An Ecological Study of an Attempt at Biological Control of

Aust. J. Agric. Res., 1974, 25, 275-92

An Ecological Study of an Attempt at Biological Control of Noogoora Burr (Xanthium strumarium)

A. J. Wapshere

CSIRO Biological Control Unit. 335 Avenue Abb6 Paul Parguel, 34000-Montpellier, France.

Abstract The recent introduction of two cerambycids, Mecas saturnina and Nupserha vexator, for the biological control of Noogoora burr, Xanthium strumavium, a weed of sheep pastures in subtropical eastern Australia, has led to the development of a verbal model of the effect of these introductions on the populations of the weed.

The model takes into account the dependence of X. strumarium on late spring and summer rainfall, the overwinter survival and germinability of the burrs, and the effect of plant density and growth conditions oil the number of stems and bur; rroduction per plant. The fact that only one cerambycid survives in each rootstock, no matter how many stems are attacked, and the small estimated reduction in burrs per plant produced by an individual cerambycid larva (15 for N. vexator, 35 for M. saturnina), indicated that successful biocontrol depended on high cerambycid populations.

On taking into account the egg production of each cerambycid, it was concluded that, although high beetle populations could develop in dense, strongly growing X. strumarium stands in coastal and subcoastal regions, they would cause only minor reductions in plant density. They would be more effective against poorly growing stands and at sites with low overwinter burr survival, provided that stands occur consistently each year.

These conclusions are dependent on the adaptability of the cerambycids to the climates of the Noogoora burr-infested region. a factor not considered.

Introduction CSIRO, in conjunction with the Queensland Lands Department, has been attempt-

ing to control the weed Noogoora burr (Xanthium strumarium L. = X. pungens Wallr.) by the introduction of two species of stem-boring, cerambycid beetles, Mecas saturnina Le Conte from Texas and Nupserha vexator Pasc. (formerly referred to as N. antennata Gahan) from India. The Mecas species was first released in the field in the Australian summer of 1962-63 and the Nupserha species in the summer of 1964-65. Since the first releases, a study of the general biology of, and interrelations between, the plant and insect species concerned has enabled a tentative forecast to be made of the outcome of this biological control attempt and the conditions under which a given outcome could be expected.

This account gives a verbal description of the system and the interrelationships involved. This verbal model is then used to study the possible outcomes of this biological control attempt.

The Characteristics of Noogoora Burr (Xanthium stvumarium) Taxonomic Note

For some time the Australian species of Xanthium that is known colloquially as Noogoora burr has been named X.pungens Wallr. or X. chinense Mill (Calvert 1930;

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Stride and Straatman 1963). Both these names are, however, synonyms of X. strumarium L. (Hooker and Jackson 1895). Love and Dansereau (1959) consider that all species of Xanthium other than X. spinosum L. are forms of X, strumarium. Recently, Kaul (1965) in India and Baloch et al. (1966) in Pakistan have demonstrated the existence of several different forms of X, strumarium. In Australia X. strumarium is of similar form throughout its range. It is nearest to the 'monsoon' type of Kaul and the 'large-leaved' type of Baloch et al. In this paper, Noogoora burr is, therefore, regarded as one of the forms of the highly polymorphic, cosmopolitan plant X. strumarium L. More recently McMillan (1971) has concluded from the photo- periodicity of the Australian form of X, strumarium (Noogoora bur) that it probably originated from Louisiana in the United States of America.

Weediness

This was reviewed by Cashmore and Campbell (1946) and the situation has changed little since then. Briefly, X. strumorium occurs both in the eastern and western States of Australia. It is, however, in the sheep-grazing plains of north-western New South Wales and western Queensland, where the summer rainfall pattern can, in some years, lead to a dense, luxuriant growth on river courses and channels, that the plant is of most importance as a weed. Here, the heavily-spined fruit, the burr, is an important contaminant of wool, becoming thoroughly enmeshed in the fleece, occasionally in such numbers that special processing is necessary before spinning. Moreover, in these areas both property and paddock (field) sizes are so large and some of the country so inaccessible that control methods with hormone sprays are difficult and costly, although the growing plant is very easily destroyed by such sprays.

Biology

Most of the following account is based on observations made and experiments carried out in the coastal and western regions of the area of Queensland bounded by Rockhampton, Longreach, Charleville and Brisbane. Very little work was done in the more tropical regions of Queensland or in northern New South Wales. Within the above area, the area east of the Great Divide is referred to as coastal, and the westward areas as western.

In Queensland and northern New South Wales, Noogoora burr is a summer annual which begins seeding in March, seeding being triggered by the shorter day lengths at the end of summer.

From April until spring or summer, the plant overwinters as a seed, within the burr, in or on the ground. Germination occurs with the first heavy spring or summer rains, usually sometimes between September and November in the coastal region, but later in the west where sufficient rains may not occur until February or March, if at all.

Once germination has taken place, growth is mainly dependent on soil moisture, especially in the drier western regions. Growth stops without rain but resumes after rain has fallen.

In both coastal and western regions the plant is usually limited to river courses, creek banks and associated flood areas. In the wetter coastal regions, the stands of plants are similar in extent and in the same place from year to year. In the drier western region, however, large fluctuations, both in growth and area covered, occur

Biological Control of Noogoora Burr

from year to year. Here, in dry years, the plants are short and occur only at the creek edges. In wet years, the plants grow actively and are found over a very much greater area.

Seed Structure and Function

The burrs of Xanthium species contain only two seeds. One of these is lower in the burr, towards the micropyle end and somewhat longer, the other is higher and smaller. In an American Xanthium species similar to Noogoora burr, the lower seed germinates first at a temperature of 22°C (71°F) but the higher seed requires a temperature of 33°C (91°F) (Crocker 1948). In Queensland, the author (unpublished data) and Mann (1965) have found a similar differentiation between the two seeds. In the field, the lower seed germinates with the spring rains, but the second seed does not germinate until the soil temperature has risen sufficiently, usually some time in midsummer.

For the above reasons seed germination depends on adequate rainfall in the spring or early summer.

Adequate rainfall has two effects. Firstly, it provides the seed with sufficient moisture to imbibe and, secondly, under Australian conditions it leads to considerable flooding, which buries the still exposed seed of the previous year beneath a layer of alluvial sand and mud. Seeds that remain on the soil surface after rain rapidly dry out again, especially in the drier western areas where early storm rains are followed by hot, drying winds.

Table 1. Effect of alluvial cover on germination of Noogoora burr seed at Samford, September 1965

Position Total No. dead or Total No. Proportion of available otherwise germinable germin- germin-

seed seeds non-germinable seeds ating ating

Covered by sand 294 82 212 187 88.2

Not covered by sand 534 359 175 46 26.3

Burial of the seed places it in a much wetter situation and allows imbibition of water to continue until germination takes place. Table 1 shows the effect of flood burial on the spring germination of Noogoora burr seed; 88 % of the lower seeds in burrs lying within 10 I-ft quadrats set on the area covered by sand germinated, but only 26 % of the seed in the area that was not covered had germinated by 1 September 1965.

Flooding is the most frequent factor producing burial, but the same effect is produced by the trampling of stock and by cultivation. In coastal regions in most years and in western regions in wet years. soil moisture is sufficient to produce growth in many places well away from creek banks after burr burial.

If germination does not take place, the seed may remain viable until the next year. Mann (1965) showed in his experiments that seed was unlikely to survive for more than a few years buried deeply in the soil. At Narrabri, N.S.W., in May 1966, on an area which normally would have carried a burr stand but which at that time did

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not because of continuous drought, it was found (Table 2) that the greater proportion of the seed remained on or near the soil surface. The surface and the first 2 in. of soil contained 98 % of the remaining viable seeds.

Therefore, the majority of seeds which survive for more than 1 year are on the soil surface and these do not germinate until buried.

In coastal regions where germination conditions for both seeds in the burr are generally good and burr stands occur in the same place from year to year, such carry- over would be relatively insignificant. In the west, however, where burr stands vary greatly in extent and regularity of occurrence, such carry-over of seeds is relatively important in allowing the plant to utilize good conditions when they occur.

Seed Survival in the Field

A series of agents and factors cause the disappearance of ripe burrs and seed from the mature plants and later from the soil surface. Rats (Rattus assirnilis Gould and related species), mice and parrots feed on the burrs.

Another factor reducing seed survival is the transport by stock (sheep, cattle and probably also the native marsupials) of burrs into areas where germination is imposs-

. ible. The proportion affected in this way is difficult to estimate but such seeds, unless buried, would lie upon the surface of the ground until destroyed by rats, mice or birds. Heavy flooding also causes disappearance of burrs.

Table 2. The number of viable seeds in two 3 ft by 7 in. transect samples of an area of non-germination

at Narrabri (N.S.W.) in May 1966

Depth of burr Total no. Total viable in soil (in.) of burrs seeds

The total loss of seed from one site by the combination of rats, mice and parrots and removal and disturbance by stock and flooding, was estimated in June 1965 by setting out 50 burrs on 114 in. square pieces of insect screening material fixed by metal stakes on a series of randomly selected spots at a wide variety of sites and then visiting the sites again in November 1965 to determine how many burrs still remained on each square of netting. The survival of burrs over winter is given in Table 3 for each site. At Samford (Table 3) on the banks of the North Pine River, of 23 sets of seeds placed out, 16 were covered by a July flood and 13 of these were almost swept clear. Only 34 % of burrs remained on the three sets covered by alluvium, although this provided the best conditions for future germination. On the region untouched by the flood, survival was very high (97.673, but here there would be a much lower germination.

Table 3 lists the overwinter survival of burrs at all the insect release sites. Survival was highest (90%) at Rockhampton where there was a good grass cover and lowest (4.2%) at Barcaldine where there was little ground cover, heavy rat populations and heavy overgrazing. The other sites were intermediate in these regards and in burr

Biological Control ot Noogoora Burr

survival. The trypetid seed fly, Euaresta aequalis Loew, previously introduced as a biological control measure (Wilson 1960), produces a small reduction of seeds; 13 % was the highest recorded in coastal regions and the percentage was usually much lower. It has not been found in western areas.

Table 3. Survival of Xanthium strumarium burrs at various sites

No. of Total burrs Position groups of *Otal surviving

no. Survival Site and 50 seeds placed burrs until period

condition over period 29 Oct. to ( %) 3-9June 1965 'laced 4 Nov. 1965

Samford, North Pine River

Samford, CSIRO station

Rockhampton, Belmont

Barcaldine Delta

Roma Clearview

(a) Effect of Flooding Flooded and

left exposed 13 650 Flooded and covered

by alluvium 3 150 Not flood-

covered 2 100 Not flooded, river

terrace well away from creek area 5 250

(b) Cerambycid Release Sites High river terrace 5 250 Lower river flat 10 500 Higher ground 20 1000

Gully region 20 1000

Bank of lagoon 10 500 Bed of lagoon 10 500

Growth and Density Relations of the Growing Plant

Noogoora burr commonly occurs on river flats in dense stands in which the individual plants grow rapidly upwards and are tall, thin and single-stemmed. Isolated plants, usually larger and multi-stemmed, also occur. It was apparent that all plants could be considered as part of a generalized system in which growth rate and density relations are inextricably mixed. At Powell's Farm, Cedar Creek, several miles from Samford, there had been in the previous year a dense stand of burr and ploughing this land produced another similar stand after rains in late 1963. Shortly after the germination of the seed, when the plants were still only a foot or so high, the stand density was reduced by pulling up unwanted plants, to produce areas having different densities of plants. Six different plant densities were produced in this way: (A) 6 ft apart, (B) 4 ft apart, (C) 2 ft apart, (D) 14 ft apart, (E) 1 ft apart, and finally, (F) 6 in. apart. The densities were so arranged that they fitted into 12 ft by 12 ft units. Thus there was one plant left in a 12 ft by 12 ft unit for density A, four plants for density B and the other four densities were set up within another 12 ft by 12 ft unit: four plants at density C in a 6 ft by 6 ft square, nine plants at density D in a 6 ft by 6 ft square, 25 plants at density E in a 6 ft by 6 ft square, and finally 25 plants at density F in a 3 ft by 3 ft square.

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Within each unit the densities were arranged at random to fit in with the basic 12 ft by 12 ft block and with the experimental unit as a whole. Finally, four rows of units were placed along the lengths of the field to give 12 replicates of each density.

Distance between X. strumarium plants (ft)

Fig. 1. Number of burrs per plant in experimental plots of Xanthium strumarium plants at different spacings and in three situations favouring growth to different degrees.

Because of the low number of plants in density A (one plant every 6 ft apart), an extra row of 10 A density plants was set up between the second and third rows of experimental units to give a total of 22 plants at density A.

Fig. 2. Number of shoots per plant in experimental plots of Xanthium strumarium plants at different spacings and in three situations favouring growth to different degrees.

Distance between X. strurnarium plants (ft)


After the density of the seedlings had been reduced to give the above experimental design, the plants were allowed to grow to maturity with only an occasional weeding in the less dense groups to remove later-germinating burr plants and other herbaceous weeds. Grass was allowed to grow between plants, as this was considered to be the natural outcome under low density burr conditions. After seeding in late March, the following measurements and counts were made on each of the plants: the number of seeds, the height of the plants, the diameter of the stem at the base of the plant,

Fig. 3. Height and basal diameter of plants in experimental plots of Xanthium strumaviurn plants at different spacings and in three situations favouring growth to different degrees.

and the number of vegetative side shoots greater than 6 in. long. Because of the topography of the land on which the experiment was conducted, consisting as it did of a high part, a shallow slope, and then a lower flat portion near Cedar Creek, the growth of the plants concerned was readily divided into three groups. On the higher level the plants grew very slowly, obviously under poor conditions; on the shallow slope the plants grew rapidly, obviously under very good conditions; and on the lower second flat, growth conditions were intermediate between the other two.

Decreasing plant density has the following important effects. It increases the size of the plant in terms of both the basal diameter and number of side branches.

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This leads to greater seed production by individual plants as density declines (Figs. 1-3). Growth conditions strongly influence the magnitude of these effects. Where vigorous growth is possible, under high density conditions, the plant develops a very tall, thin, single stem bearing few burrs, but under low density conditions it develops a very thick stem (basal diameter greater than 4 cm), and bears many side branches (more than 20) and many burrs (c. 3000). Under conditions where only poor growth is possible the plant is short and remains single-stemmed even at the lowest densities.

The height of the plants mainly reflected the conditions of growth of the three topographic areas within each and was fairly uniform at all the densities. Plants more than 4-6 ft apart are unaffected by other Noogoora burr plants and achieve the maximum size, both in basal stem diameter and in number of side branches and maximum yield, that the particular combination of nutrient and soil water conditions at the given site will allow.

The experiment was carried out under coastal conditions and, although the general principles apply to the drier western areas, the duration of growth there is dependent on rainfall, and the events described above do not necessarily proceed to completion unless growth can continue long enough. Under western conditions more than under coastal conditions, growth takes place in spurts after each succes- sive heavy rainfall. In a wet year, such spurts of growth will be more or less frequent, but in a dry year only one storm may occur and growth stops only after a short burst. The plants show only a limited development along the expected course (indicated in Figs. 1-3). Unless further rain occurs, they will seed in late March at the reduced size.

In a similar manner, since all plants of the summer crop seed and die in late March, the date of germination also determines the extent to which the above relations are fulfilled. This is of more importance in the west where the time of the first heavy summer rains varies considerably from year to year.

The Characteristics of Cerambycid Beetles Mecas saturnina and Nupsevha vexatov Taxonomic Note

There has been no recent change in the taxonomy of the Mecas species introduced into Australia from Texas, U.S.A., but there has been some confusion on the taxonomic status of the Nupserha species introduced into Australia from India. Initially the Indian species was considered to be N. antennata Gahan (Stride and Straatman 1963), but recently specimens from Brisbane originating from India and specimens from Pakistan reared from X. strumarium have been identified by Dr. Duffey as N. vexator (Pasc.) (personal communication). The specimens under the name N. antennata in the British Museum (Nat. Hist.) are much larger than the Nupserha reared from Xanthium and are too large to have developed from an X. strumarium plant. The specimens of N. vexator, however, closely resemble the cerambycid from X. strumarium in size and colour. In this communication the Nupserha species found in X. strumarium in India and Pakistan and recently introduced into Australia will be referred to as N. vexator (Pasc.), the name at present being used by Pakistan investigators (Baloch et al. 1968) for the same insect.

Biology The biology of these two cerambycids has been described by Stride and Straatman

(1963). M. saturnina attacks native North American species of Helianthus, Ambrosia


and Iva as well as Xanthium species. N. vexator is known only from the introduced plants Helianthus annuus and X. strumarium in the Indian subcontinent.

Both species oviposit at the top of the stems of Xanthium plants and the larvae on hatching bore within and down the length of the plant, feeding on the pith and adjacent tissues. Females of M. saturnina girdle the top of the stems in two places a short distance apart and oviposit between the girdles. This checks the upward growth of the plant, which responds by developing axillary shoots. The number of axillary shoots produced after girdling by M. saturnina depends on plant density and growth conditions. Poorly growing plants under high-density conditions produce no shoots and only one or two if density is less. Under good conditions plants at high density produce one or two axillary shoots after girdling. The many-branched plants at low density produce two to four axillary shoots on each girdled branch and main stem of the plant. Unless girdled again each axillary shoot will flower and seed.

N. vexator females oviposit into the surface of the stem or leaf petiole, inserting the egg below the epidermis. The hatching larvae bore upwards inside the stem or branch, which causes the death of the shoot tip in thinner stems and the production of axillary shoots. The larvae then bore down to the base of the plant.

The damage done to the shoot tip by N. vexator larvae and the number of axillary shoots produced depends on the density of the plant and on growth conditions as in the case of ovipositional girdling by M. saturnina. Shoot-tip feeding by N. vexator larvae is much less damaging than the ovipositional girdling of M. saturnina and the production of axillary shoots is correspondingly less. Thick stems and branches remain unaffected by the larvae of N. vexator but are readily girdled by M. saturnina.

On reaching ground level, the larvae of M, saturnina girdle the plant at its base and overwinter in an obligatory larval diapause after plugging the hollowed-out rootstock with woody fibres.

On reaching the rootstock, the N. vexator larva moves into the soil, after cutting its way out of the root, and uses the woody fibres to construct a cocoon attached to the root, in which the larva diapauses during the long dry season.

Both cerambycid beetles have only one generation a year. The adults emerge in early summer, the diapause being broken by a rise in soil temperature in the case of M. saturnina and by a rise in soil temperature combined with a sudden rise in humidity in the case of N. vexator (Stride and Straatman 1963). These adults mate immediately and the females lay eggs in the rapidly growing X. strumarium plants.

During this work it was discovered that ovipositing females do not distinguish between already attacked plants and those not attacked, so if many adult beetles are released inside field cages, multiple oviposition is common, particularly on plants with several side shoots. The larvae within the plants attack each other on meeting, and it was demonstrated by confining larvae together in glass tubes that the attack usually leads to the death of one larva, and sometimes both. Under field conditions the encounters between larvae probably lead initially to separation within the stem. However, when all food within the plant is consumed and the larvae descend to the rootstock at the end of the season, an encounter is inevitable and at most one larva survives to overwinter in the rootstock of any one plant, regardless of the number of ovipositions made and the number of side branches on the plant.

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The damage to the plant by the larval feeding and girdling of both cerambycids was related to the size and rate of growth of the plant.

Large, strongly growing plants at low densities were little affected by an individual larva and in many cases were not completely girdled at the base by M. saturnina. Plants of a basal diameter of less than 1-2 cm were usually girdled. Plants thicker than this were only partially girdled. Similarly the exit of N. vexator larvae from the root of this very short-rooted plant was much more damaging to thin than to thick plants.

Poorly growing plants or thin plants in very dense stands were killed outright by the downward feeding of the larvae of both species before the root was reached.

In short plants, the larvae reach the root more rapidly than in tall plants and in the thin, short plants, girdling or root exit occurs soon enough to prevent seeding, whereas in taller plants and thick-stemmed plants the larvae reach the root too late to prevent seeding.

Demonstration of the Relationship between Plant Density and Biological Control Effectiveness

X. strumarium is a short-rooted, short-lived summer annual, that overwinters from season to season as a seed within its burr. The effectiveness of a given biological control agent for this plant is directly related to the extent to which seed production is reduced. The effect that an individual cerambycid larva has on seed production depends to a considerable extent on the conditions and density under which the plant is growing.

To demonstrate more clearly the relationship between plant density and the effect of individual larvae on seed production, the following experiment was set up in the 1965-66 season at Samford. Young plants of Noogoora burr were set out in the spring in plots at four densities: (a) plants 3 ft apart, (b) plants l + ft apart, (c) plants 9 in. (2 ft) apart, and (d) plants 3 in. (t ft) apart, with 45 plants at each density. They were set out in such a way that the lower density plots served to protect and surround the high density plots. The plants at 3 in. apart were surrounded by a hessian screen which was extended upwards as they grew to simulate a continuous canopy at this highest density. The plants were allowed to grow without disturbance until the emergence of the adult cerambycid beetles in summer.

Two sets of four replicates of these densities were set up, one on the top of a river terrace where growth rates were low and the other set on the nearby river alluvial flat where growth rates were higher.

Adult beetles were allowed to mate on emergence and the mated females were placed in small gauze sleeves on the top of plants selected at random within each density. Because the beetles emerged over a long period, plants at each density were infested successively so that at any one time equal numbers of plants at each density had ovipositing females on them. The two sets of four plots were infested equally on each occasion when beetles were placed on the plants. In this way any preferential attack on any one plot, set or density was avoided.

M. saturnina females were allowed to girdle the tip of the plant and oviposit, and were then moved to another plant so that each plant was only attacked once. N. vexator adults were left on each plant for 8-24 hr and the plant examined for oviposition scars. If scars were found with signs of oviposition, the adult females


were moved to a new plant. The insects were moved in this way until they died or stopped ovipositing.

Out of the 45 plants at each density, 10 were oviposited on by a M. saturnina female and 10 by a N. vexator female. Between 7 and 10 plants of similar size, basal diameter and height were chosen as control plants at the time of the oviposition.

Table 4. The effect of cerambycid attack on the number of burrs produced per Xanthium strumarium plant

Strongly growing plants on river alluvial flat

Distance M. saturnina M. vexatov between Type of No. No. Burrs No. No. Burrs plants attack of burrs per of burrs per

(ft) plants produced plant plants produced plant

Unattacked comparison Oviposition, hatching unsuccessful Oviposition and successful larval attack

Unattacked comparison Oviposition, hatching unsuccessful Gviposition and successful larval attack

Unattacked comparison Oviposition, hatching unsuccessful Oviposition and successful larval attack

Unattacked comparison Oviposition, hatching unsuccessful Oviposition and successful larval attack 13

The remaining plants served to maintain the density within the plot and were not further observed. At the end of the summer the burrs borne by the control plants and by each plant attacked by ,M. saturnina or N. vexator were counted. The selected plants oviposited upon by each insect were divided into two groups: (a) those in which the larvae had successfully hatched, bored down the plant and in some cases girdled the base or left the root, and (b) those in which the egg did not hatch or, if it did hatch, the larvae died early after boring only a short distance and doing little damage to the plant.

The results for the upper and lower sites are given in Tables 4 and 5. In the case of the alluvial river flat, the growth rate was such that the plants at

the highest density (3 in. apart) were competing intensely with each other, and many plants remained below the canopy formed by the others and bore few or no seeds.

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Most plants in which the larvae were successful bore no seeds, but occasionally a successfully attacked plant was one of the five or so major plants in the canopy and bore a relatively large number of seeds. The results for the alluvial flat for the highest density are anomalous for this reason.

Table 5. The effect of cerambycid attack on the number of burrs produced per Xanthium strumarium plant

Normal growth on river terrace

Distance M. saturnina N. vexator between Type of No. No. of Burrs No. No. of Burrs plants attack of burrs per of burrs per

(ft) plants produced plant plants produced plant

3' Unattacked comparison Oviposition, hatching unsuccessful Oviposition and successful larval attack

14 Unattacked comparison Oviposition, hatching unsuccessful Oviposition and successful larval attack

4 Unattacked comparison Oviposition, hatching unsuccessful Oviposition, hatching unsuccessful, larval attack

Unattacked comparisoil Oviposition, hatching unsuccessful Oviposition and successful larval attack

In Table 6 the percentage reduction in burr production is given by comparison with the control plants at each density. The data are very variable, and it is apparent that a deviation of 20 % less or more than the burr production of the control plants is not significant, despite the large numbers of burrs involved at the lowest density.

On bearing this in mind, it was concluded that the effect of M. saturnina attack was only distinguishable at the highest and second-highest densities, and the effect of N. vexator was only distinguishable at the highest density.

This was expected, since N. vexator oviposition has little or no effect on the plant because only a small insertion is made beneath the epidermis. The damage is due solely to the larva boring down the length of the plant, and its subsequent emergence from the root. M. saturnina is more damaging because the oviposition girdling causes the attacked plant to fall below the canopy and to be placed immediately at a


competitive disadvantage whether or not the larva hatches. If the larva hatches, the added damage due to its boring down the plant and girdling the base causes a greater reduction in burr production.

Similarly the effect of N. vexator was almost negligible in the rapidly growing Noogoora burr stand in the river flat, whereas M. saturnina had a distinct effect here even at the second highest density. Both species clearly produced marked reductions in burr production in the stand on the river terrace. The growth rate of the plants here was similar to that commonly occurring in stands of Noogoora burr.

Table 6. Percentage reduction in seeding caused by successful and unsuccessful hatching of Mecas saturnina and Nupserha vexator larvae in Xanthium strurnarium plants at various distances apart

Strongly growing Normal growth Oviposition and plants on river on river

hatching alluvial flat terrace 3 1 + Q $ 3 1 & p $ ft ft ft ft ft ft ft ft

M. saturnina

Ovipositionf unsuccessful hatching 4 -6 15 A 17 -3 25 75 Ovipositionf successful hatching 6 12 72 A 20 15 46 73 Expected reduction fsr successful hatching

assuming loss of 35 b u r s for attack 4 15 52 100 5 16 65 100

N. vexatov

Qviposition + unsuccessful hatching -6 0 -23 A 21 3 12 45 Oviposition t successful hatching 12 -9 0 A 17 9 4 88 Expected reduction for successful hatching

assuming loss of 15 burrs for attack 2 7 22 100 2 7 28 100

*Anomalous.

For the purpose of a verbal model the effect of these two cerambycid beetles on the seeding rate of X. strumarizm can be considered as equivalent to removing a certain number of seeds from a plant whatever its size, seeding rate or density.

The following estimates for these reductions for successful and unsuccessful attack by each cerambycid are suggested after consideration of the results seen at the second and third densities on the river flat.

Reduction in burr production Attack by M. satuvnina per plant

Unsuccessful attack (ovipositional girdling only) 20 burrs Successful attack (ovipositional girdling + larval 35 burrs

feeding + basal girdling)

Attack by N. vexatov Unsuccessful attack (ovipositional insertion only) No reduction Successful attack (ovipositionali- larval feeding + 15 burrs

root exit)

The effect of one larva on burr production is obtained by subtracting the reduction caused by an unsuccessful attack from the successful. The larvae of the two species, which are very similar in size, probably produce nearly the same reduction of 15 burrs.

In Table 6 this hypothetical reduction of the burr production by the control plants at each density, if they had been successfully attacked by either species of beetle, is

A. J. Wapshere

giver1 as a comparative series of percentage values. The pattern is clearly similar to that observed. It is also clear that such small reductions would become indistinguish- able from normal plant to plant variation in burr production when, as in the lowest density, each plant was bearing on average more than 700 burrs.

The results of this experiment also indicate that a single attack by M. saturnina or N. vexator will only stop the smallest and thinnest plants from seeding, i.e. those plants which would have borne fewer than 35 burrs in the case of M. saturnina attack and fewer than 15 burrs for an N. vexator attack, irrespective of whether the small size of plant is produced under dense or under poor growth conditions. When stand density is very great, even the ovipositional girdling by M. saturnina will so affect a plant that seeding is prevented. Under natural conditions the proportion of unsuccessful ovipositions is probably small, as they are rare in field release, cages. The greater proportion observed during this experiment was no doubt due to the method of confining the insects and changing them from plant to plant. The cerambycids will have almost no effect on strongly growing plants in less dense growth, and can be regarded as totally ineffective against large, many-branched plants.

The effectiveness of both cerambycids under lower densities will depend on building up such high populations that all plants are attacked not only once but on many occasions, so that all branches are attacked; in the case of M. saturnina the one to four subsequently growing replacement axillary shoots on each branch must also be attacked so that all or nearly all seeding would thereby be prevented. On a single-stemmed plant at not too great a density, between two to four axillary shoots are produced after the first ovipositional girdling and an additional one if all these axillary shoots are girdled. Thus three to five ovipositions by M. saturnina per plant would be sufficient to prevent seeding in reasonably dense stands. A correspondingly greater number of ovipositions per plant would be necessary on many-stemmed plants growing at lower densities, because each branch of each multiply branched plant would have to be attacked three to five times by M. saturnina to prevent such a large plant from bearing any burrs.

In the case of N. vexator which is less effective at damaging the shoot tip and less effective in reducing burr production, there would need to be a greater number of ovipositions to prevent a plant seeding. The above figures suggest that approximately two ovipositions would be necessary to achieve the same effect as a successful M. saturnina oviposition. This is equivalent to 6-10 ovipositions per plant at medium density, or per stem at low densities.

Discussion and Verbal Model

A typical stand of X. strumarium in Queensland in a river flat region consists of a tall, dense central area containing many single-stemmed plants growing on the rich alluvium. On the edge of this tall central region are shorter, more dispersed plants which are also strongly growing, many-branched, and bear many burrs per plant. Further away still from the rich alluvial area, where growing conditions are poor, the plants are short, with only one or two stems even when a great distance apart, and bear few burrs at the end of the season.

With this description in mind, a verbal model of the effect of the introduction of these two cerambycids can be considered. The biology of the cerambycids is so similar that a general description suffices for both of them.


Initially, on introduction, the cerambycid population will build up until some plants become attacked more than once. Since only one larva survives per rootstock, the multiple attacks amount to a mortality. Under these circumstances and if the survival and oviposition rates of the cerambycid are very low under field conditions, the cerambycid will only increase to the level when the number of multiple attacks removes the excess larvae and the population will stabilize at this low level without affecting the Noogoora burr stand. The stand will continue to exist as described above but will now support a certain low number of cerambycids, seed reduction being negligible. On the other hand, if survival and oviposition rates are higher, under the same circumstances the cerambycid population will rise above the level of ineffectiveness and begin to reduce the seeding rate, but at the same time will also begin to suffer a considerable intraspecific mortality due to the many multiple attacks on each plant. The poorly growing plants at the edge of the stand will be the first to be prevented from seeding, followed by the central single-stemmed, high density plants.

Initially there will be a marked reduction in seeding as the plants at high density produce no or only one or two axillary shoots. The average population of burr plants in the poorly growing zone will decline rapidly, as these cannot respond by producing many branches as the population declines. The many-branched plants on the edge of the central region will initially be little affected. The plant population in the central zone will decline but, because of the good growth conditions, the plants will have more and more side branches to each rootstock as the population declines. This will increase intraspecific mortality of the cerambycid, and there will be a balance when the number of burrs produced each year gives rise to the number of plants spaced so that the intraspecific mortality removes the excess reproductive capacity of the beetle population.

It is clear that, at this stage, the spacing of the plant in the central zone will be determined by the survival and oviposition rate of the beetle and the overwinter survival of burrs.

If the survival and oviposition rate of the beetle is sufficiently high, and burr survival is neither too high nor too low, only large, many-stemmed plants will occur throughout the central and middle zone and the region of poorly growing plants will disappear, assuming the transfer of burrs from the other zone is of minor importance. This will produce a contraction of the area infested, only the best growing region now being covered with plants. This type of change in habitat range has been discussed in regard to the potential effect of the biological control organisms affecting skeleton weed, Chondvilla juncea L. (Wapshere 1971). It is also a likely event during the biological control of Noogoora burr.

To achieve biological control in the sense of reducing the occurrence of Noogoora burr to such a low level that it is not intraspecific mortality but the search for plants that causes mortality of the beetles, the density must fall well below that of plants at 4-6 ft apart. At 4-6 ft apart, plants are at their maximum size and bear the maximum number of seeds (Figs. 1-3). There is no further increase in size, branching or burr production with decrease in density beyond this point.

The data above show that, under good growing conditions, i.e. in the central alluvial flat region, the Noogoora burr plants have 20 branches and bear between 2000 and 3000 burrs, each branch bearing approximately 100 burrs. Since the winter

A. J. Wapshere

survival of seeds was normally greater than 30 % (Table 3), it only requires two burrs, each containing two seeds, to remain for each plant growing in the preceding year, for year to year replacement to occur.

The cerambycids would therefore have to attack practically all flowering branches and subsequently developing axillary shoots to prevent them from seeding. At best, three to four ovipositions by M. saturnina and six to eight by N. vexator per branch would be necessary, equivalent to some 70 and 140 ovipositions per many- branched plant for each cerambycid respectively. Since only one larva survives per rootstock, this is equivalent to a low survival of 1/70 and 11140 respectively for the whole year, as there is only one generation per year. These cerambycids have a 1 : 1 sex ratio. Hence to enable them to survive at all when the whole plant population consists of multiply branched plants, a fecundity of double the intraspecific mortality is necessary. A M. saturnina female would have to produce 140 eggs and a N. vexutor female 280 eggs.

The actual average fecundity recorded by Stride and Straatman (1963) was only 77 eggs per adult female for M. saturnina and 106 for N. vexator adult females.

It is apparent that neither cerambycid has the combination of biological properties necessary to reduce the spacing of X. strumurium plants to less than about 4-6 ft apart in the central alluvial region. Indeed, there is bound to be considerable mortality due to the dispersal of adult insects, effects of native parasites, native cerambycid larvae, and effects of flooding and digging up of the overwintering root- stocks by other animals, all of which were observed during this study.

It seems extremely doubtful whether a great reduction in infestation levels can be expected from the introduction of these insects in the coastal region where overwinter survival is above 30% (Table 3) and germination of buried seed is 80% or more (Table I).

All that can be expected under the best conditions, and assuming complete climatic adaptability. is a reduction of stand density in the central part to levels where the plants are 1-2 ft apart, and a decline in the number of small, poorly grown plants surrounding the main stands, but almost no effect on the large many-branched plants already occurring at low density. This situation will be accompanied by a considerable persistent population of cerambycids within the stand. These cerambycids do have the ability to reduce Noogoora burr stands growing under poor conditions, where plants are smaller, cannot produce so many side-shoots after attack and normally bear fewer seeds. This could be the chief observable effect of the introduction of the cerambycids in coastal regions.

The effect of the cerambycids in western regions is complicated by the large fluctuations in Noogoora burr populations which occur from year to year in certain parts of the region. In those areas where the burr population is frequently absent over one or two seasons the cerambycids will not be able to maintain a population, but the burrs will rest on the soil surface during the dry years (Table 2) and germinate to produce extensive stands after burial in wet years. There will be a mortality of seeds on the soil surface during the dry years but at the present time this is not important enough to prevent large areas being infested, often at considerable density, by the plants when wet conditions return.

In areas where the plant population does not completely disappear but where the area infested varies considerably from year to year (a thousandfold difference in area


infested and hence in plant population is relatively common), the cerambycids could maintain themselves but only at relatively low levels. This is because in the years when relatively few burr plants grow the cerambycid adults have only these to attack, so only low larval populations can result, no matter how many adults emerged from the previous year.

As an examination of the climatic data for these western regions has indicated that one year in four is a dry year and in one year in eight there is a severe drought, there would be little chance for the cerambycid population to recover to levels which would reduce Noogoora burr populations.

In those areas where burr populations occur at approximately the same level from year to year the same arguments apply that have already been used in discussing the biological control effect of the cerambycids in the coastal region. However, the survival of burrs is less in the western areas than on the coast. Table 3 shows that the survival at the CSIRO Samford Station, a typical coastal infestation, averaged nearly 60%, whereas at Roma, a typical western infestation, the average was 44% and at Barcaldine, even further inland, survival was as low as 4%.

In areas with burr survivals of 44 % as at Roma the cerambycids would have almost exactly the same effect as described for the coastal regions. At burr survivals as low as 4 %, as at Barcaldine, the cerambycids could be more effective since a considerable reduction in plant population could be achieved by a much smaller reduction in burr production produced by the cerambycids.

The above description of the effectiveness of these cerambycids against Noogoora burr assumes that they are adapted to and can maintain themselves at high levels under typical coastal and western conditions. The adaptability of these insects to the climates of these regions will be discussed in a future publication.

Acknowledgments The author would like to thank Mr R. Kassulke for invaluable technical assistance

during the major part of this work and the other personnel who were from time to time involved in it. He also expresses his gratitude to the Division of Tropical Agro- nomy, CSIRO, for the help provided, and for laboratory and field facilities at the Cunningham Laboratory and at the Samford Research Farm.

References Paloch, G. M., Dim, I. M., and Ghani, I. M. (1966). Biology and host-plant range of Oeobia

vevbascalb Schiff. (Pyralidae: Lepidoptera), an enemy of Xanthium strumarium L. Tech. Bull. Commonw. Inst. Biol. Control 7, 81-8.

Baloch, G. M., Mohyuddin, A. I., and Ghani, M. (1968). Xanthium stvumuvium-insects and other organisms associated with it in West Pakistan. Tech. Bull. Commonw. Inst. Biol. Control 10, 103-11.

Calvert, J. (1930). Noogoora burr-botanical classification. J. Coun. Sci. Industr. Res. Aust. 3, 183-1.

Cashmore, A. B., and Campbell, T. G. (1946). The weeds problem in Australia: a review. J. Coun. Sci. Industv. Res. Aust. 19, 16-31.

Crocker, W. (1948). 'Growth of Plants.' (Reinhold: New York.) Hooker, J. D., and Jackson, B. D. (1895). 'Index Kewensis.' Vol. 11. (Clarendon Press: Oxford.) Kaul, V. (1965). Physiological ecology of Xantlzium stvumarium Linn. (1) Seasonal morphological

variants and distribution. Trop. Ecol. 6, 72-87. Love, D., and Dansereau, P. (1959). Biosystematic studies on Xanthium: taxonomic appraisal

and ecological status. Can. J. Rot. 37, 173-208.

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Mann, J. (1965). Noogoora burr-seed germination. Working paper, Australian Weeds Conf., Toowoomba, 1965, Vol. 1, pp. 12-14.

McMillan, C. (1971).-Photoperiod evidence in the introduction of Xanthium (cocklebur) to Australia. Science (Wash. D.C.) 171, 1029-31.

Stride, G. O., and Straatman, R. (1963). On the biology of Mews saturnina and Nupserha antennata, cerambycid beetles associated with Xantlzium species. .dust. J . 2001. 11, 446-69.

Wapshere, A. J. (1971). The effect of human intervention on the distribution and abundance of Chondrilla juncea L. Proc. Adv. Study Inst. Dynamics Numbers Population, Oosterbeek, 1970, pp. 469-77.

Wilson, F. (1960). A review of the biological control of insects and wezds in Australia and Australian Nzw Guinea. Tech Commun. Commonw. Inst. Biol. Control. No. 1, pp. 1-102.

Manuscript received 10 August 1973

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