S6 LOWE: EFFECTS OF WEATHER ON APHIDS IN CROPS AND PAST1)ftE~

SOME EFFECTS OF WEATHER ON APHIDS IN CROPS

AND PAS T U RES I N CAN T E R BUR Y, NEW Z EA LAN D

A. D. LOWE

Entomology Division,Department of Scientific and Industrial Research, Lincoln'

INTRODUCTION

Pest insects are frequently held in check bynatural means. The level of this control isinfluenced by the supply of food, the activityof parasitic organisms, including entomophag-ous insects and disease pathogens, and weather.Any enquiry into the importance of weatherin insect ecology must include an examinationof the effects of temperature (including freez-ing), humidity, wind (direction and velocity),precipitation (rate and amount), light (intens-ity and duration), and evaporation. The effectsof these factors are interrelated and all reacton other controlling influences. The impor-tance of weather in this connection is wellillustrated by the work of Andrewartha andBirch (1954). In R 14-year census of thripsin a garden in Adelaide they showed that 78per cent. of the variation in annual peak num-bers was correlated with meteorological varia-tions.The investigation of aphid ecology in New

Zealand has reached the stage where hypo-theses about general and detailed effects ofweather can be made, but these still needconfirmation by controlled experiment.Weather appears to be an important factor,and this paper reports some of the preliminaryfindings, and relates these to overseas hypo-theses of aphid behaviour.Aphids are small, soft-bodied, polymorphic

insects caDable of doing damage in their ownright. They are also important because oftheir ability to spread virus diseases betweenand within crops.Seventy-three species have been recorded in

New ZeaJanrl mainly by Cottier (1953) com-pared with New York State which has 303(Leonard 1963), Israel, 207 (Bodenheimer andSwirski 1957), and Britain, 500 (Johnson,1962). ApProximatelv 2,700 species have beendescribed throughout the world.The aphid life-cycle is involved and consists

of a lll1mber of generations annually. Alate(winged) forms which undertake dispersalflights are regularly produced, with peaks of

flight in mid-spring and late autumn. Samplingof the air-mass with traps and the study ofindividual aphid species on their host.plants inreplicated plots are methods used in aphidstudies. Both have been employed in anecological study carried on at several sites inCanterbury over the last seven years. Intrapping, 42 species have been captured, ninebeing taken in considerable numbers in allseasons and at all sites where traps have beenmaintained. Seven of these are known to beimportant vectors of crop viruses. The cropsaffected are potatoes, brassicas, cereals, legwnes,and carrots. Species may have a limited or awide range of hosts, and it is importantecologically to note that present agriculturaland horticultural practices provide an unbrokencontinuity of crops which the economicallyimportant species colonise.

EFFECT OF THE PHOTOPERIOD- TEMPERATUHE

INTEHACTION

The "classical" life-cycle of aphids has beendescrihed from Europe. It includes the pro-duction of sexual forms in autumn fcHawed byan egg stage in winter. Lees (1964) deter-mined that the stimulus leading to productionof sexual forms is the onset of short days withlow temperatures. New Zealand's latitude issuch that this does not OCCurin most economic-ally important species. The aphids survivethe winter as apterae in sheltered positions ontheir host plants. It is this continuous vivipar-ous parthenogenicity within populations ofintroduced species that is the dominant effect ofclimate on aphids under New Zealand condi-tIOns.The cabbage aphid (Brevicoryne brassicae

(L.) ) and the cereal aphid (Rhopalosiphumpadi L.) may be regarded as typical of those

,.Postal address: C/- Crop Hesearch Division, PrivateBag, Christchurch, New Zealand.

LOWE: EFFECTS OF WEATHER ON APHIDS IN CROPS AND PASTURES

thus affected. Other types of aphid life-cyclesknown in New Zealand are summarised, withexamples, in Fig. 1. Against this generalpattern detailed effects of weather can bestudied.

FIGURE 1. Diagram showing the main cycles

of aphid behaviour in New Zealand.

EFFECTS OF TEMPERATURE, HUMIDITY, AND

RAINFALL ON POPULATION PATTERN AND

FLIGHT IN Two SPECIES

Population pattern

A detailed study of the relationship of thecabbage aphid to its host plants has been under-taken at several sites in Canterbury. Thisspecies colonises all br8ssicas and a typicalpopulation pattern on plants is shown in Fig.2 (A), taken from Lamb and Lowe (1961).Figure 2 (B) shows the numbers of alatesdeveloping on the same plants, and Fig. 2 (C)the number of alates taken on traps locatedwithin the study plots. There is a relationshipwithin these three sets of results and betweenthem and the weather. A t present thisrelationship can be stated only in generaltenus pending further analysis, but the patternruns chronologically under field conditions asfollows: A spring population begins to developon seed-crops of- chou moellier as the tempera-ture rises in about mid-July, the populationincrease becoming apparent after 3-4 weeks.Within this population alates begin to appearin early September and increase in proportionto the whole population from then onwards.

These alates fly to emerging seedling brassicasin late spring, and before the chou moellierplants dry off in early December, farmers'sowings of rape, turnips, and swedes haveemerged to receive an initial colonisation.Alates give birth to live wingless nymphs anda population of apterous forms develops as aresult of favourable temperature. The upwardtrend in numbers is regularly interrupted by asta ble period or a slight decline in autumn,when day-temperatures rise beyond the upperthreshold of development-62°F. (Lamb 1961).Lamb interpreted this figure--surprisingly lowfor an upper threshold-as evidence that otherfactors were operating in the field to reducethe suitability of the host plant as a food sourcefor aphids. These factors in turn were relatedto the low humidity and rainfall at this time,causing high evaporation and transpirationrates.

As temperatures fall and autumn rain allowsincreased plant growth, the upward trend inaphid numbers resumes and a population peak

FIGURE2. Seasonal trends at Lincoln in meannumbers of the cabbage aphid: (A) and (B)on plants, (C) all traps, all as log (n + 1),1958-59 season. After Lamb and Lowe (1961).

87

TABLE 1. Effect of temperature on generation time, total life span, and fecundity of the cerealaphid on barley in the laboratory. After Vi1laneuva and Strong, 1964.

Lowest threshold for development = 46<>F. Highest threshold for development = 78.8<>F.

Temperature Generation time Life Span Young per Number of(OF.) (days) (days) female Aphids tested

55.4 21.7 + 1 44.9 + t6.6 30.5 + 13.4 1273.4 6.6 + 0.7 20.5 :t 4.5 42 + 17.5 2978.8 5.2 + 0.4- 18.3 + 5.9 50.4 + 9.8 2286 5.8 + 0.7 13.7 + 3.7 10.9:t 4 t6

88 LOWE; EFFECTS OF WEATHER ON APHIDS IN CROPS AND PASTURES

is usually reached in March or April. Thedecline in numbers from this peak is rapid,due to a complex of factors. High autumnhumidities favour propagation and growth offungous diseases, and Entomophthora aphidisHoffman can be epidemic at this stage. Tem-

peratures then drop towards the lower thres-hold for development in aphids (approximately45°F.) which restricts the aphid populationsto low levels during the winter. Under experi-mental conditions the whole process takes placeon the one plant from initial sowing in Octoberto seeding and ripening in December of thefollowing year.The reactions of various species of aphids to

weather are quite distinct, and comparison ofthe above trends in cabbage aphid populationswith those in populations of the cereal aphidreveals specific differences. The temperaturerelationships of the cereal aphid have beenstudied under controlled conditions by Vii-laneuva and Strong (1964) and some of theirdata are shown in Table 1. These indicatetemperature effects similar to those alreadyshown as typical of the cabbage aphid. Thereis an unbroken succession of host-crops avail-able to both species. The notable differencesbetween field populations of these two aphidsare (1) the very low numbers of the cerealaphid present during the summer-autumnperiod of high temperatures, possibly causedby a relatively low upper temperature thres-hold, and (2) the almost total absence of diseaseas a controlling factor in populations of thecereal aphid (see below).

An unexplained phenonlenon occurs in therelationship of autumn and spring numbers ofcereal aphids trapped in Canterbury during sixyears (Fig. 3). Alate cereal aphids fly in lateautumn on to seedling wheat crops, and givebirth to nymphs which become the nucleusfor spring populations. Data for the years1963 and 1964 showed large differences in

survival rates between autumn and spring.Sampling gave no evidence of biological controland meteorological records failed to show differ-ences in mean temperatures between the twowinters. Rainfall records, however, werenegatively correlated wilh aphid survival rates,and this strongly suggests weather plays a part.

CEREAL APHID - LINCOLN

FIGURE 3. Total cereal aphids taken on 3 trapsat Lincoln at the end of autumn and the begin-ning of spring flights, 1959-64.

Flight

The production of winged forms in apopulation is of prime importance in dispersalof aphids and hence in the spread of virusdiseases. Recent work (Johnson 1965) hasconfirmed that tactile stimuli lead to the pro-duction of alates. Once they have developed,take-off and dispersal is almost entirely amatter of suitable weather. After moultingto the winged stage, alate aphids go through abrief period of maturation on the plant. Theright combination of temperature and lowwind speed leads to take-off after which thealate is strongly attracted to ultra-violet-bluelight and deliberately flies upward. It is thencarried involuntarily on wind currents. During

LOWE: EFFECTS OF WEATHER ON APHIDS IN CROPS AND PASTURES

flight biochemical changes take place andinduce a landing response. Flight then becomesdeliberate again, the aphids are attracted togreen-yellow light, and land on fresh greenplants. Johnson (1962) showed that there isa diurnal bi-modal peak (Fig. 4) in the flightpattern and interpreted the first peak as take-off of aphids which moult overnight and receivethe stimulus to fly early in the day. Heregarded the second peak as being caused byan accumulation of aphids moulting duringthe day but not flying until they had gonethrough a maturation ("teneral") period, eventhough the flight stimulus may be present allday. He states "As the population grows,successive batches of winged aphids comestraight into migration. . . . These are not seenbecause they are usually taken high into theair. On calm days, especially when there aretemperature inversions, these immense popula-tions become concentrated in the lower layersof air, and that is when we usually see them."This has been deduced from the results oftrapping on a vertical transect, with segrega-tion of the catch at hourly intervals. Thepattern appears to be directly applicable toCanterbury where population peaks coincidewith the occurrence of Fohn winds and rela-tivel:v shallow temperature-inversion layers.

HOURLY CHANGES IN AERIAL

DENStTY OF APHIS FABAE SCOP.

AT CROP LEVEL.

FIGURE 4. Daily flight curves of Aphis fabae,showing bi-modal peak. After Johnson, C. G.(1962).

89

Local data on the cereal aphid reveal bothsustained and short flights. With other species,it has been taken in moderate numbers morethan 100 miles off -shore by ships sailing to theAntarctic (Gressitt et al. 1961). It wiU alsocolonise, presumably by short flights, paddocksadjacent to a known source (Lowe 1964). Bothtypes of flights are reflections of weather suit-able for take-off-in the first instance forsustained flight, in the second for early landing.

Take-off in the cabbage aphid occurs attemperatures in the crop of from 58°F. upwardsand at wind-speeds of under 7 m.p.h.

Such flights may be affected by heavy rain,which sometimes virtually destroys the alatepopulation just before flights or directly afterlanding, causing failure of the species to dis-perse or to establish on fresh crops.

SOME INDIHECT EFFECTS OF WEATHER

The indirect effects of weather on aphidecology are manifold. No local studies havebeen made of those arising from soils andplants, except insofar as they are correlatedwith weather. However, in studies of brassicassusceptible and resistant to colonisation, thecabbage aphid and the green peach aphid,which colonise these plants, have been shownto increase at different rates on leaves of differ-ent age. The green peach aphid shows amarked preference for aging, yellow leaves butthe cabbage aphid prefers fresh leaf growth.Where yellowing, and hence change of leafconstituents, is a result of weather adverse toplant growth (as in drought), differentialpopulation growth in the two aphid species isin turn a reflection of weather operatingthrough host- plant condition.

The entomophthoraceous fungus mentionedas the main cause of a drop in autumn popula-tions of the cabbage aphid, is directly affectedby autumn weather.' Under similar conditionsno such control appears to be exerted onpopulations of the cereal aphid, though thedisease is present in an incipient form. Plantstructure in the different host crops may playa part in the modification of the macro-climate,and the consequent behaviour of the fungus.The micro-climate of these crops would pro-bably repay further study.

.

90 LOWE: EFFECTS OF WEATHER ON APHIDS IN CROPS AND PASTURES

An understanding of the effects of weatheron aphids in crops and pastures is incompletewithout a knowledge of the relationship ofmeteorological factors to the biology andecology of associated entomophagous insects.Our knowledge of these relationships is limited

at present. An example of effect of tempera-

ture is found in the predaceous ladybird

Coccinella undecim.punctata 1. This insectappears as an adult later in the spring than thestart of population growth of the cabbage aphid.The latter begins in mid-July when meantemperatures are in the upper forties. The

aphid has a distinct advantage in being ableto increase to pest numbers early in the growing

season when free from this predator. Furtherknowledge of this kind may give a reliableguide to suitable beneficial insects for aphidcontrol in crops.

THE COLLECTION AND ANALYSIS OF DATA

What methods are to be employed to analysethe data have yet to be decided. For instance,severAl effects of temperature on the cabbageaphid have been noted as fncts observable in

the field. Ontside the generallv accepted thres-

holds (45-75°F.) within which multiplicationtakes place. little is known about upper andlower lethal thresholds, or about the cumulative

effects of temperature. A auesti(1ll of obviouser.onomic significance in Canterbury is theeffect of frost on aphids. This is related to thelower lethal temperature of the snecies involvedas well as to the micro~climate of the individualhost crop and cannot at present be answered.Normal meteorological values expressed asdaily or weekly means are too general to permit

detailed analysis. eOlltinuous records of tem-perature though easily obtained represent a

Herculean task in analysis if cumulative effectson different species in different seasons or sitesare studied. This is only one aspect of theeffect of weather on a species. Weather hasa marked influence on population fluctuations,and evaluation of all effects, direct and indirect,

on the several species involved will requiremany years of data collection and experimentalanalysis.

REFERENCES

ANDREWARTHA,H. G., and BIRCH, L. C., 1954. The

distribution and abundance of animals. University

of Chicago Press, Chicago, Ill.

BODENHEIMER, F. S., and SWIRSKI, E., 1957. Thetlpilidoidea of che Middle EasC. The Weismann

Science Press of Israel, Jerusalem.

COTTIER, W., 1953. Aphids of New Zealand. N.Z.

D.S.l.R. Bull. t06.

GRESSITT,J. L., LEECH, R. E., LEECH, T. S., SEDLACEK,

J., and WISE, K. A. J., 1961. Trapping of air-borneinsects in the Antarctic area (Part 2). Padf.

Insects 3, 559-62.

JOHNSON, B., 1965. Wing polymorphism in aphids, II.

Interaction between aphids. Entomologia Expl. Appl.

8, 49-M.

JOHNSON, C. G., 1962. Aphid migration. New Scient.15, 622-5.

LAMB, K. P., 1961. Some effects of fluctuating tempera-

tures on metaholism, development, and rate of

population growth in the cabbage aphid. Ecology42, 740-5.

LAMB, K. P., and LOWE, A. D., 1961. Studies of theecology of the cabbage aphid on brassica field crops

in Canterbury, New Zealand. I. Introduction andpopulation study, 1958-9. N.Z. J. Agrie. Res. 4:

619-42.

LEES, A. D., 1964. The location of the photoperiodic

receptors in the aphid Megoura viciae Buckton. J.

Exp. 8iol. 410 It9-133.

LEONARD, M. D., 1961. A list of aphids of New York.Proc. Rochest. Acad. Sci. 10: 289--432.

LoWE, A. D., 1964. The ecology of the cereal aphid inCanterbury. Proc. 17th N.Z. Weed £& Pest Cont.

Coni., 175-182.

VILL NEUVA, J. R., and STRONG, F. E., 1964. Laboratory

studies on the biology of Rhopalosiphum padi. Ann.

Amer. Ent. Soc. 57: 609-13.