing more than

2.2 mg (Fig. 18 ), it is suggested that a flattened curve would be more

representative. Two factors which may contribute to the shape of the curve

in this pert of the range are the few data available and the typical "tail"

characteristic of the quadratic curve.

Adult weight increase during the pre-reproductive period.

68 of the adults used to evaluate pre-reproductive periods were

re-weighed after parturition. The range of weights was 0.60 - 2.99 mg with

a mean weight of 1.794 mg and standard error 0.070 mg. Adults increased in

weight by 0.41 - 1.81 mg during the pre'reproductive period and in three

instances by more than 100 per cent. Examination of percentage transformation

of weight increase, based on initial adult weight, revealed a significant

negative association between weight increase and initial weight (r = 0,595,

p <0.001).

The data is expressed as microgrammes increase per hour in Fig.19.

Two linear regression lines, calculated for two overlapping weight groups

with weights ..c2.05 mg and 1".1.95 mg gave

and

Y

y

2.05

1.95

=

=

- 10.539

103.129

4. 33.760)c

- 19.758x

(F = 74.78, 1/41 P =4;0.001)

(F = 6.59,11/ 26

P <0.05)

where Y is weight increase in microgrammes and x is initial adult weight in

milligrammes. The lines indicate that the rate of increase in weight of small

and large aphids is not as great as that of those weighing initially about

2 mg. The weight increase of approximately 0.055 mg/hr by aphids of 2 mg is

similar to that of 0.057 mg/hr for A. pisum quoted by Cartier (1960). Small

92.

aphids apparently compensate for larval deficiency by their increased pre-

reproductive period, while large aphids may not require to rectify larval

deficiencies.

The second degree curve corresponding to

Y = - 42.779 + 83.598x - 17.758x2

is plotted in Fig. 19and a summary of the analysis shown in Table 15 .

TABLE 15 . Test of significance of departure from linear regression.

Items Degree of Sums of

Mean Square

Freedom Squares

Deviationsfrom linear

Deviations from quadratic

Deviations removed by fitting second degree

F = 22.511

66

65

1

10.355

7.692

2.663

P C 0.001.

0.118 ***

The same general trends are shown when data are treated as proportional

increase in weight i.e. percentage increase per hour Increased

scatter reduces the value of the regression for weight groups " <2.05 mg"

where the regression coefficient (0.3910), is not significant, but for group

" >1.95 mg" the slope is significant (b = 1.4924, P <0.001).

Conclusions

Pre-reproductive periods may vary inversely with aphid size. Within

one clone a- n extended pre-reproductive period can be attributed to poor

nutrition as a larva and is a time of recuperation, whereas the different

values obtained by Harrington (1941) and Cartier (1960) were probably due to

differences in biotype. Campbell (1921) considered that temperature was the

most important factor but, since his rearing was done in an outside insectary,

he could only state that the pre-reproductive period varied from about 1 day

in the warmer months to 6 days in the cooler months of the year.

93.

Not all aphids have long pre-reproductive periods. For example, at

20°C. Aphis fabae, reared under favourable conditions, begins to reproduce

1 - 2 hours after moulting to the adult (Banks and Macauley, 1963), and

shows little increase in weight during this time. In contrast the shortest

period recorded for A. pisum was 16 hours at 20°C. Adult apterae of aphids

are considered to be ceotenous; perhaps A. pisum moults to the adult at a

more juvenile physiological stage than does A. fabae th-,ugh the variability

imposed on apterae of A. pisum could perhaps occur with A. fabae reared

in unfavourable conditions.

b• Effect of adult weight on fecundity, longevity and reproductive period.

Data on these three factors were obtained using insects kept in the

glasshouse (ref p. 27). Adult apterae were confined in leaf cages on young

broad bean leaves and their larvae counted and removed daily.

Fecundity increased with increase in weight of the newly moulted adult

(Fig. 20A), Y = 55.778 + 12.949x 0.001), but results were variable

(b t 3.482). Longevity (Fig.20B), and reproductive period (time from moult

to last-born larvae) (Fig.20C) were not affected by the weight of the newly

moulted adult.

Data obtained at a constant temperature of 20°C are presented in

Figs. 21A,B & C.Again fecundity increased with weight, Y = 68.527 + 19.663x

(p <0.001), but the reproductive period also varied inversely with weight,

Y = 23.904 - 2.709x (1).< 0.01). Longevity was not affected significantly

by weight.

Slopes of the regression of fecundity on weight of the newly moulted

adult (b1 = 12.949 ± 3.482 and b2 = 19.663 ± 3.995) were not significantly

different. However, further comparisons made between the experiments done

FIG. 20. The effect of adult weight on fc::cuneity (A),

longevity (B) and reproductive period (C) of

glasshouse-reared apterous virginoparae A. pisum.

120

110

100

90

FEC

UN

DIT

Y.

80

70

60

50

40

30

20

10

0

• g

B 00

• • P *•••• • • •

• • •

• • • • • • •• • •

• • • • • • *

•

•

I I I I 2 3

• 30

z 0 • 0

h 20 • • -A- I • a • w • wa 0 441) -• a. atfir%

• ••

• • • • • • .• • • •

•

• • • • • •

C

FIG, 20, 95.

• • • A

•

• •• • • • • •

• • • • • • •

• • •

• • • •

• • • •

• • • • •

•

e•

Y= 53.778 + 12.949 x

• •

•

•

•

•

•

•

WM.

1 2 3

40

>: F. 30 > o

>. ^J 20 Z 0 O s"

10

0

I 2 3

ADULT WEIGHT. (mg)

FIG. 21. The effect of adult weight on fecundity (A),

longevity (B) and reproductive period (C) of

apterous virginoparaa A. pisum reared at 20°C.

30

2 0 6 ..... o . i et. la ci. s, 10

0

•

[ I

•

1 2

• • •_ w . • • -•-•*etritl • •• —'-----Tr---"IL—JL--E—f----' . • •

I I I I

3

RE

PR

OD

UC

TIV

E

i

C

130

120

110

100

90

80

70

SO

50

40

30

20

10

0

40

30

20

10

0

FIG. 21 . —

IMO

• • Is

NM •

I.—

INIO•

I

...••••• • ...,..........4)>,...

• • • lb

• • • •

•

•

Y= 23.904 — 2.709x

I I

•

•

• I

I

—

PM.

I

I • I I

1

• . • • •

• • • • do •• • • • •

•

I I

•

2

do 07 — — - —

I

A •I • •

•

FEC

UN

DI T

Y .

I 1 3

B • - S •

•

i I

97.

•

1

2 3

ADULT WEIGHT . (mg)

98.

in the glasshouse and constant temperature room, showed that mean fecundities,

84.85 larvae and 98.91 larvae respectively, and mean longevities, 22.425 days

and 26.31 days respectively, were significantly different at P =

where the mean reproductive life was significantly increased from 16.89 to

19.72 days respectively with P = -e„ 0.001.

c.Reproductive characteristics of four adult weight groups.

The study of embryo size described in Section 2 (p.51 ) showed that

the first embryos from small adults reared in crowded conditions, were

significantly smaller than those from large aphids reared uncrowded. This

suggests that there is a correlation between size of first instar larvae and

size of parent, but small adults have a relatively lone pre-reproductive

period (p. 88), during which large gains in weight occur. This period of

recuperation from adverse rearing conditions could have involved increase

in size of the embryos and consequently an increase in larval size. The

experiment described below was done to obtain details of numbers and sizes

of successive first instar larvae produced by adults of various sizes, and

to make an estimate of the total biomass. of larvae produced by them.

In order to simplify the data, parent adults were divided into four

weight groups based on their weights within 2 hours of moulting to the

adult stage, namely:

1. Lass than 1.0 mg; 2. 1J0 - 1.5 mg; 3. 1.5 - 2.0 mg and 4. more than

2.0 mg. The sizes of first instar larvae were based on lengths of the 3rd,

4th and 5th antennal segments, which were measured separately to correct for

antennal curvature, and the measurements were summed. The term antennal

length will be used to describe the summed lengths of segments 3 - 5. The

total biomass of first instar larvae was calculated from the linear

99.

relationship between antennal length and weight of newly born larvae

determined in Section 1 (p.45 ). The regression of weight on antennal

length for a single larva was adapted to

Yb = n (-0.09583 + 0.18550R),

where n is the mean total number of larvae produced per adult and Tc is the

mean larval antennal length per weight group, and gives the calculated mean

total biomass per adult (Yb) in milligrammes.

Results

Mean values characterising each weight group are listed in Table 16 .

TABLE 16 .

Weight group.

Limits of group (mg)

No. of parent individuals

Mean weight (mg)

Mean fecundity (No, of larvae/adult)

Mean reproductive period (Days)

Daily numbers of larvae born/adult (per day of reproductive period)

Mean length of first instar antenna (mm)

Mean total biomass of larvae produced per adult (mg)

Mean longevity of adult parent (days)

A B C D

1.00 1.00 - 1.50 1.50 - 2.00 >2.00

2 4 7 6

0.78 1.33 1.75 2.28

80.00 93.25 110.85 117.00

21.5 22.0 18.7 17.6

3.72 4.24 5.93 6.65

1.153 1.177 1.134 1.162

9.44 11.42 12.70 14.01

30.50 30.75 26.00 26.40

Fecundity and longevity of adults

The largest individuals (groups C and D) began reproduction within

24 hours of the final moult, the smaller (group B) between 24 and 38 hours,

and the smallest (group A) between 48 and 72 hours after final mount; these

100. results confirm those described on p. 88., At the time of parturition

adult weights had increased by averages of; A = 135%, B = 72%, C = 71% and

D = 60%; thereafter weights remained steady, but increased slightly towards

the end of reproductive and during the post-reproductive period (Fig. 22).

Fecundity increased with increased weight, whereas the reproductive period

and longevity were inversely related to weight at the time of the final

moult (Table 16). By comparison, the results of the previous experiment

(p, 93) suggested that only the reproductive period and not longeveity

varied inversely with weight> There was an inverse relationship between

reproductive period and rate of reproduction, thus mean daily larval

production by groups A, B, C and D respectively, reached peaks in 8.0, 6.5,

3.6 and 3.5 days after the final moult.

Size of larvae from adults of different weight groups.

The general shape of the larval size curve, based on antennal measure-

ments, applies to all weight groups and is illustrated in Fig. 25. First-

born larvae of all adults were smaller than subsequent larvae but peak size

was reached more rapidly in progeny of the larger adults than in those of

the smaller ones. The size of the first-born larva was correlated with

the weight of the newly moulted adult, even where adults had fed for

relatively long periods in the pre-reproductive periods. However, as

shown in Fig. 25, the later-born larvae of the smaller adults increased in

size until they were as large as those produced by the larger adults;

Fig. 25 shows that the mean size of larvae from small adults increased to

the maximum during the first 6 days of reproduction. Thus, small adults

are able to recover to the extent of producing full-sized larvae, although

fewer are produced than by larger adults. Curves of larval size of groups

101.

A and D (Fig.25 ) show troughs indicating occasional production of a group

of relatively small larvae. This may be caused by disturbance to the adults

when they were weighed every four days or by plant-changingevery eight days,

which also disturbs the adults, or byaging of some of the adults whose last

progeny were relatively small.

A measure of differences in productivity of different sized adults is

the product of the total number of larvae they produce and the weight of

those larvae at birth. The biomass (pp.45 and 99 ) thus obtained gives a

more realistic comparison than either number or size of larvae. Fig.26

shows the daily production of larval biomass; larger, 'fitter' aphids are

more productive in the first 5 to 6 days. Smaller adults take longer to

produce their first, relatively small, larvae and do not become as productive

as the larger adults until about the sixth day;thereafter differences are smal

and irregular.

The mean totals of biomass produced per aphid show that productivity

increases with weight of the parent (Table 16 ) but the weight of larvae,

expressed as a ratio of the newly moulted weight of the parent, decreases

with size. Thus, adults of groups A, B, C and D respectively, gave birth

to larvae whose total weights were 12.1, 8.6, 7.3 and 6.1 times their own

weight. However, this does not allow for important increases in weight during

the pre-productive period and ratios recalculated as proportions of weights at

parturition were 5.2, 5.0, 4.3 and 3.9 for groups A, B, C and D respectively.

These ratios provide further evidence of the ability of small adults to

recuperate. It is interesting that productivity of the larger A. pisum is

similar to that of good quality apterae of A. fabae which produced larvae

totalling 2.64 mg (i.e. 3.7 times their own weight of 0.71 mg just before

reproduction) (Banks and Macauley, 1964).

FIG. 22. Mean weights of apterous virginoparae A. pisum

during adult life: (1) <1.0 mg ( X ); 2 1.0 - 1e5 mg

+ ); (3) 1.5-2.0 mg ( • ); >2.0 mg ( 0 ).

FIG. 23. Mean daily number of larvae produced by four weight

groups of apterous virginoparare A. pisum. Weicht

groups as for Fig. 22 above.

FIG. 24. Accumulated daily mean number of larvae produced

by four weight groups of apterous virginoparae

A. pisum.

Weight groups as for Fig. 22 above.

O

x x

• +

4

AD

ULT W

EIG

HT

(mg

).

15 - 10 -

110 -

NU

MB

ER O

F LA

RV

AE.

10 -

s - o -

10 -

5 -

0 - 5 -

FIG. 22. 103.

AC

CU

MU

LATE

D T

OT

AL

0 I i I

11/1111111111111111

li• MI

F1G.23 .

hill - I III

1.

111 I I 1 I II I •ft

I I/ I I/ I I/ I/ I I 1 111 1. i= 1 ri 00 0 00

0 0;••••••••• F1G.24 .

iu • 100 0 . -

0 • ft 90 - + + + + + 0 • + + •

80 + 0 • +

++ w X X X X

70- + X X X - 0• + x

60 - + X 0 • X

+ x o

• + x

_ 50

40- 0 • + X X

• +

S + X X 20- +

X 10 - • + x + x 0 -

i e i I I I i 1 1 i 0 4 v 12 16 20 24 28 32 ADULT AGE (Days).

1

30

FIG. 25. Daily mean lengths of antennae (segments 3+4+5)

for first instar larvae produced by weight groups

of apterous virginoparae A. oisum.

Weight groups: <1.0 mg. ( >(); 1.0-1.5 mg (♦ );

1.5-2.0 mg ( •); >2.0 mg ( C)).

FIG. 26. Mean total biomass of first instar larvae produced

per c'ay by four weight groups of apterous virginoparae

A. pisum.

Weicht groups as for Fig. 25 above.

NM,

FIG. 25.

1115 .

0 4 8 12 16 20 24

0

4 128 16

20

24 ADULT AGE ( Days ).

BIO

MA

SS

(mg )

.

FIG. 26.

1.50

1.40

1.30

1.20

1.10

1.00

0.90

0.50

0,10

0.A0

0.80

0.70

0.60

Q30

0.20

AN

TEN

NA

L L

EN

GTH

(mm

).

1.16

1.14

1.12

1.10

1.08

1.06

1.04

..02

1.00

0.98

0.96

0.94 492

106.

d The effect of first instar size on larval orowth and adult size.

In uncrowded conditions newly-moulted small adult aphids can make

some compensatory growth by extending the pre-reproductive period during than

which relatively greater weight gains occur/during the pre-productive period

of larger adults (pp. 07 and 91). Some recovery also continues during the

reproductive period since the new-born first progeny of small adults are

undersized but later progeny are larger at birth and at about the seventh day

of the reproductive period are as big as those produced by large adults.

It is probable that small adults kept crowded would continue to produce

undersized progeny at birth. Undersized progeny may occur commonly in

nature and it is important to determine whether such small progeny can make

up for initial undersize during their subsequent larval development.

Three apterous adult virginoparae obtained by larval crowding and mg

weighing 0.83 (A), 0.83 (B) and 0.75/(C) were put individually in leaf cages

and allowed to reproduce. Four larvae were taken from each adult on

successive clays throughout the reproductive period and reared in groups of

four (i.e. uncrowded) in leaf cages. The exuviae of the developing larvae

were collected. The resulting adults were kept in the leaf-cages and allowed

to reproduce for one day when they and their larvae were collected and

preserved in alcohol and glycerine. Antennal measurements were made of all

larval exuviae and of the adults and their first progeny, i.e. the second

generation first-instar larvae,

Comparisons between the mean lengths of antennae of groups of four

individuals in the first instar and subsequently as adults (Fig. 27) indicated

a direct size relationship. The typical increase in larval size during the

first four to five clays of reproductive life (cf. p. 100) was reflected in

107,

corresponding increases in adult size, and troughs in the larval size curve

nearly always coincided with troughs in the curve of the adult size. The

regression of adult size on first instar larval size (Fig. 28) was significant first

P 4:0.001). Thus relatively small/instar larvae became relatively small adults

and vice versa.

Thus it would appear that small larvae are not capable of becoming full

sized adults but examination of the ratio of size increase, i.e,

Length of adult antenna Length of first instar antenna,

shows that small larvae increase in size relatively more than larger ones and

the negative regression of growth ratio on first instar size (Fig. 29) is

significant (P4( 0.001).

These results were confirmed by a further comparison between the

development of small and large first instar larvae. The average measurements

of larval exuviae and of adults which developed from the ten smallest and ten

largest groups of four larvae are given in Table 17.

The antenna' curves (Fig. 30) were fitted using the orthogonal poly-

nomial procedure (Fisher and Yates, 1953) where the generalised regression is

"St — 0C ÷

andig2 being the slope and curvature coefficients respectively. The

estimates of the parameters, b1 and b2 are given in Table 18.

Comparisons made between the curves of antenna' growth showed that the

slopes ( b1) of small larvae curves were greater than those of the large larvae

in respect of segments 3 + 4 ( P <0.02), 5 (P.:0.001) and total length (P4:0.02

In addition the depression of the curve ( b2) was less for small larvae than

large larvae for segments 3 + 4 (P4:0.05), 5 (P47.0.01) and total length

(P<0.05). Thus the overall rate of size increase was greater for small

108.

larvae and the rate of increase was maintained at a more uniform rate

between instars, and did not decrease in successive instars as much as for

the large larvae.

Thus some, but not complete compensation, for initial undersize can

occur during larval development providing food, aphid density and other

conditions are satisfactory for larval development.

Unfortunately, the adults were not weighed at emergence nor were their

fecundities determined so that the importance of the observed size differences

in productivity cannot be assessed directly. However, an estimate of

fecundity can be made from the calculated relationship between antennal length

and weight (p.39) and between fecundity and weight 93). The estimated

weights of the adults from small and large new born larvae (Table 17) were

1.718 mg (9520 confidence limits, 1.667-1.769 mg) and 2.016 mg (1.962-2.070 mg)

respectively, and the corresponding estimated mean fecundities were 102.3

progeny (89.9 - 115.4) and 108.1 progeny (93.7 - 123.4). Thus there was an

indication (not significant) that the adults from the larger new born larvae

were more fecund.

Comparisons made between the size of new born larvae and the size of

their parents, the F1 adults, showed that there was no correlation (P,-0.10),

(ice. the smaller adults did not produce smaller larvae than the larger ones).

This indicates that size recovery from size decrease due to crowding was

completed within one generation (i.e. from new born larva to new born larva )

but there were possibly small decreases in fecundity. This contrasts with the

need for two generations to be completed before aphids recover from size

decreases caused by high temperature (25°C) (p.69).

109.

TABLE 17. Mean antennal measurements (mm) for larvae and adults developing from small and large first instar larvae of A. pisum.

Instar I II III IV Adult

Antennal segments

Small larvae 3 + 4 0.1945 0.3659 0.6661 1.1480 1.9355

5 0.1718 0.2681 0.4014 0.5675 0.7745

6 0.5013 05,7054 0.9087 1.1213 1.3453

Total 0.8676 1.3394 1.9262 2.8368 4,0553

Large larvae 3 + 4 0.2418 0.4595 0.7915 1.3436 2.0434

5 0.2087 0.3364 0.4843 0.6588 0.8130

6 0.5905 0.8564 1.0836 1.3092 1.4548

Total 1.0410 1.6523 2.3594 3.3116 4.3112

TABLE 18. Estimated regression coefficients, b1 and b2' of larval growth curves for small and large larvae of-A. Risum.

Antennal segments y b1 b2

Small larvae 3 + 4 0.80452 0.24924*** 0.00850 **

5 0.58198 0.16337*** - 0.01009**

6 0.93710 0.10587*** - 0.01122*

Total 1.28216 0.16653*** - 0.00403 ns.

Large larvae 3 + 4 0.87700 0.23198*** - 0.01459*

5 0.65207 0.14730** - 0.01833**

6 1.00370 0.09674** - 0.01795

Total 1.35271 0.15368*** - 0.01287*

ns. - not significant * P40.05 Pc0.01

*** P.<0.001.

FIG. 27. The effect of first instar size on adult size.

Relationship between length of first instar antenna

(segments 3+4+5) and length of adult antenna (segments

3+4+5+6) for the progeny of three small apterous

virginoparae A. pisum. First instar0; adult •.

FIG. 27.

I 1-0 Z as ...

4 Z Z W Z 4

/._

1.1

i; qt— 1'0 4-

,.. 0 E Z E '''' g-

40 0.9

CC I'

0.0

0.7

r

—

,—

— 13 z 0 .— 4

1 —

—

1•••

4.7

4.6

4.5

4.4

4.3

4.2

4.1

4.0

3.9

3.1

3.7

3.6

3.5

3.4

.-

r- Im•

Imp

DAYS OF THE RE PRODUCTIVE PERIOD.

B. C.

1 I I I I I I I I I J 1 1 1 1 1 i 1 5 10 15 20 1 * 10 15 20 25 1 5 10 15 20 25

A.

The effect of first instar size on adult size.

FIG. 26. Regression of length of adult antenna (segments

3+4+5+6) on length of first instar antenna

(segments 3+4+5).

FIG. 29. Regression for ratio of increase in length of adult

antenna on length of first instar antenna,

AD

ULT

AN

TEN

NA

(mm

).

113.

•

FIG.28. • •

• lip At ft.

• • • • • • •

Y= 1.700 1. 2.5840 x I

• • • •• • • • •

so •

4.60

4.40

4.20

4.00

3.80

3.60

0.80 0.90

• •

•

•

•

• • • •

•

• • • •

•

• • •

••

• • •

• e •• •

• •

"IA •

0.80 0.90 1.00 1.10

FIRST INSTAR ANTENNA (mm).

FIG.29. 4.8 - • •

4.7

4.6 •

e. 4.5

4.4 I- OC 4.3 • •

• •• 4.2

0 4.1 CD

4.0

3.9 Y= 6.1396- 1.8510x

3.8

3.7

3,6

FIG. 30. Comparisons between antenna' growth of small (S)

and large (L) A. pisurn larvae.

AnteAnal segments: 3+4; 5; anc 6; also total

length.

LT ST

L 344 ,S 344

L 6 S6

L5 S5

FIG .30 .

1.70

1.60

1.50

1.40

1.30

1.20

1.1 0

1.00

0.90

0.60

0.50

040

0.30

0.20

0.1 0

0

115.

1 1 1 1 1 1 2 3 4 ADULT.

1NSTAR.

116.

e. The effect of crowding stress during the adult stage on fecundity and

longevity of different sized aphids.

Experiments in the other parts of this section (pp. 93 to 119 ) show

that small adult aphids produced by crowding or by high temperature during

the larval stage, are less fecund and produce smaller larvae initially than

large adults: those data were obtained in conditions where the adults were

well nourished and were kept singly (i.e. during the adult stage they were

not under stress). In nature, however, stresses from crowding for example,

are more likely to continue and increase as the adults age in a growing

population. For example, it is known that establishing colonies of Aphis fabae

with 2, 4, 8, 16 and 32 adult virginoparae per plant, drastically reduced mean

productivity per adult as crowding increased above a population of 8-16 aphids

per plant (unpublished data, Way and Banks, 1959), and that crowding of adult

Drepanosiphumip?.atnoidesretarded embryo development and reproduction, wherdas

release from crowding stress permitted embryo development and allowed daily

larval production to approach the normal (Dixon, 1963). Preliminary work

was done to indicate effects of crowding during the adult stage on A. pisurn,

adults of different sizes.

Individuals of a pink strain of A. pisuni were used to crowd small

and large adult apterae of the normal green form in leaf cages. Densities

of 1, 5 and 20 adults per cage were compared using ratios of 1:0, 1:4 and

1:19 green to pink adults. Progeny of both pink and green adults were removed

daily and numbers of green larvae recorded. The numbers of pink adults were

kept constant by replacing dead aphids when necessary.

TABLE 19. Fecundity and longevity of adult Acvrthosiphon pisum under different conditions of crowding.

Small aphids Large aphids

A. No. aphids per cage 1 green 1 green + 1 green 1 green + 19 pink 19 pink

No. insects studied 6 6 6 6

Mean weight (mg) 1.39 1.40 2.64 2.72

Range 1.23-1.57 1.23-1.65 2.21-2.91 2.24-3.16

Mean longevity (days) 12.3 3.0 15.0 5.0

Range 4-32 2 - 4 2-26 4 - 6

Moan .fccuneity 41.3 7.7 76.2 26.5

Range 9 - 98 0 - 19 29 - 123 18 - 33

B. No. aphids/cage 1 green 1 green +

4 pink

No. insects studied 6 6

Mean weight (mg) 1.23 1.23

Range 1.01-1.36 0.97-1.38

Mean longevity (days) 21 13.2

Range 11 - 24 4 - 22

Mean fecundity 81.3 54.0

Range 55 - 103 6 - 83

117.

118.

The green adults, including the uncrowded ones, varied greatly in

longevity and fecundity (Table19 ) suggesting that general conditions were not

optimal. Fungal disease appeared among adults crowded 20 per cage ( 1 green and

19 pink) and undoubtedly caused the reduction in their viability and was trans-

mitted to the uncrowded treatments. The data of table 19iltvee from a later

experiment when conditions were better.

The results show that crowding 20 per cage drastically reduced fecundity

of both small and large insects, while crowding small adults 5 per cage had less

effect on their productivity. Crowding 20 per cage had relatively more effect

on the smaller than on the larger adults, reducing the fecundity to about 17%

and 35% respectively, of small and large adults kept singly. The fecundity

of small aphids kept at a density of 5 per cage was reduced to about 66% of

single adults. Reduction in fecundity at this low density of 1.5 aphids per

square cm of leaf area could have important repercussions on population

build up in the field and further consideration will be given to this in the

discussion

Conclusions

Fecundity of the adult and total weight of larvae produced increased with

increases in weight of the newly moulted adult; longevity was less affected by

weight. During the first six days of reproductitte life, size of larvae born and

mean daily number of larvae born also depended on the weight of the parent, but

after this period differences between those of small and large adults decreased

presumably as a result of compensatory development by the smaller adults. The

delay in peak productivity by smaller adults may indicate that recovery from stress

as larvae was not completed in their lengthened pre-1 reprodbctiv_ period ( q).100

but continued until at least the sixth day of adult life.

119.

The small larvae produced by undersized adults are able to

compensate partially for their small size by having greater rates of size

increase than large ones, but they are not able to recover size completely

since small first instar larvae develop into smaller adults than large ones

and vice versa. However, in conditions favourable for larval development,

recovery from size decrease caused by crowding, is completed in one

generation.

The implications of recovery from stress will be cohsidered further

in the Discussion.

120.

B. EFFECTS OF SIZE VARIATION CAUSED BY TEMPERATURE

The effect of tem erature on adult life and pattern of reproduction.

Introduction

It has been shown in previous parts of this section (pp.93 & 100) that

small adults produced by crowding are less fecund and, initially, give birth

to smaller larvae than large adults reared under uncrowded conditions. This

part is concerned with a study of fecundity of adults and size of their larvae,

produced under various temperature regimes, and to determine whether adults

of different sizes, reared under conditions of temperature stress, react in

the same way as individuals of equiValent size, reared in crowded cultures.

A. &isum was reared at temperatures varying from 10°C. to 2800 (see

Section 2 ) and the size, fecundity and longevity of adults, and size

of their progeny, were determined. The adults were kept in individual leaf-

cages and larvaetwere counted and removed daily. Measurements were made of

the third, fourth and fifth antennal segments of first instar larvae; the

separate measurements of each segment were summed and is referred to below

as the antennal length. Fecundity of the adults was determined at the

temperature at which they were reared and sometimes also at 20°C.

Results

A summary of mean values is given in Table 20 and more detailed daily

observations of numbers of larvae and their mean antennal lengths are presented

graphically in Figs. 31 to 37

Extremes of temperature during larvae and adult periods of a single

generation reduced fecundity (Table 20) in the order 15°:-..25°:74,.10°,11....28°C.

Continuous treatment at 25oC. for three generations led to decreases of

fecundity in successive generations, and the F3 failed to reproduce. Fecundity

TABLE 20. Summary of mean size and fecundity of adults reared at various temperatures.

Rearing Temp. Mean weight Mean pre-re- Mean re- Mean Mean temp. and at which fecundity at moult productive productive Mean larval

generation tested No. (mg) fecundity period (days) period longevity size (mm)

10°C F1 ( 10°C

( 20°C o

15oC F1

15 C

25°C oC F1 25°C

20°C

F2 25°C

°C20

25°C F3 o 20 C

2 generations) 0 at 25, 1 at ) 20 C 20 C. )

o ( 28°C 28 F1 ( 20°C

2

6

3

3

3

3

3

3

3

3

8

4

2.42

2.11

2.41

1.91

1.77

1.02

1.07

0.46

0.33

0.52

1.24

1.24

20.5

67.6

114.3

32.3

104.0

31.6

60.0

0

5.3

5.7 .

8.75

7,25

3,5

1.1

3.0

1.3

2.0

2.3

3.6

-

11 days

10.6 days

2.0 days

3e6

9.5

11.0

25.3

5.0

20.3 25.0

10.3

17.0

.., Q.

*

3.0

4.5

10.5

14.1

33.6

5.3

16.3

25.0

1205.31

25.0

5.6

5.3

1.046

0.957

10

1.114

1.051

0.942

1.048

0.960

0.986

0.851

0.885

* Values not given because reproduction sporadic over long periods.

FIG. 31. Mean daily number ane size of first instar larvae

produced by apterous virginoparae A. pisum reared at

10 C. when kept as adults at 10°C. (• and II)

and 20°C. (0 and 0).

Fig. 32. Mean daily number and size of first instar larvae

produced by apterous virginoparae A. pisum reared

and kept at 15°C.

FIG. 31. 123.

4 6 8 10 12 14 16 111 20 22 24 26

DAYS AFTER FINAL MOULT.

1.10

1.00

0.00

0.x0

IMO

— E E

IM

WO

UM.

- ...., a - 1- - a - ce

iu

0 0 0 0 Z a. W cc

12 10 8 6 4 2 0

'OM

1.00

1 et Z Z w I- Z 4 dr a I-U) z i-in Z IT.

DA

ILY

NE

T

FIG. 32.

2 4 6 8 10 12 14 16 18

. Mean daily number and size of first instar larvae

produced by apterous virginoparae A. pisum

FIG. 33. Reared for one generation at 25°C. and kept as

adults at 25°C (0 and II ) and 20°C. (0 and 0 ).

Fig. 34,- Reared for two generations at 25°C. and kept as

adults at 25°C (111 and • ) and 20°C. (0 and 0).

FIG. 35. Reared for three generations at 25°C. and kept

as adults at 20°C.

FIG. 36. Reared for two generations at 25°G. and for one

generation at 20°C. and kept as adults at 20°C.

F10.33.

14

10 12

B

4 2—

2 . 4 6 II 10 12 14 16 18 20 22 24 O

0

1.1•11.11.

I- O 110

J

J 4

• 1.00 Z E 4(

4 I- In ▪ 0.90

I..

I'

4 0

100

0.90

1.00

W;

4

NE

T R

EP

RO

DU

CT

IVE

FIG.34.

125.

1,10

140

8-- 6 - 4 - 2 - o —t

o 2 4 6 8 10 12 14 16 18 20 12 FM.35.

n 12 14 16 18 20 22 24 26

2ro 0 2 4 6 8 10 12 14 16 18 20 22 24 26

DAYS AFTER FINAL MOULT.

2r_ 0 0 2 4 FIG.38.

r-8--t B 10

FIG. 37. Mean daily number and size of first instar larvae

produced by apterous virginoparae A. pisum reared

at 28°C. and kept as adults at 28°C, (0 and II )

° and 20C. (0 and 0)

FIRST INSTAR ANTENNAL LENGTH. (mm)

o a a a

r 1 DAILY NET REPRO-

DUCTIVE RATE.

128.

increased when individuals reared as larvae at 25°C and 10°C were transferred

to 20°C as adults. When newly-born larvae of F2

adults reared at 25°C. were

transferred to a rearing temperature of 20°C., their fecundity did not differ

appreciably (mean of 5.7 larvae per adult) from that of adults reared at

25 oC. for three generations and then kept at 20

oC. as adults (mean of 5.3

larvae per adult) (Table 20 ).

F1

adults reared at 2SoC. reproduced for an average of only 5 days,

produced an average of 32.3 larvae, and then died (Table20 and Fig. 33).

The low fecundity of individuals reared and kept at 25°C. contrasts with

that of adults of similar size reared as larvae at 25°C. and then transferred

to 20oC. as adult (mean 104.0 larvae). Increased reproductive periods and

longevity of F2 adults reared and kept at 25°C. suggest acclimatisation of

the clone; however, adult size (Section 2 p.68 ) and fecundity decreased.

The F3

adults, although sterile when reared throughout at 25°C., reproduced

sporadically when transferred as adults to 20°C. (Fig.35 ). When reared

and kept at 20°C., F3 generation aphids, whose parents were reared at 25°C.

for two generations, also reproduced sporadically and had a mean fecundity

of 5.7 larvae (Table 20 and Fig. 36 ). Adults were generally short-lived

when reared throughout life at 25°C., but individuals reared at 25°C. as

larvae and removed to 20°C. on becoming adult, were uniformly long-lived

(about 25 days).

Adults reared at 15oC. produced the most uniform sized larvae (Fig. 32

Larvae of parents reared throughout at 10°C. increased in size during the

first three days of the parents' reproductive life, but decreased unevenly

thereafter. When individuals reared at 10°C. as larvae, were transferred

to 20C. as adults, their first instar progeny increased in size through

129.

the reproductive period and reached a high maximum mean antennal length

of 1.164 mm on the 13th day after the parent became adult (Fig.31 ).

Larvae produced by adults reared at 28C. were small and decreased to a

minimum mean antennal value of 0.733 mm. Continuous rearing at high

temperature had a cumulative depressant effect on larvae size (Figs.33-35)

but there was recovery when the aphids were released from temperature stress

(cf. 10°C. above), by transfer to 20°C. at the time of moulting to adult.

The F3

adults reared at 25°C. and kept at 20°C., produced larvae which were

variable in size and smaller than larvae produced by the F3 adults reared

throughout at 20°C. (from F1 and F2 generations at 25°C. )(Figs 35 & 36 ).

Conclusions

For A. pisum, there seem to be relatively narrow limits of temperature

between which fecundity and reproductive pattern are normal, confirming the

findings of Kenton (1955); only 15°C. and 20°C, produce what is considered

here as a stable pattern in successive generations and at all the other

temperatures which were examined, fecundity and size of larvae were impaired.

The harmful effects of rearing at 25°C. for one to three generations, and

at 28oC. for one generation, were not immediately nor completely rectified

when high temperature stress was relieved. From the experiments described

above, it is known that at least one generation at 20oC. was necessary to

enable the population to recover from rearing at 25C. Although adults

recover full size after two generations at 20°C. (Section 2 p.69), it is

not known whether fecundity and size of offspring would have been normal since

temperature appears to produce effects independent of size. This was shown

by F1 adults reared throughout at 25°C. and at 10°C., where fecundity was

approximately 80 larvae less than expected of aphids of equivalent size

reared at 20°C. Further work is required to investigate the harmful

effects of temperature on adult fecundity observed in the present work.

130.

SECTION 4.

The effect of body size on the ability of adult apterous virginoparae

of A. pisum to survive stresses.

131.

132.

INTRODUCTION

In Section 2 it was shown that small and large adults differed in

their reactions to stress, Thus, the fecundity of small adults was affected

more than large adults by crowding. However, crowding is only one of the

stresses to which adults in the field may be subjected, and other stresses,

such as starvation, could reduce the survival value of the individual. The

experiments described in this section were aimed at an assessment of the

resistance of adult aphids of different sizes to some stress factors other

than crowding, which might affect the survival of adults. The stress factors

studied included: -

a, an insecticide - DDT

b. starvation

Co high temperature, and

d, low humidity.

Preliminary experiments were done to determine the effects of these.

It was hoped that the tests would be followed by more detailed studies, but

lack of time and, in some cases, technical difficulties prevented further work

being done.

A. The effect of body weight on the resistance of A. pisum to topical

222lication of DDT.

Adult apterous virginoparae of varying weights, obtained by different

amounts of crowding during their larval stages, were taken from the cultures

0-6 hours after moulting, kept over moist filter paper for 2 hours and then

weighed. Each weighed aphid was anaesthetised with carbon dioxide and treated

topically with a known dose of DDT dissolved in 0.25 IA of acetone. The doses

of DDT corresponded to a geometric series of 0.15, 0.09, 0.054, 0.032, 0.018,

133.

0.019 and 0.0066 pg active ingredient per aphid. "Control" aphids were

treated with CO2 and acetone. The doees were applied in the LD50 range

indicated by preliminary tests.

After treatment each aphid was transferred to a 1 inch diameter leaf

disc cut from a mature broad bean leaf which was kept turgid by means of the

technique described in Section 1 (p. 35 ). Counts of "killed" insects were

made 24 hours after treatment. "Killed' aphids included those which were

moribund and those which were badly affected (i.e. unable to make co-oreinated

movements). Counts made after 48 hours showed that adults exhibiting these

symptoms always died. Difficulties in obtaining enough freshly moulted

adults at one time, and time taken to deal with each insect, made it necessary

to spread each experiment over a week in order to accumulate sufficient data.

Each daily batch represented a random cover of weights and doses such that

"day effect" did not bias the results.

Two series of experiments were done. The first, with aphids, ranging

in weights from 0.90 to 3.70 mg, held at 25°C after treatment, and the second

with aphids, varying in weights from 1.51 to 3.70 mg., held at 20°C. after

treatment.

For analysis of the first experiment, the range of weights was divided

into three groups 0.90 - 1.50 mg (Group 1); 1.51-2.30 mg (Group 2) and

2.316.3.70 mg (Group 3), while the second experiment comprised only the two

largest groups.

The mid-point weights of the groups were in geometric series, namely

1.20 mg; 1.90 mg; and 3.00 mg. Kills were corrected for "control" mortality

by Abbott's formula (1925). 1-C P P = x 100;6,

1 C

134.

where P is the corrected kill, P1 is the observed kill; and C is the

"control" kill.

The dosage responses followed typical sigmoid curves and were analysed

by probit (Finney, 1947) of the general form

Y = a + bx.

Responses at 25°C (Experiment 1) sucgested that advantage would be gained

by fitting a probit plane following the general form

Y = a + blxl, + b2x2

where x1 is log dose and x2 is log weight (Finney, 1943 and 1947).

Results

a. Experiment 1. Treated aphids held at 25°C.

The results are presented graphically in Fig. 38and tabulated in Table 21

In Fig. 38 each circle represents a single aphid examined 24 hours after

treatment. An open circle denotes a live aphid and each solid circle a "killed"

aphid. The line was drawn through the calculated median lethal doses for the

mid-point values of each weight-group.

Table 21 lists the dosages and corrected dosage response data for each

weight-group. The high control mortality recorded for the smallest weight-

group (Group 1) may be because the group was more sensitive to carbon dioxide

anaesthetisation and to acetone. Unfortunately, no "untreated controls" were

used, but in another experiment with small aphids (p.157 ) handling alone

caused only 3.85% mortality. In this DDT work the use of carbon dioxide and

acetone was unavoidable being inherent factors in the treatments.

Table 22 summarises the analyses of the probit lines, illustrated in

Fig. 392 and gives the calculated LD50 values with attached 95% confidence

limits.

Dosage responses of three size groups of A. pisum to topical applications of DDT.

Mid-point of weight group (mg)

Dosage (pg/aphid) ,_ treated

Group 1 1.20

No. insects Corrected % mortality

Group 2 1.90

No. insects treated

Corrected % mortality

Group 3 3.00

No. insects Corrected treated o mortality

0.1500 1 100 32 10C 33 81.82

0.0900 2 100 38 6E.64 28 64.29

0.0540 20 80.55 32 45.83 27 33,33

0.0320 32 71.65 31 33.61 29 17.79

0.0180 34 58.06 26 S.33 2 0

0.0109 31 36.69 2

0.0656 31 3.82 3 0

Control 35 22.86 39 7.69 11 0

TABLE 21.

Fia. 38 The effect of DDT as a topical application on adult

A. pisum of varying body weights.

•, insects which died; 0, insects which recovered

or were unaffected.

0.1500.-

0.0900

O

1.0540 a.

re o..0

324

0 0

W pc0182 0

0.0109

0,0066

— •

FIG .38 .

• •• ...one • • hi sem • es. N •• 0 •• 0

• IIP• • • % SA4110% Se3 83 co • • gib • oe

•• eesio•oiDep. qi• 99 if6o lb o o o o

• coo 88410 511•40 %% Imo Vo co • oc54 qh d o

8

• • el& 46* 8. o289% c6o 9, o °ode) % % o cQ90

0 0 soot co 0 alo 8%

0 o • oo80815Wo libca 0 0 0

1.00. 2.00 3.00

ADULT WEIGHT ( mg ).

!III!! I tittiii l t II

Fici.39 Relationship between dosage of topically applied

DDT and probit of kill at 25°C for three weight

groups of adult A. pisum.

Weight groups: 0, 0.90 - 1.50 mg: X, 1.51 - 2.30 mg;

0, 2.31 - 3.70 mg.

FIG. 39.

7.0 t

6.0

PR

OB

IT K

ILL

,

5.0

4.0

3.0 x x 0 4, , 4 I I I 1 I I 1 0.111 1.04 1.26 1.51 1.73 1.94 2.18

DOSE DDT.

( Log io pp+ 3 ).

139,

•

a

22, Summary of analyses and LD50 values.

14C.

TABLE

Mid-point of weight group (mg) 1.20 1.90 3.00

Parameter b 2.274 ± 0.514 3.03 ± 0.491 3.022 ± 0.563

-X2

LD(Log pg +3)

LD50 (lag/aphid)

95Z confidence limits (kg)

0.3680 (NS.5DF) 4.0659 (NS.5DF) 1.2962 (NS.3DF)

1.125 ± 0.066 1.712 ± 0.041 1.860 ± 0.044

0.0178

0.0515 0.0725

0.0173 - 0.0184 0.0429 - 0.0619 0.0594 - 0.0883

LD50 adjusted

for weight (pg/Mg) 0.148 0.0271 0.0242

NS.- not significant.

The data of Table 22 show that the LD50's increased with weight

in the ratio of 1;2.9:4.1 for the groups with median weights of 1.20, 1.90

and 3.00 mg respectively. The largest group was only 3,4 times more

resistant than the intermediate size-group. When doses were calculated in

terms of unit weight the LD50's increased in the ratio of 1;1.8:1.6 for

groups 1 to 3 respectively. On this basis the larger aphids were equally

resistant whereas the smallest ones were still significantly more

susceptible.

Fitting of a_probit plane to the dosage response data.

Following Finney (1943) a probit plane,

Y = a blxl + b2x2

where x1 is the log dose + 3 and x2 the log median weight for each group,

was fitted to the data by reiteration until the best fit was obtained. The

plane is illustrated in Fig. 40 where

Fig.40 Probit plane relating resistance at 25°C of adult

A. pisum to DDT in relation to body weight.

Weight groups: 0 , 0.90 - 1.50 mg;

X 1.51 - 2.30 mg; 0 , 2.31 - 3.70 mg.

0.0 ...

3.0 /Mb

7.0

5.0 ...

Fl G. 40.

1+2-

2.0 ,.. Y=1.11146 + 2.1312 xi - 5.3110x2

I 0 1.0

I I 2.0 3.0

LOG WEIGHT + LOG DOSE + 3.

143.

Y = 1.8146 + 2.8382x1 - 5.3110x2' and

b1 and b2 had standard errors of ± 0.2578 and 0.7050 respectively. Table 23

gives a summary of the analysis of variance of the plane.

TABLE 23 . Analysis of variance of the plane.

Degrees of Freedom

Sums of Squares

Mean Square

Fitting the plane 2 121.954 60.977*

Residual error 16 1.623 0.101

Total 18 123.577

F = 603.7 Probability .e: 0.001.

There is no single value for the median lethal dose for a probit

plane since solution of

2.8382x1 - 5.3110x2 = 3.1854

for any pair of values of xi and x2 will yield an LD50. However, by

substitution, as many values of xi as required can be found corresponding to

known values of x2 from

x1 = 3.1854 + 5.3110x2

2.8383

Using the logarithms of the median weight for each group three

solutions were obtained which corresponded to expected values of LD50 for

each weight-group. The calculated figures are shown in Table 249 together

with those obtained from separate probit solutions in Table 22 above.

One of the effects of plane analysis is to standardise the distances

between the lines, which are proportional to the differences between

corresponding values of xi or x2 (Finney, 1947). This has had the effect

144.

TABLE 24. Median lethal doses (MLD) for the weight groups of A. pisum

calculated from

A. Probit plane data, and

B. Discrete analysis of each group by probit lines.

Weight MLD per Ratios be- MLD per mg Ratios between weight

group aphid (pg) tween MLD's of aphid (pg)

corrected MLD's.

A 1 0.0187\ .1/4 ;2.17\

2 0.0406/ "4.72 >2.17 /

3. 0.0882 /

0.0156

0.0214

0.0294

1.37

1.37

B 1 0.0178\ \ 0.0148 -N,

, \2.89\ „..>1.83 ... " 2 0.0515 ......._ / ;;.4.07 0.0271 CN,..„... 1.41 '

-,...0.89 ....,.,,,, ;:- 1.64

3 0.0725'7 0.0242'

of reducing the ratios of resistance between adjacent weight-groups to a

common figure of 1.37 (Table 24 above) by minimising the differences between

groups 1 and 2 and maximising the difference between groups 2 and 3 as compared

with the ratios calculated from median lethal doses based on individual

regression lines. Therefore, although both methods of analysis indicate that

larger aphids require greater doses of insecticide than smaller aphids to

produce a standard kill of 50%, probit plane analysis obscures the fact,

indicated by separate analyses of individual weight-group responses, that

although resistance increases with weight, the increments of resistance

between weight-groups are not constant. Further discussion will therefore

be based only on data from individual regression lines.

145.

b. Experiment 2. Treated aphids held at 20°C.

The relationships between groups 2 and 3 were investigated further

in another experiment where aphids were held at 20°C. after treatment.

Conditions, other than post-treatment temperature, were identical with those

of experiment 1 above.

The experimental results are summarised in Tables 25 and 26 9 and

the calculated regression lines shown in Fig.td

TABLE 25 . Dosage responses of two size groups of A. pkum to topical applications of DDT.

Weight Group

Dose (fag/aphid)

2

No. of insects % Mortality

3

No. of insects % Mortality

0.15 7 100 23 78.3

0.09 30 83.3 29 86.2

0.054 35 74.3 27 48.1

0.032 27 48.1 29 27.6

0.018 17 41.2 -

Control 26 0 21

TABLE: 26 . Summary of analyses and LD50 values.

Weight groups 2 3

Parameter it, 2.092 ± 0.500 2.459 ± 0.662

2 1.3373 (NS.3DF) 5.1740 (NS.2DF)

LD50 (log pg + 3) 1.457 ± 0.093 1.712 ± 0.056

LD50 (pg/aphid) 0.0287 0.0575

Ratio of resistance 1.80

95% confidence limits

(pg)

0.0188 - 0.0436 0.0401 - 0.0662

LD50 adjusted for weight (pg/mg)

0.0151 0.0172

Ratio of resistance

1.14

Fig.41 Relationship between dosage of topically applied

DDT and probit of kill at 20°C for two weight

groups of adult A. pisum.

Weight groups: X, 1.51 — 2.30 mg; 0) 2.31 — 3.70 mg.

7

FIG .41.

147.

7,

6

4

3

1.26 1.51 1.73 1.94 2.111

DOSE DDT.

(log10 tog+ 1 /

148.

The results confirmed that aphids of the larger group require greater

doses of insecticide per individual than the smaller aphids to produce a 50%

kill9 while the doses per milligramme of body weight, were almost equivalent

(larger aphids required only 13% more insecticide per milligramme of weight.).

The lower values for the median lethal doses when treated aphids were

held at 20°C. as compared with 25°C. illustrated that, as with other insects,

DDT has a negative temperature coefficient of action. Thus, for group 2

aphids the LD50 was 0.052 pg/aphid at 25°C. compared with 0.029 pg/aphid at

20°C., while the LD50tfor the largest group were 0.073,pg and 0.0521pg per

aphid at 25°C. and 20°C respectively.

The weight adjusted median lethal doses have so far been based on the

mid-point weights of individual weight-groups. Calculation of the true mean

weights showed that these were very similar to the mid-point weights for

groups 1 and 2 but not for group 3 (Table 31 ).

The recalculated values show that the adjusted median lethal dose for

group 3 of both experiments were overcorrected by using the group mid-point

weights. However, the adjusted doses do not affect the conclusions that the

median lethal doses, per milligramme of body weight, for groups 2 and 3 are

not significantly different and that group 1 aphids are more susceptible than

the larger ones. Thus, although median lethal doses, per individual and per

unit of body weight, increase with body size they do not increase at a

uniform rate.

Table 27 Comparisons between MLD's based on mid-point and True mean weights

25°C 20°C

Weight group 1 2 3 2 3

Mid-point weight of group (mg) 1.20 1.90 3.00 1.90 3.00

True mean weight of group (mg) 1.23 1.89 2.70 1.92 2.86-

MLD's and 95% confidence limits

based on mid-point weights 0.0148 0.0271 0.0242 0.0151 0.0172

(fag/mg body weiyht). 0G0144 - 0.0153 0.0225 - 0.0326 0.0198 - 0.0294 0.0098 - 0.0229 0.0134 - 0.0221

MLD's and 95% confidence limits

based on true mean weights 0.0145 0.0273 0.0263 0.0149 0.0180

0.0141 - 0.0150 0.0227- 0.0328 0.0220 - 0.0327 0.0098 - 0.0227 0.014 - 0,0231 (fag/mg body weight)

150.

B, Survival time under starvation conditions at high and low humidities at

low temperature.

A constant temperature of 10°C was chosen as the experimental tempera-

ture at which to starve newly moulted adult apterous virginoparae. The

insects were kept either individually zt low humidity over phosphorus

pen-Vvide or at high humidity over water in the jars described in Section 1

(p. 34).

The metabolic rate of the aphids was low at 10°C so that they took a

relatively long time to die. In these circumstances it was possible to

examine the aphids at intervals as long as 6 hours without introducing

significant error in estimates of survival time.

a. Survival at low humidity

Fig. 42 shows the results of the experiment as a scatter diagram of

individual wirvival times plotted against the adults' weight just after

moulting. The regression line

Y = 29.98 + 17.72x (b ± 2.002)

removed a significant proportion of the total variation (PeC 0.001) where

1 mg of additional weight led to increase in survival time of 17.72 hours.

In Table 28 the mean survival time is given for 0.50 mg weight-groups.

The results show that larger aphids survived starvation for a

considerably loncrr time than smaller ones.

b. Survival at high humidity.

For convenience and ease of collection of newly moulted adults, only

two size groups, large and small, were starved at 10°C over water. The data

are shown in 2 scatter diagram (Fig. 43) and the mean weights and survival

times of each group are given in Table 29.

151.

Table 28 Mean survival times of 0.50 mg weight-groups of apterous

virginoparae A. pisum starved at 10°C at low humidity.

Weight groups (mg)

> 1.00

1.01 - 1.50

1.51 - 2.00

Number of

Insects

6

22

12

True mean weight (mg)

0.913 ± 0.011

1.203 ± 0.031

1.716 ± 0.047

Mean survival time (Hours)

39.33 ± 2.98

51.97 ± 3.04

59.85 ± 4.13

2.01 - 2.50 23 2.253 i 0.032 72.79 i 4.25

2.51 - 3.00 27 2.741 + 0.025 80.92 ± 3.48

3.01 - 3.50 21 3.234 ± 0.031 81.26 ± 4.32

3..1 ± - 4.00 5 3.664 0.060 99.80 ± 9.71

+ 1 standard error

L

Relationship between survival time and body weight of

adult A. RtlLUm at lOoe at low humidity.

- - - - ---

SUR

VIV

AL

TIM

E (H

ours

).

140

120

100

88

60

40

20

0

1==.

.,M.

FIG 42.

,Imm•

P.....

.--

1-.-

1••

•

• •

• • • •• ••

so •

• II

• $ ••

I

•

• so • • • 1

• • •

• • •

• • •

• ••• 4119 • • •0

Ili • • • • • • se V• • 0 •• • • • •

• s • • ••o • • • • •

19 = 29.98 + 17.72 x

I I

•

• •

• •

fil•

•

• •

I •• 4,1 4.1 •

0 1 2 3 WEIGHT (n16 ).

4

Fict.43 Relationship between survival time and body weight

of adult A. pi,-,um starved at 10°C at high humidity.

•

• 0

SU

RV

IVA

L T

IM

E (

Ho

urs

). 400

300

200

100

0 1 3 4

FIG.43.

500 .-- •

2

WEIGHT ( mg ).

155.

• • • 10 •

• •••

• I •

•

1

•

• • •

• I.

•

•

• • •

156.

TABLE 29 . Mean weights and survival times of two sizes of apterous virginoparae starved over water at 10°C.

n = 16 for both groups.

Weight groups.

Mean weight (mg)

Mean survival time (hours)

Small

le.""t78 ± 0.050

8. 2 . CO -+ 9.95

2.796

372.50

Large

+ - 0.125

+ -18.39

t = 10.253 for 30 D.F. P4:0.001.

The summary of the analysis of survival data by "t" test (Table :0

shows that the two size groups differ significantly (P<0.001) in their

abilities to survive under starvation conditions.

Both experiments showed that survival times increased with weight.

The first, with a drying agent, measured ability to survive desiccation

rather more than ability to survive starvation, whereas the second, with

atmospheric humidity maintained at n high level, was more a measure of ability

to survive starvation. The amount of increase in survival time with increase

in weight was relatively less at low than at high humidity (i.e. x 1.4 at

low R.H. compared with x 2.4 at high R.H. for a doubling of body weight).

The smaller adults seemed to be relatively less able to survive starvation

than desiccation

C. The ability of different sized adult virsinoparae um to survive

at high temperature.

In preliminary studies of survival at high temperature it was found

that the time of death could not be determined accurately even when the

insects were observed continuously. The problem of determining death applies

to most arthropods (Busvine, 1957) and is often replaced by the use of some

other response, such as knockdown. However, the use of knock-down as the

end-point was found impracticable also since many aphids, apparently

157.

moribund, regained mobility, and the times to knock-down appeared to bear

no relation to weight. It is possible that weight does not affect time to

knock-down but it was felt that the inaccuracies in assessing this end-point

could account for a large part of the variation observed. Accordingly the

methods of the experiment were changed to assess the ability of the aphids

to recover from standard times of exposure to high temperature.

The effects of short periods of high temperature were tested using

freshly moulted adults, reared under varying degrees of crowding, divided

into three groups :0.60-1.60, 1.60-2.40 and above 2.40 milligrammes weight.

Each adult was exposes'to 38.8°C 0.1oC. at high humidity over water in the

hot water bath (see p.34) for one of six periods varying from 12 to 37

minutes where the intervals between times of exposure increased by a constant

proportion. Treated aphids and controls were kept on broad bean leaf discs

(see p. 29), and mortality was assessed after 24 hours at 20°C. The results

are summarised in Table 30 .

TABLE 30__. Percentage kills (control corrected), of aphids exposed to 38.8°(e over water. (Number of insects in parenthesis).

Log time exposure (mins)

0 1.079 1.176 1.273 1.370 1.467 1.564

Weight group (mg)

0.8 - 1.6 3.85 44.3 43.7 44.0 68.0 100.0 (26) (28) (24) (26) (13) (7)

1.6 - 2.4 0 15.0 17.4 14.1 39.3 72.7 (20) (20) (46) (64) (56) (44)

>2.4 0 0 0 • 0 21,,9 61.9 100.0 (5) (4) (2) (18) (32) (21) (10)

The data do not fit a sigmoid response curve. The percentage mortalities

corrected for control mortality, were transformed to unweighted angles and curve

were fitted using orthogonal polynomials (F:g. 44); the quadratic term was

found the best fitting curve (P x:0.05). It is apparent that for each weight

Fici.44 Relationship between time of exposure to high

temperature anc angle of percentage mortality for

three weight groups of A. pisum.

Weight groups: A3 0480 "' 1.60 mg; B, 1.61 — 2.40 mg;

C, > 2.41 mg.

A 0 90

1.564 1.467 1.079 1.176 1.273 1.370

EXPOSURE TIME,

( log 10 minutes )

FIG . 44 .

A. Y = 52.08 + 11.08 I; + 5.91 f;

B. Y =33.30 + 0.t0r + 4.00c C. Y = 28.32 + 9.05 r + 3.41 L 80

159,

70

60

20

10

0

50 ..; _. 2 45

40 W ...I 0 Z Ct

30

1600

group the kill remains constant for exposures of 12 - 18 minutes but further

increases in exposure time caused proportional increases of mortality. The

amount of mortality was inversely related to body weight for all exposure

times. Interpolating from the plotted curves (Fig. 44) LT50

values of

20.33, 25.61 and 27.87 minutes respectively for groups A, B and C were

obtained. The differences between median lethal times for the three weight

groups are similar to those for DDT ( p; 140 ) in that the smallest group

was quite well differentiated from the next which was less well separated

from the largest. However, the differences between individual group

mortalities caused by exposures of 12 - 18 minutes are large.

It would seem possible that the temperature used in this experiment

might upset metabolic balance (Wigglesworth, 1951), the effect of which is

independent of weight, but if weight reflects general 'fitness' then small

insects would be expected to die more quickly. However, weight reflects

body size which is an important factor of heat tolerance since smaller

animals have a relatively greater surface area, and thus, theoretically, have

a greater potential for cooling the body by evaporation. Unfortunately, in

this experiment the insects were kept at a high humidity which would prevent

cooling by evaporation and therefore it would not test whether the greater

ratio of surface area to mass would benefit the smaller aphids at lower

humidities. Broadbent and Hollings (1951) showed the importance of the

effect of humidity on survival of several species of aphids at high tempera-

tures; aphids were more resistant to high temperatures when the relative

humidity was low. Thus the difference, between tolerances of large and

small aphids to high temperature might be relatively less at low humidities

if the larger ratio of surface area to mass confers an advantage on the

161.

smaller aphids, and would probably reward further study.

Conclusions

To summarise the results in this section:

(1) Resistance of adult virginoparae to DDT increases with

weight of the adult, but does not increase at a uniform rate

for equal increments of weight.

(2) Smaller aphids seem relatively less able to survive

starvation than desiccation at low temp,rature (10°C ).

(3) Mortality of adults exposed to high temperature (38.8°C.)

and high humidity increases with the period of exposure,

and for any period of exposure increases with decrease in

weight.

SECTION 5.

An examination of the differences in growth

rates of individuals reared under stress

conditions!,

3.62

163

Introduction.

It was shown in Section 2 that small adults produced by crowding

had larger antennae than those of similar weight produced by starvation.

It was suggested that the relatively longer antennae of the crowded

individuals resulted from stress, perhaps through starvation, which did

not operate until relatively late in larval development compared with

stress from the starvation treatment which was continuous from second

instar to adult. Furthermore. it has been suggested that rearing at

high temperature may decrease L-2ze of insects because food intake or

assimilation c:innot maintain rate of synthesis required by relatively

high rates of mritabolism at high temperatures — i.e. high temperatures

may induce a state of starvation with effects on growth comparable to

those caused by a true starvation treatment.

These hypotheses were tested in preliminary experiments in which

measurements were made on successive instars of individuals starved or

crowded at 2000. and individuals reared at a high temperature (28°C).

a• Initial experiment to determine which characters should be measured.

It was indicated (p.36) that appendages were probably the most

useful characters because they could be measured in the exuviae.

Accordingly segments 3,4,5 and 6 of both antennae and all the tibiae on

both sides of the insect were measured in exuviae of successive instars

and adults of 16 aphids reared at 20°C in uncrowded conditions. The mean

of the two measurements for each character (one for each side of the

body) was calculated (cf. p.37).

The mean antennal and tibial measurements of the 16 larvae in

successive instars and adults are in Table 31.

164

Table 31 Mean antennal and tibial measurements (mm) of instars of

A. pisum reared uncrowded at 20°C.

Instar Antennal segments

I IT. III IV Adult:.

3 + 4 0.2214 0.4438 0.7871 1.2874 2.1048

5 0.2178 0.3365 0.4894 0.6565 0.8483

6 0.5842 0.7728 0.9957 1.1817 1.3283

Total 1.0234 1.5601 2.2722 3.1257 4.2813

Tibiae.

1 0.4429 0.656 0.9326 1.3541 1.8644

2 0.4265 0.6411 0.9395 1.3572 1.9059

3 0.5765 0.8680 1.2723 1.8929 2.7170

Curves were fitted to the growth data (cf. p. 107) and are given

in Fig.45 and the estimated coefficients of slope (JD') and curvature

(b2) in Table 32.

165

Table 32 Estimated growth regression coefficients (b1 and b2) for

instar morphometric data of Al_pisum reared uncrowded at

20°C.

Antennal segments. bl b2

3 + 4 - 0.01506 * 0.24155 xxx

5 0.14712 *** - 0.01359 **

6 0,08940 *** - 0.01293 **

Total 0015447 *** - 0.00827 *

Tibiae

1 0 15638 "K - 0.00383 ns.

2 0.16260 *** - 0.00467 ns.

3 0.16865 *** - 0.00260 ns.

Heterogeneity removed by fitting coefficients: ns. - not significant;

* P.;0.05; ** - 13(0001; *4 - P<0.001.

166

The analyses indicate that tibiae have constant growth rates with

negligable curvature while the antennal segments demonstrate a diminish-

ing growth rate with increase in age. However, the antennal segments

3 + 4 have the greatest growth activity and might be expected to show

influence of stress, e.g. crowding or starvation, more than the other

characters measured. For this reason, and also because of ease of

measurement, measurements of growth in this section have been based on

antennal segment growth, particularly the segments 3 + 4.

b. The effect of crowding on growth.

In Section 2 (p.37) it was shown that adults reared as larvae at a density

of 4 per leaf cage were significantly heavier and had significantly

longer antennae than those reared 64 per cage (P(0.001). The mean antennal

length of both groups are presented in Table 33. The growth curves of

the antennae were calculated (Fig.46) and the relative growth rates

(b1 and b2) (Table 34) were compared by 'tf test.

Table 33 Mean antennal measurements (mm) of larvae and adult

167

A. pisum reared at densities of 4 and 64 per leaf cage

at 20°C.

(number of insects measured in parenthesis).

Instar Antennal

segments

I

(12)

II

(10)

III

AAaga

IV

(11)

Adult

(10) (10)

3 + 4 0.2267 0.4227 0.7330 1.2547 2.0535

5 0.2107 0.3372 0.4829 0.6593 0.8917

6 0.6107 0.8364 1.0704 1.3112 1.5622

Total 1.0481 1.5963 2.2863 3.2252 4.5074

64/cage

(32) (29) (32) (26) (39)

3 + 4 0.2281 0.4106 0.7256 1.2246 1.9363

5 0.2121 0.3280 0.4701 0.6422 0.8410

6 0,5987 0.8045 1.0372 1.2622 1.5060

Total 1.0389 1.5431 2.2329 3.1290 4.2833

168

Table 34 Estimated growth regression coefficients (b1 and b2)

of A. pisum crowded and uncrowded during development.

Antennal segments. log y b1 b2

4/cage 3 + 4 0985158 0.23866 ** - 0.00849 *

5 0.66094 0.15444 *** - 0.01193 *

6 1.00939 0.10113 *** - 0.01002 *

Total 1.34.904 0.15724 *** - 0.00580 ns.

64/cage 3 + 4 0.84144 0.23228 *** - 0.00947 *

5 0.64941 0.14883 *** - 0.01180 **

6 0.99551 0.09968 *** - 0.00917 **

Total 1.33621 0.15373 *** - 0.00605 ***

ns. - not significant; * - P<0.05; ** - P<0.01; *** - F<0.001.

Comparisons made between the corresponding bl and b2 coefficients of

crowded and uncrowded aphids did not show any differences significant at

less than P = 0.05 and the only possible differences were between the bl

coefficients for segnents 3 + 4 and total length (P10.10). However, in

all instances the JDl coefficients were larger for uncrowded than for

crowded aphid growth curves. Thus, although consistent differences in

slope did exist they were not sufficiently important to be separated by

the methods of experimentation and analyses used. By marking the aphids

it might have been possible to follow the develcpment of individuals and

increase the precision of the analysis and separate the apparently small

differences in growth rates.

169

C. The effects of starvation on growth.

Exuviae were collected during the starvation experiments (p.60)

from larvae starved 12 hours per day from second instar (Group D 1/2)

and from unstarved 'control' larvae (Group A 1/1). The antennae of the

exuviae and adults were measured (Table 35)9 growth curves calculated

(Fig.47) and the relative growth rates (b1 and b2) (Table 36) were

compared by 't' test.

Table 35 Mean antennal measurements (mm) of larvae and adult

A. pisum starved and unstarved during development.

(number of insects measured in plrenthesis).

Instar I 11 III IV Adult Antennal segments (17) 15) (16) (14) (13)

Unstarved

3 + 4 092529 0.4699 0.8525 1.3888 2.1000

5 092390 0.3761 0.5427 0.7103 0.8730

6 0.6653 0.8897 1.1220 1.3102 1.4587

Total 1.1571 1.7357 2.5171 3.4093 4.4317

Starved (9) (8) (6) (5) (4)

3 + 4 0.2537 0.4750 098130 1.2148 1.6587

5 0.2395 0.3739 095188 0.6600 0.7717

6 0.6581 0.8639 1.0623 1.2174., 1.3587

Total 1.1513 1.7128 2.3942 3.0922 3.7891

170

Table 36 Estimated growth regression coefficients (b1 and b2)

of A. pisum starved and unstarved during development.

Antennal segments

Unstarved

log Y b1 b2

3 + 4 0.89410 0.23092 *** -0.01612 *

5 0.69614 0.14135 ** -0.01816 *i

6 1.02071 0.08499 ** -0.0137i- **

Total 1,37684 0.14609 *** -0.01109 **

Starved

3 + 4 0.85906 0.20386 ** -0.02380 "

5 0.67431 0.12630 ** -0.02063 4w

6 0.99990 0.07785 ** -0.01226 **

Total 1.34858 0.12913 ** -0.01449 **

- P(0.05; ** -F(0.011; *** -P<0.001.

The 't1 test of the differences between growth rates (Table 36)

showed that starvation significantly decreased the general slope of the

line (b1) for segments 3 + 4 and 5, and total length (P(0.001) and segment

6 (P(0.01), and increased the curvature of the line (b2) for segments 3 +

4 (F<0.05) and 6 (P(0.02) and total length (P(0.02). Starvation would

seem to have decreased the general rate of growth of all segments but had

most effect as the sixth antennal segment.

d. The effect of high rearing temperature on larval growth:

Adult apterous virginopavae from an uncrowded culture at 20°C were on

placed,four-inch broad bean seedlings at 28°C and allowed to reproduce

171

for four hours. The number of first instar larvae was reduced to about

four per plant (a total of 30 larvae) to avoid crowding. The larval

exuviae and adults of these larvae were collected and their antennae

measured (Table 37) and growth curves fitted to the measurement means

(Fig.48)

Table 37. Mean antennal measurements (mm) of larvae and adult

A. pisum reared at 28°C.

Instar Antennal segments

I II III IV Adult

3 + 4 0.2245 0.4054 0.6920 1.0710 1.7269

5 0.2072 0.3208 0.4395 0.5543 0.7161

6 0.6126 0,,8030 0.9515 1.0509 1.1751

Total 1.0443 105292 2.0830 2.6762 3.6181

Table 38. Estimated growth regression coefficients

A.pisum reared at 28°C.

(b1 and b2) of

bl b2 Antennal segments log, V

3 + 4 0.81325 0.21940 *** - 0.01007 ns.

5 0.61286 0.13147 ** - 0.01380 ns.

6 0.95239 0.06827 ** - 0.01207 ns.

Total 1.30140 0.13233 *** - 0.01007 ns.

ns. - not P(0.001. significant; ** - P<0.01; ***

172

The results of the analyses (Table 38) show that fitting the linear

expression accounted for significant proportions of the heterogeneity but

that the quadratic term did not remove significant amounts of error. This

suggests that high temperature acts equally on the growth of all instars and

that the effect is not intensified with time, but there seems to be some

evidence of diminishing growth rates of antennal segments 5 and 6 (Fig.48).

e. Comparisons of the growth rates of antennal segments of crowded. starved and

high temperature reared A. pisum.

The growth rates of the three groups studied were compared and the

differences analysed by 't' test. The results are summarised in Table 38.

Table 38 Summary of the analyses of differences between antennal growth

rates of aphids reared crowded, starved and at high temperature.

bl

Starved Crowded

Antennal segments 3+4 5 6 Total 3+4 5 6 Total

High temperature At V V A V V V V

0.02* ns. ns. ns. 0.05 0.05 0.01 0.01

Crowded A A A A

0.001 0.001 0.001 0.001

High Temperature V V V V A A A A

042 ns. ns. ns. ns. ns. nse ns.

Crowded V V V V

0.001 0.05 ns. 0.001.

t Coefficient larger (A) or smaller (V) than coefficient of heading

group above. * - probability level.

173

The comparisons made above (Table 38) show that starvation had the

greatest effect on antennal growth rates. Compared with crowding, the

antennal growth curves of starved individuals had decreased slopes (P(0.001)

and the growth rate decreased more between successive instars (segments 3+4

and total length, P< 0.001; segment 5, p <0.05) Like those of starved

aphids, the antennal growth rates of individuals reared at high temperature

were also affected more than crowded ones since the slopes of the curves

were significantly less (segment 6 and total length, P <0.01; segments 3+4

and 5, P <0.05), but the decreases in growth rates between successive instars

were not significantly different although they were consistently greater at

high temperature. Crowded and high temperature reared individuals had

similar growth rates of antennal segments 5 and 6, and total length, but the

growth curve of segments 3+4 had greater slope and less curvature (P< 0.02)

on individuals reared at high temperature.

Conclusions.

The evidence given above shows that starvation and high temperature

have the same effects as antennal growth, with the exception of segments

3+4 which appear to be more sensitive to starvation treatment than to high

temperature. Thus, starvation and high temperature affect antennal growth

similarly in all instars suggesting that a similar kind of stress is opera-

ting from an early stage in development. By contrast, crowding stress at

the density studied (64 per cage) does not seriously affect antennal growth

rates and does not seem to be more effective in decreasing antennal size

late in development. This contrasts with the effect on adult weight which

was drastically decreased by rearing larvae at high densities, which might

be explained by weight being more sensitive to crowding than the antennae,

174

or perhaps because weight can be affected at a later stage of development

when antennal size is already determined (cf.pp. 52 and 57). However the

differences between the effects of high temperature and starvation, compared

with crowding, can be clearly seen: the first two decreasing size contin-

uously from an early stage of development and the latter causing slight

size decrease throughout development,

Fid.45 Growth curves of antennae and tibiae of A. bisum

reared uncrowded at 20°C.

Antennal segments: 3 + 4; 5; 6 and 3 + 4 + 5 + 6

combined (T).

Tibiae: 1;2;3.

FIG . 45.

1.70

1.60

1.50 ,-

1.40

1.30

1.20 .-

1.10 .-

1.00

0.90

0.80

0.70"-

0.60

0.50 .-

0.40 ^-•

0.30

0.20-

0.10

LE

NG

TH

( lo

gio

mm

).

MEM

I I I 1 2 3

INSTAR •

0 1 4 ADULT.

176,

Fic.46 Growth curves of antennae of A. Pisum reared

crowded art uncrowded at 20°C.

111•lIl •ga crowded; uncrowded.

Antennal segments: 3 + 4; 5; 6; and total length (T).

1.60

1.50

1.40

1.30

1.20

co 1.00 0

0.90 I I- 1.9 0.80 Z w —1 0.70

0.60

0.50

e.

e e 6 ▪ ....." .0..

3+4

FIG • 46.

178.

1.70 VIM

,—.

,—.

0.40

0.30

0.20

0.10

0 1 2 3 4 ADULT.

INSTAR .

Fia.47 Growth curves of antennae of A. pisum reared

starved and unstarved at 20°C.

- - starved; .T.--.-------unstarved.

Antennal segments: 3 + 4; 5; 6; and total length (T).

1 0 1 2

FIG. 47.

1.70

T moo '11) ao• ..• ri.

1.40 ...

1.30

1.20

1.10

1.00

0.90

0.80

0.70 LENG

TH

( log

iom

m +1

).

3+4

0.60

0.50

0.40

0.30

040

0.10

I N S TA R .

I ADULT.

1.60

1.50

MM.

11111M,

i 1 3 4

I 80.

Fio.48 Growth curves of antennae of A. pisum reared at

high tempeY'ature.

Antennal sogments: 3 + 4; 5; 6; and total lenoth (T).

FIG. 48.

1.60

1.50

1.40

1.30

.--

E E 1.00 to ...

to o 0.90 ..-. I 0.80 I- tD 0.70 Z 'al ...1

0.60

0.50

0.40

0.30

0.20

0.10

1 2 3 4 ADULT.

0

INSTAR.

102.

184

a. Crowding: its meaning with reference to A. pisum and its mode of action.

It was suggested (p.50) that A. pisum might be more sensitive to

crowds than aphid species such as Brevicorvne brassicae and Aphis fabae

because it does not form such dense aggregateias these other species and,

therefore, is less likely to be adapted to living under crowded condi-

tions. It is not possible on the basis of the work done to compare

directly the reactions of these species to crowding, but the effects of

crowding on A. pisum and the possible ways in which the crowd might

operate can be discussed.

The work described shows that A. pisum is group sensitive since growth

is stimulated at low densities (1.25 individuals per square centimetre of

leaf surface) in a similar way to that reported for Acheta domestics

larvae (Chauvin, 1958) and Dysdercus fasciatus larvae (Hodjat, 1963),

but, like D. fasciatus (Giles, 1958) group tolerance is also low. Thus

fecundity of small adults was decreased at a low density (1.55 individ-

uals/sq.cm) and growth of larvae and size of the resulting adults were

also decreased at a low density (5.0 individuals/sq.cm): such densities

occur commonly on vetch in the field. Further increases in rearing

density during the larval stage decrease adult size even more drastically.

Thus, the deleterious effects of increasing density first occur at a

density which does not appear to be a crowd as defined in the Oxford

English Dictionary, viz. "a number of persons (or things) gathered so

closely together as to press upon each other". Thus, it is convenient to

redefine the term in the context of the work discussed here as a density

at which individual aphids are harmed by the presence of other individuals.

1a5

For larvae of A. pisum a rearing density of 5.0 individuals/sq.cm is a

crowd since it decreases the amount of growth while for the adults,

1.55 individuals/sq.cm is a crowd since fecundity and longevity are

decreased at this density

The ways in which individuals in a crowd operate against other individ-

uals are not clear but there are several possible modes of action which,

it was suggested, could cause size decrease (p.50) and will be discussed

further. Crowding might cause size decrease in one or more of the follow-

ing ways:

1. By damaging the host plant (including direct feeding damage, con-

tamination by excreta and exuviae and decrease in photosynthesis) and

thereby restricting food intake or decreasing the quality of food;

2. By mutual contamination with excreta and exuviae;

3. By mutual disturbance which interferes with feeding;

4. By mutual disturbance which might upset neuro-homeostatic mechan-

isms without necessarily decreasing food intake.

At the highest density studied (20 individuals/sq.cm) (p.52) serious

damage to the host plant was evident near the end of the aphids develop-

ment, the symptoms included necrosis and wilting of the leaf area occupied

by the aphids in the leaf cage. It is reasonable to assume that in cir-

cumstances such as these sap pressure and sap flow was greatly reduced

and that aphids were existing in starvation conditions. However, a later

experiment (p.57) showed that equivalent size decreases occurred at high

density even when the host plant was changed regularly in the absence of

symptoms of host plant damage. Furthermore, size decreases occurred at a

lower rearing density (5.0 individuals/sq.cm) when damage to the host

la6

plant was not visible. Thus, the condition of the host plant is not the

most important factor governing size of A. oisumo

However, a somewhat different attribute of the host plant may be

important, that is the ability of the plant to provide sufficient nutrients

to support aphid populations above a critical density. Kennedy and

Stroyan (1959) suggest that aphids may induce premature senescence in the

host plant, thus enhancing the quality of the food, and that aphids might

act as 'adventitious sinks' draining water and solutes from the surround-

ing plant tissues. Thus, a group of aphids feeding together may be more

effective in promoting senescence and/or increasing the rate ®f sap flow

to the'sinkl Nevertheless, plants probably have a limited capacity for

supplying sap so that the demands of a critical number of aphids may

exceed the supply leading to semi-starvation of some or all of them. A

measure of the extent to which a standard area of leaf can be utilised by

A. pisumis the biomass of aphids produced on it. Using the data of

Table 5 (p.54) the following mean totals of biomass of aphid survivors

per square centimetre of leaf surface were calculated for adults reared

at three larval densities: 3.00 mg at a density of 1.25 individuals/sq.cm;

10.10 mg at 5.00/sq.cm; and 26.50 mg at 20.00/sq.cm. Thus a square

centimetre of leaf can yield as much as 26 mg of aphid which indicated

that aphids reared 5.00/sq.cm failed to utilise food capable of producing

about 16 mg of aphids yet, individually, they still suffered since they

were smaller than those reared 1.25 individuals/sq.cm. Thus food supply

was not the size-limiting factor in the low-density crowds but might have

been important to the individual at high rearing densities. Nevertheless

it seems that the high-density population made better use of the available

food.

187

Comparisons between size of individuals starved 12 hours daily and

those reared at high density show that growth patterns were different.

Thus2 weight was affected almost equally by both treatments but antennal

length was decreased more by starvation than by high-density crowding,

indicating that the factors acting against growth are not the same in

both rearing conditions.. It was suggested (pp. 52 and 57) that the

differences might be explained by starvation being continuous whereas

crowding stresses intensified as larvae increased in size and as space

per individual decreased. In the latter circumstances weight was decreased

but antennal length was already partly established and thus lelatively

slightly affected. Examination of antennal growth (p.116) did not provide

evidence of stress intensifying later in development and, perhaps, for

future work it might be more expedient to use weight as the yardstick of

growth in order to pin-point the time in development when crowding

stresses are most severe. The importance of starvation in the crowd

cannot be determined from the present series of experiments but it might

be expected that some individuals would be forced off the leaf surface

and be unable to feed.

Contact between individuals has been shown to affect larval growth of

Bupalus piniariu5 (Gruys, unpublished data - Klomp, 1964) at very low

density, and might be important also to A. pisum. The frequency of con-

tacts between individuals depends on the density of the population and

on the size and activity of individuals. At a density of 5.0/sq.cm con-

tacts between individual A. pisum are probably infrequent in early instars

but later in development the space available is filled more effectively

and contacts more common: many more contacts would be expected at a larval

188

density of 20/sq.cm and also at an earlier stage in development. The

influence of mutual contact on development might explain the small weight

decreases at 5.0 individuals/sq. cm compared with those at 20 individuals/

sq.cm.

Mutual contact could have several different effects on the aphids.

Individuals could be so disturbed as to stop feeding completely and with-

draw their stylets, or they may reduce the rate of sap uptake, or stop it

tempJrarily without withdrawing the stylets. All these responses decrease

the amount of sap ingested and would cause semi-starvation which would

probably become more severe as crowding stress increased. However, it is

known that aphids do not feed continuously (Banks a.id Macauley, 1964) and

the starvation experiments showed that adults reared from larvae starved

6 hours daily were not very undersized, which suggests that aphids have a

reserve of feeding time, possibly equivalent to 6 hours enforced starva-

tion daily, and are capable of compensating for short periods of starva-

tion. The starvation experiments also indicated that starved larvae are

less easily disturbed than unstarved ones which could mean that crowded

larvae, which are starved, are disturbed less by the presence of others

and continue to feed unless physically forced off the leaf surface. Both

these factors, i.e. reserve feeding time and alteration of behaviour,

could minimise some of the effects of crowding.

Physical contact between individuals in a crowd could be supplemented

by visual stimulus, providing visual images of disturbance factors.

Another way in which the effects of the crowd might be conveyed to

individuals might be a build-up of ectohormone ('pheromones'. Karison and

Butenandt, 1959) in the crowd when neither sight nor physical contact

would be required. A mechanism such as this, could explain why size

109

decrease occurred at low density (5.0 individual/sq.cm) where presumably

there was little contact and no food shortage. It could also explain why

size decrease at high density is not wholly caused by starvation.

Thus, from the results and this discussion there would appear to be

some stress factor in crowds which cannot, with any confidence, be attri-

buted wholly to shortage of available food nor to disturbance, although

both may contribute to the final result. Stress induced by crowding has

been well studied for some mammals (Munday, 1951; Wynne-Edwards, 1962)

and was associated with such internal changes as adrenal hypertrophy

inducing hyperactivity. Aphids like A. pisum may be subject to similar

alterations in physiological homeostasis causing unbalance in the organism;

certainly aphids which have been starved or crowded are more active, off

the host plant, than unstarved or uncrowded 'controls'. Such individuals

may well be unable to use ingested food efficiently and also, by spending

more time walking around the leaf cage, wastefully utilise stored food

reserves.

There is scope for further experiments to elucidate the kinds of

mechanisms which elicit the size and behaviour responses caused by

crowding.

b, Temperature as a stress factor.

The effects of temperature on size of A. pisum are more predictable

than those of crowding since increases in temperature decrease size con-

forming with the size responses to temperature shown by many other insect

species ( f• p. 16). The effects observed can be attributed to two

possible causes, viz. 'semi-starvation' (Uvarov, 1931) and 'heat injury'

(Lees, 1959).

(90

It is feasible that rapid growth rates at high temperature cannot be

sustained by the food intake and that a situation comparable to that

caused by starvation results. The analysis of antennal growth rates at

high temperature compared with growth rates as affected by starvation

indicates that the growth patterns are the same (p.172), but this might

indicate that both stresses are continuous, affecting each instar equally,

without confirming a closer analogy between the two causes.

Comparisons between adults of equivalent size produced by crowding

and high temperature reveal important differences which appear independent

of the weight attained and suggest that 'heat injury' is inflicted on

individuals reared at high temperature. Thus, aphids reared at 28°C were some of

heavier than, those reared at high density and yet, even when kept as

adults at 20°C, produced dead progeny or progeny which died soon after

birth, whereas those reared at a slightly lower temperature (25°C) for

two generations were slightly larger than those reared at 28°C, but were

moderately fecund when kept at 20°C0 However continuous rearing at 25°C

also appeared to cause 'heat injury' apart from size decrease, because

progeny of adults reared at 25°C for two generations were unable to grow

as well as progeny of smaller, crowd-reared adults and developed into

undersized adults (p072). The third generation aphids reared at 25°C

require the appreciation of another concept besides heat injury, that of

minimum developmental weight (Long, 1953). For every species there is,

presumably, a minimum size which must be exceeded for the individual to

develop and reproduce. It is probable that third generation adults

produced at 25°C were near or below the minimum developmental weight and

would have not reproduced however well they were treated as adults.

191

Lees (1959) reared' individuals of iooura viciae at 25°C of one-tenth

normal weight which were inviable even when transferred to 20°C. This

parallels the results with A. pisum described in this thesis.

Thus, high temperature would seem to act directly on the individual

by causing injury to it which increases in severity with successive gener-

ations. Perhaps, as Imai (1933) suggested, high temperature causes

growth "to proceed with less integrated completeness". The present

work does not indicate where the major effects are directed in the organ-

ism nor does Imai's statement help since it covers the whole mosaic of

developmental and homeostatic mechanisms which govern the organism's

growth processes and organisation.

The size instability of the sixth antennal segment relative to the

size of the third antennal segment and third tibia at high temperatures

is analogous to the variability of the wing length compared with femur

length of Drosophila (Imai, 1933) and of Schistocerca oreparia (Dudley,

1964). The differences in the relative rates of change of lengths of the

sixth and third antennal segments and third tibia of A. pisum can be seen

in Figs. 14,15 and 16, and the b coefficients are compared below with

those for a size range of individuals reared ± various levels of crowding

(cf Figs 7 and 8).

192

Antennal segments 6 on 3

Tibia 3 on antennal segment 3

Tibia 3 on antennal segment 6

Crowding Temperature

Line of size Recovery decrease line

1.1531 ± 0.0681 1.8081 ± 0.0732 1.4541 ±0.0826

2.0690 ± 0.0840 1.9491 ± 0.0658 1.9641 ± 0.0817

1.6525 ± 0.0587 0.9657 ± 0.0625 1.2229 ± 0.0789

± 1 standard error.

Comparisors between the slope coefficients show that the rate of change

in tibial length relative to third antennal length was the same under both

sets of conditions (crowding and temperature) i.e. a lmm change in the

length of antennal segment 3 was accompanied by a 2mm change in tibial

length. However, the rate of change of antennal segMent 6 relative to the

other measurements varied between crowding and temperature. Thus, when

compared with antennal segment 3, segment 6 had a relatively greater rate

of change in length for both the line of decrease (P 0.001) and the line

of recovery (1)-(0.002), while the length of tibia 3 changed more than

that of the sixth antennal segment in relation to crowding than to

temperature (P(0.001).

Dividing the temperature effects into two parts demonstrates that size

decrease of the characters measured does not proceed at uniform rates

throughout successive generations, neither does size recovery from size

decrease caused by high temperature proceed along the same pathways of

relative size changes. By contrasts crowding causes uniform rates of

relative size change throughout the size range of individuals studied

193

but does not lead to such marked decreases in size as high temperature

c. The possible ecological significance of size variation in A. pisum.

It is of interest to consider to what extent size change as induced by

crowding, temperature,photoperiod, starvation and other environmental factors

are of adaptive significance and are of value to the individual or population

of A. pisum.

Individuals are more able to recover from size decrease caused by crowd-

ing than from size decrease caused by high temperature. The recovery of a

small crowd-reared adult can occur partly in its relatively long pre-reprodtc-

tive period during which there is relatively greater weight increase than in

large adults from uncrowded larvae. Furthermore, in the reproductive period

there is evidence of recovery in the increase in size of the new-born larvae

(p.f. 100). Ioreover, the relatively small progeny produced early in the

reproductive period of small adults can grow at a greater rate than normal-

sized new-born larvae, but they do not become full-sized adults (p.107)

Nevertheless these adults were able to produce normal-sized new-born larvae.

Thus the small progeny of undersized crowd-reared adults were able to make

more size recovery during their development and as adults they then produced

larvae of normal size, i.e. size recovery was completed in one generation

after release from crowding stress. By contrast, small adults reared as

larvae at 280C. had prolonged pre-reproductive periods but did not increase

in weight and produced few and minute new-born larvae which died soon after

birth. There was no increase in size of new-born larvae on successive days

of the reproductive period (Fig.37). Fi and F2 adults from larvae reared at

25°C were dale to make some compensatory weight increase when transferred as

adults to 20°C. The size of their progeny, especially of the F2 adults,

194

increased on successive days of the reproductive period (Figs.33 and 34).

However, first-born progeny of adults reared at 25°C for two generations

which were transferred to 20°C were themselves incapable of making normal

growth and developed into small adults. These adults produced few young and

their first=born larvae were small though they developed into nearly full-

sized adults. Thus, two generations werr required to recover nearly normal

size after rearing for two generations at high temperature.

From the data obtained to date it can be stated that recovery from crowd-

ing stress or from moderately high rearing temperatures of short duration can

be effected within two generations under satisfactory conditions of tempera-

ture and sensity. Continuation of the stresses leads to further size decreases

with corresponding decreases in fecundity and longevity which may become

irreversible at the limit where non-viable individuals are produced. The

question then arises as to the possible significance of variation. The results

of experiments described in this thesis show that small adults produced by

crowding are less able than large ones to cope with stresses such as starvation

at high and low humidities at low temperature, short periods of exposure to

high temperature and treatment with an insecticide. The insecticide experi-

ments showed that not only were small aphids more susceptible on the basis of

dose per individual but also when dose was adjusted for body weight. Thus,

large size appears likely to be advantageous to the individual in all circum-

stances.

It can be argued, however, that the success of a population, as distinct

from that of the individual, can be measured more by the number or biomass of

viable individuals it can produce per unit space however variable the size of

the individual. In a previous part of this discussion (p.196) it was shown

195

that crowded aphids produce many more offspring and much more biomass per

unit of leaf area than those reared at the low density required to produce

maximum-sized individuals; this, in terms of the population, could be inter-

preted as a greater success, being a more efficient utilisation of available

food both in terms of numbers produced and of biomass. Clearly, there must

be a limiting density above which biomass will not increase. Thus, Oatman

(1960) showed that the larval biomass per plant of Liriomyza pictella (Diptera)

did not increase when the larval density was raised ablve a critical level

although more, but smaller, larvae were produced.

Decreased fecundity frequently occurs in animals and is advantageous

in situations where potential food supply is low or where overcrowding exists

for example, (Lack, 1954). Thus the lower fecundities of small adults would

be less likely to lead to self-destructive over-exploitation of the habitat

than the higher fecundities of large adults. The differences in rates of

increase of two weight groups, viz. c1.5 mg and '1.5 mg (calculated from the

reproductive rates and generation intervals of aphids used in the comparisons

made between four weight groups in Section 3, p.98) demonstrate the effects

of size differences on the potential rate of population build up and are

summarised below.

Weight groups of founder-aphid <1.5 mg. 71.5 mg.

Net reproductive rate. 87.5 108.1

Innate opacity for increase per two-day period. 1.895 2.242

Total aphids which would be attained after 30

days of unlimited increase. 7,700. 81,000.

196

The rates of increase given above would not be maintained for long since

size and fecundity decrease at low densities which would lead to slower rates

of population build-up.

This evidence of the subtle capacity of A. pisum for self-regulation

supports modern theory on self-regulation of numbers (Wynne-Edwards, 1962).

To be efficient, mechanism of self-regulation should be sensitive to popula-

tion density, i.e. density

dependent in operation (Nicholson, 1933

1958), having increasingly depressive effects on population density increases

and vice-versa as population density decreases. Milne (1957) argued that only

a "perfectly density dependent factor...." could " control increase ®f

numbers endlessly " and that this requirement was satisfied in a popula-

tion only by",.the competition between its own individuals, i.e. its own

intraspecific competition...". Thus in A. pisum there appears to be a

mechanism for its self-regulation, which satisfies the stringent requisite of

ry Milne's definition. Account must also be taken of

acomplementa mchanism for

self regulation, i.e. production of alatae, which has been shown to be density

dependent in some aphids (Davidson, 1929; Bonnemaison, 1951; and others) and

may provide a valuable safety-valve for rapidly expanding populations of

A. pisum as well as providing essential emigrants.

The two major responses to changing density, viz. elate production and

size variation, with consequent reduction in fecundity, together seems to

provide the necessary mechanism whereby the aphid canquickly exploit, but not

dangerously over-exploit, its ephemeral food supply.

197

SUMMARY.

Crowding., temperature and photoperiod as causes of size variation.

1. The size of adult apterous virginoparae of Acvrthosiphon pisum

is increased when they are reared as larvae at low densities (1.25

individuals per sq. cm of leaf surface) compared with when they are reared

singly.

2. When the larval rearing density is increased to 5.0 individuals per

sq. cm adult size decreases and at 20.0 larvae per sq. cm the mean adult

weight is about half that of adults from larvae reared at a density of 1.25

individuals per sq. cm. Antennal length is less affected by increase in

rearing density than is weight.

3. Crowding three successive generations of the aphid does not decrease

size of the adult (weight, third antennal segment length and embryo length)

more than does similar crowding for one generation.

4. When crowded larvae are transferred to fresh host plants at regular

intervals during larval development the moulting adults are no larger than

those reared constantly on one plant. This indicatesthat deterioration .of

the host plant is not the primary cause of size decrease.

5. Larvae starved for 6 hours daily from the second instar produce adults

which are slightly but significantly smaller in terms of weight and antennal

length than adults from unstarved larvae. 12 hours starvation daily decreases

adult weight to about half that of unstarved 'controls'.

6. There are highly significant correlations between weight; lengths of

third, fourth, fifth and sixth antennal segments; lengths of first, second

and third tibiae; length of cornicle, body length; and body width of different

198

sized adults reared at various levels of crowding. A single regression

line can be fitted to the size array of each pair of characters.

7. The optimum constant temperature for the development of large adults

probably lies between 15 and 20°C. Constant temperatures of 10, 25 and

28°C decrease larval and adult size. Adults reared for three generations at

25°C become smaller in each successive generation.

8. The weight and certain morphometrics of adults reared at 10, 15, 20,

25 and 28°C are highly correlated. Changes in the relative size proportions

of some of the adult characters measured in the second and third generations

at 25°C and in the first recovery generation from size decrease caused by

25°C necessitate the fitting of two regression lines to represent relative

size changes throughout the complete size array.

9. Photoperiod has a slight but significant effect on size.

10. Tibial growth in larvae is exponential with equal rates of increase

in size between instars, but the rate of antennal growth decreases between

successive instars. Crowding causes less decrease in the rate of antennal

growth than starvation or high temperature.

Recovery from size decrease.

1. Recovery from size decrease caused by crowding is accomplished in three

successive stages.

a. Small adults have relatively longer pre-reproductive periods

than large adults during which they make relatively greater weight gains.

b. Small adults produce smaller new-born progeny during the early

part of their reproductive period than large adults, but after the

seventh day of the reproductive period they are able to produce new-

born progeny which are as large as those produced by large adults.

199

c. Although the smaller larvae develop into smaller adults than

do larger larvae they grow relatively more during their development

than the large larvae. The new-born progeny of these smaller adults

are normal-sized. Thus, recovery from undersize caused by crowding

is completed in about one generation.

2. Recovery from undersize caused by high temperature is much slower than

recovery from undersize caused by crowding. Small adults reared for two

generations at 25°C have long pre-reproductive periods when placed at 20°C

but weight increases are small. Their first progeny are undersized and are

incapable of normal growth and develop into small adults. These adults have

very long pre-reproductive periods (about 6 days) with slight weight increases,

and produce small larvae which develop into almost normal-sized adults. Thus,

two generations are required to recover from such effects of high temperature.

Effects of size variation on adult biology.

1. Fecundity of crowd-reared adults compared with those reared uncrowded

is correlated with their weight just after moulting, but longevity and length

of the reproductive period does not differ between large and small adults.

2. There is a linear relationship between weight and antennal length of

new-born first instar larvae. This relationship can be used to assess the

biomass of new-born larvae produced by different sized adults. The total

biomass of new-born larvae produced increases with increase in weight of the

newly-moulted adult. Large adults produce more larval biomass per day in the

early part of the reproductive period than do small crowd-reared adults, but

later in the reproductive period differences are small and variable.

200

3. Fecundity is affected by temperature. Adults reared at 10°C produce

few and undersized progeny when kept at 10°C but when transferred to 20°C

produce more progeny the size of which is increased on successive days of the

reproductive period. The fecundity of adults reared and kept at 15 and 20°C

is related to their weight. Adults reared and kept at 28°C produce a few

undersized progeny the size of which is decreased on successive days of the

reproductive period. Adults reared for one or two generations at 25°C produce

more and larger progeny when transferred to 20°C. Third generation adults

reared at 25°C are inviable when kept at 25°C and even when transferred to

20°Cproduce only a few small progeny which do not live. Adults reared for

two generations at 25°C and one generation at 20°C produce few progeny, and

these are small and variable in size.

4. Adult crowding decreases fecundity and longevity. Small crowd-reared

adults are affected more by crowding than are large adults.

Effect of size variation on resistance to stress.

I. Large adults are more resistant to DDT applied topically than are small

adults both in terms of dose per individual and dose per unit body weight.

Large adults are more resistant than medium-sized adults in terms of dose per

individual, but the two groups are equally resistant when MLDs are considered

in terms of dose per unit weight.

2. Small adults are less able than large ones to survive periods of star-

vation at 10°C in low and high humidities. Small adults are relatively more

able to survive starvation at high humidity than at low humidity.

3. Large adults are more resistant to periods of high temperature (380C)

than are small and medium-sized adults. Exposure to this temperature for

201

less than 18 minutes kills a proportion of individuals which decreases

with increase in weight but which is not related to the period of exposure.

The ecological significance of size variation.

1. Although large size is advantageous to the individual it is suggested

that in some circumstances, 0,1. overcrowding or when food is in short supply,

small size can benefit the population.

2. It appears that A. pisum has a mechanism for self-regulation based on

size decrease at high density accompanied by decreased fecundity and on

production of elate migrants which, acting together, prevent eliminative

competition.

202

ACKNOWLEDGE! ENTS.

I wish to acknowledge the following persons for their help during

the course of this work.

My supervisor, Mr. M.J. Way, for his invaluable suggestions and

discussion of the research problem and for his criticisms of the

manuscript.

Professor 0.W. Richards for providing facilities in his Department

at Imperial College Field Station, Ascot.

My wife for her encouragement, support and patience and for her help

with many of the text figures.

Mrs. G.T. Sarney and Mrs. E. Gough for typing this thesis.

Mr. H.O. Devitt for preparing the negatives for the plates and

assisting with their final preparation.

The technical staff at the Field Station who helped in the

construction of apparatus.

This work was begun while in receipt of a Ministry of Agriculture

Studentship and the manuscript was completed while assisting research

financed by the Agricultural Research Council.

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