Larval gregariousness and neonate establishment of the eucalypt-feeding beetle Chrysophtharta agricola (Coleoptera: Chrysomelidae: Paropsini)

The selective advantage offered to individuals living within groups may relate to natural enemy defence, but in leaf feeding insects may also relate to overcoming plant defences, especially with respect to feeding establishment. We conducted a series of experiments focusing on neonate larval survival, examining the effect of group size and leaf age on the survival of a eucalypt-feeding beetle, Chrysophtharta agricola , which formed groups of up to 43 larvae on the foliage of Eucalyptus nitens in the field. In the laboratory, in the absence of natural enemies, we found that initial density, leaf age and damage to the leaf margin significantly affected larval survival. Survival of solitary first-instar larvae on young foliage was around 80% whereas on older foliage it was around 11%. Prior damage to the leaf margin significantly increased survival on older leaves to around 61%. Initial larval density also affected survival, although mortality was always significantly higher on older leaves. On older leaves the larval group size above which mortality increased no further was over two-fold that on young leaves. Observations of group feeding behaviour at each instar showed that the majority of larvae (75.7%) were aligned facing away from the feeding site and that only around 7.5%, or just 1–2 larvae per group, fed at any one time. Feeding larvae chewed the leaf edge by straddling the leaf margin. Measurements of leaf margins showed that older leaves had significantly thicker leaf margins and 'thickness' ratios (leaf margin to leaf lamina proper). In the field, approximately 85% of all larvae occurred on the first two expanded leaf pairs, and larval mortality was highest between eclosion and establishment of the first instar. However, beetles apparently did not adjust clutch size according to leaf age.     

The consequences of larval aggregation in the butterfly Chlosyne lacinia

1. Females of Chlosyne lacinia (Geyer) (Lepidoptera: Nymphalidae, Melitaenae), the bordered patch butterfly, clump eggs in a few large clusters on their host plant, Helianthus annuus. Resulting larvae form sibling aggregations to at least the third instar.
2. The effect of group size on survival and development of C. lacinia larvae was tested experimentally in the field. Larvae developed faster and survived better in larger groups.
3. The effects of various predator guilds (ground-dwelling arthropods, aerial arthropods and avian predators) on survival of larvae was then tested while controlling group size. Ground-dwelling arthropods, mainly fire ants Solenopsis xyloni , reduced larval survival greatly but other solitary invertebrate and avian predators did not alter survival. Group defences and aposematism of C. lacinia larvae are probably ineffective against predatory ants that attack en masse and recruit other colony members.
4. In laboratory experiments, two possible mechanisms underlying faster development of larvae in larger groups were tested: (i) overcoming the physical toughness of host plant leaves, and (ii) social stimulus to feed. Results support the physical toughness hypothesis but not the social stimulus hypothesis.
5. Feeding in large groups by C. lacinia larvae confers multiple advantages, including protection from solitary predators and increased feeding efficiency because grouped, early-instar larvae can initiate feeding wounds on tough sunflower leaves. These advantages of larval gregariousness, coupled with reduced desiccation at the egg stage, apparently outweigh disadvantages of aggregation, such as interference and exploitative competition among larvae.     

Life table studies of the walnut leaf beetle,Gastrolina depressa (Coleoptera: Chrysomelidae), with special attention to aggregation

Summary In order to make clear the relationship between the mortality processes and aggregation on the walnut leaf beetle,Gastrolina depressa, life tables of individual colonies were developed and the major mortality factor and the mode of its action were investigated. Furthermore, the influences of the larval colony size on the survival rate and the developmental period were also investigated in the laboratory. In the laboratory, when the larvae were reared on suitable (soft) host plants, the larger the group size, the faster the development. When the larvae were reared on unsuitable (hard) host plants, the larger the group size, the higher the survival rate. In the field, one of the major mortality factors of the egg stage was physiological death. However, there was no particular relation between the egg mass size and hatchability. On the other hand, another mortality factor of eggs, predation byA. hexaspilota, tended to act more on the larger sized egg masses. The mortality of the 1st instar larvae decreased with the increase of colony size. During the larval stages, the major mortality factor was predation byA. hexaspilota andP. japonica. Both predators tended to attack the colony in an all-or-none manner. The number of male adults was much less than the number of females in the Ina populations.     

Aggregation facilitates larval growth in the neotropical nymphalid butterfly Chlosyne janais

1. Larvae of Chlosyne janais (Lepidoptera: Nymphalidae) feed gregariously as early instars on the shrub Odontonema callistachyum (Acanthaceae). During the fourth instar, aggregations break up and larvae feed as solitary individuals.
2. The hypothesis that aggregation increases growth rate was tested by raising larvae on intact plants in the field in different group sizes and measuring their daily growth.
3. There was a striking effect of group size on larval growth whereby larvae more than doubled their weight gain by feeding in large rather than small aggregations on intact plants in the field.
4. This group-feeding advantage was lost altogether if larvae were raised on excised leaves in the laboratory, suggesting that large aggregations may facilitate growth either by inducing a nutrient sink or by overwhelming an induced allelochemical response in the plant.
5. Although larval survival was higher in cages that excluded enemies than in exposed aggregations, there was no influence of group size (experimentally manipulated) on short-term survival in the field. However, there was a weak positive relationship between short-term survival and the size of naturally occurring larval aggregations in the field. These data provide mixed support for the notion that gregarious feeding promotes defence against natural enemies.
6. Although the group defence hypothesis warrants further investigation, feeding facilitation is clearly an important factor contributing to the aggregation behaviour of C. janais larvae.     

Intraspecific interference among larvae in a semivoltine dragonfly population

Summary This study focuses on ways that the size distribution of individuals influences the types and intensities of competitive interactions within a population of aquatic arthropod predators. Three field experiments and one laboratory experiment were designed to test for feeding interference, interference mortality, and dispersal effects within and between larval size classes of the primarily semivoltine dragonfly Tetragoneuria cynosura in Bays Mountain Lake. One field experiment documented the temporal pattern of colonization of large-mesh cylinders by the small, first-year-class larvae during a 30-day period; the results are consistent with passive (density-independent) colonization. A second field experiment examined the effect of large, second-year-class larvae at densities of 1 or 3 per cylinder (14 or 42 m^-2) on colonization by small larvae; this colonization was inhibited at the high density of large larvae. In the laboratory experiment, when larvae of the two size-classes were together in the same aquarium, small larvae moved around less than when by themselves (dispersal inhibition). Thus the inhibition of colonization observed in the field may result from interference mortality, rather than from a flight response to the presence of larger conspecifics.To evaluate this interpretation, the third field experiment measured the in-situ functional response of large larvae to each other and to their small conspecific prey. Results suggest a type 1 (linear) functional response, with feeding inteference among large larvae. Moreover, the interference mortality inflicted by larger larvae on smaller conspecifics was apparently more intense on larger individuals within the small size-class. Taken together, the three field experiments and a statistical power analysis show how colonization and interference interact to determine the local density of small larvae, and why such interference effects are difficult to detect experimentally in the field.     

Movement and feeding patterns of Epilachna cucurbitae Richards (Coleoptera:Coccinellidae) on pumpkin and zucchini plants2

The oviposition patterns of adults and the movement and feeding patterns of larvae of Epilachna cucurbitae on two species of cucurbits, Cucurbita maxima cv Queensland Blue and C. pepo cv Blackjack, were studied in the field and laboratory. The physical and nutritional characteristics of host plant leaves of different ages were described. Younger leaves had higher nitrogen contents but were less abundant, smaller and had higher trichome densities than older leaves. The development of first instar larvae was delayed by the leaf hairs on young and mature pumpkin leaves which prevented larvae from reaching the leaf surface to feed First instal larvae developed more quickly on leaves rich in nitrogen. Neither the total developmental time of larvae nor the size of pupae was affected by leafage because larvae on poor quality leaves compensated by eating more. Female beetles oviposited on all but the youngest and oldest leaves of the host plant. The trichomes on young leaves prevented females from attaching eggs to the leaf surface. First instar larvae remained where they hatched, but older larvae were more mobile, Changing feeding sites frequently and moving progressively to younger, more nutritious leaves. Final instar larvae moved onto adjacent vegetation to pupate. The adaptive significance of these patterns is discussed in relation to the nutritional value, hairiness and abundance of host plant leaves of different ages and the physical limitations of different larval instars.     

Understanding gregariousness in a larval Lepidopteran: the roles of host plant, predation, and microclimate

Abstract.  1. Many moth and butterfly larvae are gregarious early in development, but become solitary in late instars. This ontogenetic variation in behaviour is probably the result of temporal changes in the costs and benefits associated with gregariousness. This study provides observational and experimental evidence that, in one particular moth species, a series of different ecological factors influence larval behaviour at different times during development.
2. Field observations show that young caterpillars of the limocodid Doratifera casta form large aggregations while foraging, but that mature larvae are largely solitary.
3. A field experiment revealed that individual first to third instar larvae in larger groups develop more rapidly, but that group size had no detectable influence on survival. The developmental advantage associated with gregariousness is affected by host plant species, but not by predator exclusion, suggesting that group living in these cryptic early instar larvae promotes feeding facilitation, but does not provide individuals with protection from natural enemies.
4. Laboratory experiments revealed that aposematic fourth instar caterpillars in large groups were less likely to be attacked by a generalist insect predator than those in small groups.
5. Field observations provided no evidence that group living affects body temperature, suggesting that microclimatic factors do not favour gregariousness in this species.
6. It is concluded that gregariousness in D. casta confers at least two different advantages on larvae at different stages early in development, but that these advantages disappear, or are outweighed by costs associated with intraspecific competition, in final instars.     

Population dynamics ofLuehdorfia japonica Leech (Lepidoptera: Papilionidae)

Mortality processes from egg to final instar larval stage are examined in the papilionid butterfly Luehdorfia japonica for two generations in a natural populations. Special attention is given to the effects of initial egg cluster size on the survival rate by the 3rd instar. Mean egg cluster sizes for the two generations were 11.3 and 10.7. The hatching rate was not affected by the egg cluster size. Up to 3rd instar, larvae in a group were more likely to die en masse rather than to die individually, whereas most larvae in later instars died independently. The egg clusters of the average (and the most frequent) size class (11±1) had the second highest survival rate until the 3rd instar. A slightly higher survival rate occurred in clusters a little larger (14±1). The average size clusters were least likely to be exterminated and about 70% of them produced at least one individual surviving to the 3rd instar. Factors affecting the observed mortality patterns were discussed.     

紫茎泽兰粗提物对草地贪夜蛾生长发育及繁殖的影响

为明确紫茎泽兰粗提物对草地贪夜蛾生长发育及繁殖的影响,在实验室条件下分别利用4个不同稀释浓度（20、15、10、5倍）的紫茎泽兰粗提物处理玉米叶片后喂养草地贪夜蛾初孵幼虫,记录各虫态的发育历期、存活率、重量、成虫寿命以及后代性比、F2代卵孵化率、F2代卵的发育历期。结果表明,随着紫茎泽兰粗提物浓度的升高,草地贪夜蛾各虫态的发育历期延长,5倍稀释液处理下蛹的发育历期最长,达20.27 d,10倍稀释液处理下幼虫的发育历期最长,为21.46 d;随着处理浓度的增加,各虫态的重量逐渐降低,且存活率呈下降趋势,其中粗提物对蛹存活率的影响最大,对1~3龄幼虫存活率的影响较小;随着处理浓度的升高成虫的寿命均缩短,且雌成虫寿命长于雄成虫;随着处理浓度的升高,草地贪夜蛾的产卵前期和F2代卵发育历期延长,产卵期缩短;单雌产卵量、后代性比和卵的孵化率降低。总的来说,紫茎泽兰粗提物对草地贪夜蛾的生长发育及繁殖均造成消极影响。5倍稀释液对草地贪夜蛾的生长发育及繁殖的影响最显著,可作为草地贪夜蛾防治研究中的重要植物源杀虫剂材料。     

Group size in a gregarious tortoise beetle: patterns of oviposition vs. larval behaviour

In laboratory and garden experiments, we tested for the existence of adaptive patterns of oviposition and larval behaviour regarding group size in the gregarious tortoise beetle Chelymorpha varians Blanchard (Coleoptera: Chrysomelidae: Cassidinae) on its host plant Calystegia sepium L. (Convolvulaceae). Specifically, we addressed the following questions: (i) Which is the more frequent egg cluster size? (ii) Does cluster size fully predict larval group size? (iii) Are newborn larvae attracted or repelled to conspecific groupings? and (iv) Which is the group size associated with enhanced larval development and adult mass? We found that the mean cluster size was 21.4 eggs. Egg hatch time was significantly shorter in larger clusters. A regression analysis of larval group size against cluster size showed non‐significant results. Thus, original cluster size did not totally determine the larval group size. The mean larval group size was 17.1. Choice tests in an experimental arena showed that larvae clearly preferred leaves of a host plant rather than moistened papers, and that larvae preferred a small group of conspecifics (four larvae per leaf) over larger groups (12 or 20 larvae). Empty leaves of the host plant showed an intermediate level of preference. Development time and beetle performance (adult mass) were affected by larval group size. Larvae in the smallest group (one per leaf) took four more days to attain adulthood than larvae in the larger groups (12 and 20 larvae). Adult C. varians reared in the 12‐larvae group were significantly larger than those reared at the other densities. Comparison of patterns across experimental groups, excluding the 12‐larvae group, showed a tendency for a greater final mass with slower developmental rate.     

Effects of Melia azedarach, (Meliaceae) fruit extracts on the leafminer Liriomyza huidobrensis, (Diptera, Agromyzidae): Assessment in laboratory and field experiments

Control of the widely distributed pest Liriomyza huidobrensis, is complicated due to the protected habit of the leafmining larvae, and their resistance to insecticides. The effects of Melia azedarach, (Meliaceae) fruit extracts against adults and larvae of L. huidobrensis, were investigated. In the laboratory, leaves of Cucurbita, sp. infested with first and third instar larvae were treated with different extract solutions. Larval and pupal survival, as well as wing‐spread of adults, were ssessed. Female adult behaviour towards the extract was also analysed in terms of number of feeding punctures and number of offspring left on treated/untreated leaves. In the field, an infested Vicia faba, crop was sprayed four times at weekly intervals with plant extract, water, and a blank solution. The number of adult leafminers and parasitoids emerging from sampled leaves from each treatment were compared. The laboratory tests showed translaminar action of the extracts, which negatively affected leafminer pupal survival, while body size was not affected. The extracts also deterred feeding by adult females and may also have caused reduction in oviposition rates. All solutions and concentrations tested had similar effects. In the field, extract effects were consistent with those from laboratory trials, number of pupae and pupal survival being lower on treated plants. Percentage parasitism was not affected by plant extract treatment, suggesting a selective activity.     

Biological characterization of an English granulovirus from the summer fruit tortrix moth, Adoxophyes orana

Adoxophyes orana granulovirus (AdorGV) was isolated from overwintering larvae in an orchard in Kent, in the UK. The developmental time of each A. orana instar was determined by measuring the size of the head capsule. The susceptibility of the larvae to the English isolate of AdorGV was evaluated in laboratory bioassays using inoculation by microdroplet feeding and applied dose assays. A series of bioassays were performed to determine LD(50) and ST(50) values for first, fourth and fifth instar larvae. The median lethal doses ranged from 30 occlusion bodies in first instar to 1.36 x 10(6) in fifth instar. The median survival time decreased the later the larvae were infected and ranged from 37 days in first instar to 24 days in fifth instar. Approximately half of the infected larvae released a discharge rich in occlusion bodies from their posterior end prior to death. Approximately 85% of larvae attempted pupation and died as larva-pupa intermediates.     

温度和食料对白眉野草螟幼虫生长发育的影响

【目的】白眉野草螟Agriphila aeneociliella(Eversmann)是近年在我国小麦上新发现的一种害虫,以幼虫在小麦茎基部取食危害,造成缺苗断垄,对我国小麦的安全生产构成潜在的威胁。本研究旨在明确温度和食料对其生长发育的影响,对该害虫的预测预报和有效防控具有重要的指导意义。【方法】在光周期14L∶10D,RH 70%±5%的条件下,设置系列恒定温度,用小麦作饲料,记录和分析不同温度下幼虫各龄的发育历期、存活率,明确其发育起点温度、有效积温;设置温度25℃,光周期14L∶10D,RH 70%±5%的条件,分别用小麦、玉米和人工饲料饲养,分析不同食料对其生长发育和成活率的影响。【结果】在恒温(13~29℃)范围内,白眉野草螟幼虫发育历期随温度升高而逐渐缩短,存活率没有明显差异;在恒温33℃,该虫不能完成幼虫期生长发育而死亡。不同食料饲养后,幼虫各龄发育历期存在显著差异,顺序为取食小麦取食玉米取食人工饲料,尤其是1-3龄幼虫差异最为明显,取食小麦、玉米的4-6龄幼虫发育历期差异未到达显著水平,但显著低于取食人工饲料的幼虫。【结论】白眉野草螟幼虫具有很强的温度适应能力,不同温度对其发育历期具有显著的影响;在目前白眉野草螟发生危害区的主要粮食作物中,小麦为其最适宜寄主,室内条件下取食玉米也能完成幼虫期的生长发育。本研究为制定白眉野草螟在我国的潜在发生危害区提供了理论数据,为田间种群动态变化的预测预报和综合治理提供了技术支撑。     

Hatchling morphology and adaptation to host plants during juvenile stages in the butterfly <Emphasis Type="Italic">Pieris napi</Emphasis>

The adaptations of young insect larvae to factors causing mortality may not depend on whole-body size but may instead depend substantially on the size of specific body parts. Using two closely related plant species, Arabis flagellosa, which has leaves that are tougher and difficult to digest, and A. gemmifera, which has leaves that are softer and difficult to convert, we showed that larvae of the butterfly Pieris napi exhibit specific adaptations through changes in body-part size based on different traits of their host plants. For a given egg size, the head widths of hatchlings from eggs collected from A. flagellosa were significantly larger than those of hatchlings from eggs collected from A. gemmifera. In addition, larger heads were accompanied by smaller abdomens in hatchlings originating from A. flagellosa, whereas the opposite pattern was observed in hatchlings from A. gemmifera. The time to completion of the first feeding on leaves of A. gemmifera was not affected by either egg or head size regardless of the original food plant. However, the time to completion of the first feeding on A. flagellosa decreased with increasing head size of hatchlings, regardless of the original food plant. Furthermore, even though egg sizes did not differ between treatments, larvae originating from A. gemmifera and A. flagellosa exhibited similar weight gain on leaves of A. flagellosa, whereas larvae originating from A. gemmifera gained more weight than larvae from A. flagellosa when reared on leaves of A. gemmifera. These results suggest that selection in young larvae for adaptations to environmental conditions may operate on specific larval body-part sizes.     

嘴壶夜蛾的形态、生活史及昼夜节律

嘴壶夜蛾Oraesia emarginata (Fabricius)是危害水果果实的重要害虫之一, 其成虫和幼虫取食不同的寄主植物, 可以作为理想的嗅觉研究模式昆虫。为了全面地了解嘴壶夜蛾的形态特性和生物学特性, 本实验通过室内饲喂和红外摄像机观察, 对嘴壶夜蛾各虫态的外部形态、 发育以及昼夜活动节律进行了系统研究。结果表明： 嘴壶夜蛾的各龄幼虫可以通过体色, 体表色斑的类型、 位置和数量, 以及腹足的数量进行区别。通过蛹的生殖孔和成虫触角能够很好地区别雌雄。在室内饲养条件下, 嘴壶夜蛾的寿命为53.18±1.70 d, 存活率为63.62%±2.15%, 其中幼虫的发育历期最长, 存活率最低, 卵的发育历期最短, 存活率最高, 雄成虫的存活时间显著长于雌成虫（P=0.008）。6龄幼虫的发育历期（5.29±0.15 d）显著长于其余各龄幼虫（P<0.001）。同一龄发育中期幼虫的体长和体重显著大于将蜕皮幼虫和刚蜕皮幼虫（P≤0.037）; 第2-6龄刚蜕皮幼虫的体重和体长与前一龄发育中期幼虫之间没有显著差异（P≥0.106）。幼虫在光期的孵化、 蜕皮和化蛹比例高于暗期, 而成虫在暗期的飞行、 产卵比例高于光期, 成虫的飞行随着暗期时间的增加而逐渐变少, 光期成虫的飞行主要在开灯之后1 h。成虫交配集中在暗期的第3-5 小时。本研究结果有助于制定有效的嘴壶夜蛾防治措施, 而且为嘴壶夜蛾作为嗅觉研究模式奠定基础。     

Influence of larvae of Gastrophysa viridula on the distribution of conspecific adults in the field

1. Previous laboratory bioassays indicate that exocrine glandular secretions of larvae of Gastrophysa viridula repel conspecific adults and deter them from feeding and oviposition. The present study was conducted to investigate the influence of larvae of G. viridula on conspecific adults in the field.
2. Within the G. viridula population studied, two generations were observed in a year. Occurrence of the different developmental stages overlapped temporally.
3. Some individual plants of Rumex obtusifolius , the host of the G. viridula population studied, grew so close to each other that they were considered as a plant group. When investigating the spatial distribution of larvae and adults within such plant groups, larvae were rarely found on plant groups on which adults were feeding.
4. A field experiment revealed that adults avoided plants of R. obtusifolius infested by conspecific larvae of the second and third instar. Adults still avoided these damaged plants when larvae had left them for pupation.
5. Gastrophysa viridula avoided oviposition on leaves and plants infested by conspecific larvae. Larvae of the second instar significantly deterred oviposition when present at a high density (33.3 larvae/dm²), whereas larvae of the first instar did not deter oviposition of conspecifics at either density tested. The oviposition deterring effect was also observed when just exocrine glandular secretion of larvae of the second instar was applied to the leaves in amounts equivalent to a density of 33.3 larvae/dm².
6. Availability of food ( R. obtusifolius ) largely exceeded its exploitation in each group of plants examined. This may be due to either the spatial separation of adults and larvae or the low population density observed on these plant groups.     

Relationships between body size,reproductive traits,and food resource in three species of tropical flesh flies

Intra- and inter-specific comparisons in progeny size and clutch size were used to identify possible selection pressures acting on three sympatric species of ovoviviporous flesh flies. Both progeny and clutch sizes increased with body weight of the female parent in all species. There was no evidence for a simple trade-off between the two traits. In one species, the larger the female adult the smaller the relative biomass of larvae per clutch (total weight of larvae/body weight of female parent). Female flies produced similar progeny and clutch sizes over successive batches. Variation in progeny size did not influence either larval survival or duration of larval and pupal development. However, larger first instar larvae tended to form larger pupae in two of the species. The experiments testing the effect of progeny size on pupal weight under limited food conditions suggest a selective advantage for larger progeny size.     

Effects of colony size on larval performance in a processionary moth

1. Some lepidopteran species have larvae that live gregariously, especially in early instars. Colony‐living species may benefit from improved protection from predators, thermoregulation, and feeding facilitation, for example. 2. While many studies have compared solitary and gregarious life styles, few data exist as to the relationship between size of the larval colony and larval performance in gregarious species. The present study was aimed at understanding the importance of colony size for growth and survival of the northern pine processionary moth (Thaumetopoea pinivora) larvae. 3. Field studies, comparing three different sizes of colonies of T. pinivora larvae, showed that individuals in larger colonies had a higher survival rate compared with those living in smaller colonies and also a faster growth rate. 4. The higher survival rate of large colonies was attributed to improved protection from predacious arthropods. 5. In early spring, the young larvae bask in the sun to increase their body temperature. In field experiments the thermal gain was higher in large colonies, and individuals in such colonies also grew faster. As growth rate was not affected by colony size when the ability to bask was experimentally removed in a laboratory experiment, the higher growth rate of the larger colonies was probably due to improved thermoregulation rather than feeding facilitation. 6. The size of larval colonies of gregarious insects depends on natural mortality events as well as on female oviposition strategy. Our results show that decreasing colony size can lead to a reduction in growth rate and survival. It is therefore important to understand whether or not small colonies will benefit equally from the gregarious behaviour.     

Evaluation ofMicrothrix inconspicuella, [Lepidoptera: Pyralidae], a potential biological control agent forEmex australis in Australia,carried out in apple orchards in South Africa

Host specificity tests carried out in the laboratory in Australia during 1977, showed thatMicrothrix inconspicuella Ragonot could develop on young apple leaves (Harley et al., 1979). Field studies in unsprayed apple orchards in South Africa showed that some feeding occurred, but fewer than 40% of late instar larvae developed to adults when confined in sleeves on apple tree branches. No feeding or survival occurred in large field cages or in the open. Adults which developed from apple fed larvae were smaller, deformed, occasionally mated and laid fertile eggs but their progeny did not feed or develop on apple fruit or leaves. In conclusion,M. inconspicuella larvae did not develop on apple fruit or leaves in the field, damage was mainly limited to apples already injured and feeding on leaves was minimal. Under normal pest control practicesM. inconspicuella populations did not survive on any part of the apple tree or onE. australis growing under the trees.      

Changes in spatial distribution pattern during the larval stage of the fall webworm,Hyphantria cunea drury (Lepidoptera: Arctiidae)

Summary The changes in spatial distribution pattern during larval stage of the fall webworm,Hyphantria cunea were quantitatively investigated in the field experimental populations. The female adult deposits eggs as a cluster and the hatchlings make a compact colonial-web. In this period, the all-or-none type mortality which is characteristic in gregarious insect species was occasionary recognized before spinning a compact colonial-web. Once making a compact colonial-web, the larvae feed the leaves in the colonial-web up to about 5th instar. In this period, the movement of larvae occurred due to the local food shortage in a colonial-web and the expansion of colonial-web. As the larvae developed, the colonial-web was separated into several small groups. These larvae began to disperse about 5th instar. In this period, the local food shortage seems to be an important trigger for the larval dispersal. The mean concentration of larvae on leaves abruptly decreased, and finally the larvae became solitary at the 6th or 7th instars. The dispersal process in later larval stage is not necessarily due to the complete food shortage. The dispersal prior to the occurrence of food shortage may be a safety mechanism to protect the larvae from the food shortage.