Phenology of nutritional differences between new and mature leaves and its effect on caterpillar growth

Abstract. 1. We determined the phenology of the shrub Spiraea latifolia Ait. Bork. (Rosaceae), which has indeterminate shoot growth, and the effects of phenological changes in leaf quality on growth rate of the early-spring feeding buckmoth caterpillars, Hemileuca lucina Hy. Edw. (Saturniidae).
2. Leaves, regardless of whether they were newly expanded or several weeks old, were tougher later in the growth season (mid-June) than similarly aged leaves collected earlier; correspondingly, water and nitrogen content for leaves of all ages declined through the larval period. By July, newly expanded leaves had no more nitrogen than mature leaves.
3. Relative growth rate of third instar larvae fed new leaves or a mixture of new and mature leaves in early June was higher than that of those fed mature leaves, and efficiency of conversion of digested food to biomass was higher for larvae fed new leaves than for those fed mature leaves or a mixture.
4. In another experiment, larvae were reared on new leaves through the fourth instar and then fed a diet of new, mature or a combination of new and mature leaves, a regimen that was similar to the phenologies of both plants and caterpillars in the field. There was no difference in time to pupation or pupal weights among these treatments.     

Direct and indirect effects of predatory wasps (Polistes sp.: Vespidae) on gregarious caterpillars (Hemileuca lucina: Saturniidae)

Summary We examined how predation by vespid wasps,Polistes dominulus andP. fuscatus, affected the behavior, growth rate and survivorship of aggregated caterpillars ofHemileuca lucina (Saturniidae). Although these larvae can exhibit a variety of defense and escape behaviors, in general larvae reacted to wasp attacks by clinging to the hostplant. Neighboring larvae in the aggregation responded by leaving the feeding site and moving to the interior or base of the plant. To determine wheter wasp attack affected the behavior and growth of the caterpillars that escaped, a field experiment was conducted with treatments of: 1) larvae exposed to wasps, 2) larvae protected from wasps, and 3) larvae protected from wasps but with the attack of wasps simulated (=harassment). Over just one instar, protected larvae gained significantly more weight than the harassed larvae, which in turn weighed significantly more than the larvae that escaped the wasps. The behavior of attacked and harassed larvae differed from that of the protected larvae; the disturbed larvae often fed in smaller groups and in shaded portions of the plant where only mature leaves were available. A laboratory experiment showed that at 35° C (daytime temperature) larvae had significantly higher relative growth rates and significantly shorter instar duration than larvae reared at 25° C. Our results suggest that wasps, in addition to killing caterpillars, indirectly affect larval fitness by slowing larval growth, at least in part by forcing larvae into cooler microhabitats where leaves are of lower quality.     

The causes of colour and colour change in caterpillars of the poplar and eyed hawkmoths (Laothoe populi and Smerinthus ocellata)

Caterpillars of the poplar and eyed hawkmoths (Laothoe populi and Smerinthus ocellata respectively) were reared under different conditions in order to determine why final instar caterpillars vary in colour. Poplar hawkmoth caterpillars normally rest on the undersides of leaves. Dull green and redspotted caterpillars are genetically determined polymorphisms. Caterpillars that are not dull green, however, can become white when fed on Populus alba or yellow-green when fed on Salix fragilis. Experiments showed that it is the reflective qualities of the leaves that determines which colour the caterpillar develops: if the young larva sees white then it becomes white, but if it sees green, grey or black then it becomes yellow-green. Young eyed hawkmoth larvae always developed into grey-green final instar caterpillars under our rather poorly-illuminated rearing conditions, but when reared on wild plants in white muslin sleeves they became whitish-green. In this species also it appears that colour of the final instar is determined by the reflectance of the substrate perceived by the young caterpillar.     

Temperature and food quality effects on growth,consumption and post-ingestive utilization efficiencies of the forest tent caterpillar Malacosoma disstria (Lepidoptera: Lasiocampidae)

Temperature and food quality can both influence growth rates, consumption rates, utilization efficiencies and developmental time of herbivorous insects. Gravimetric analyses were conducted during two consecutive years to assess the effects of temperature and food quality on fourth instar larvae of the forest tent caterpillar Malacosoma disstria Hübner. Larvae were reared in the laboratory at three different temperatures (18, 24 and 30 degrees C) and on two types of diet; leaves of sugar maple trees Acer saccharum Marsh. located at the forest edge (sun-exposed leaves) or within the forest interior (shade-exposed leaves). In general, larvae reared at 18 degrees C had lower growth rates and lower consumption rates than larvae reared at the warmer temperatures (24 and 30 degrees C). Moreover, the duration of the instar decreased significantly with increasing temperatures. Type of diet also affected the growth rates and amount of food ingested by larvae but did not affect the duration of the instar. Larvae fed sun-exposed leaves consumed more food and gained higher biomasses. Values of approximate digestibility and efficiency of conversion of ingested food were also higher when larvae were fed sun-exposed leaves. Higher growth rates with increasing temperatures were primarily the result of the shorter stadium duration. The higher growth rates of larvae fed sun-exposed leaves were possibly the result of stimulatory feeding and consequently greater food intake and also a more efficient use of food ingested. This study suggests that the performance of M. disstria caterpillars could be enhanced by warmer temperatures and higher leaf quality.     

Development and feeding efficiency of Malacosoma neustrium larvae reared with Quercus spp. leaves

Observations on larval development of Malacosoma neustrium were conducted both in a cork oak stand and in the laboratory by using leaves of different host trees (cork oak, holm oak and downy oak) as food source. Instars were determined using head capsule and frass measurements. In the field the larvae progressed up to the fifth instar before pupating, and the increase in head capsule width followed Dyars Rule with a rate of increase (R.I.) value of 1.74. The same number of instars was determined for the larvae reared with cork oak (R.I. = 1.73) and holm oak (R.I. = 1.70) leaves. The caterpillars reared with downy oak foliage completed larval development in five, six and seven instars and the R.I. values obtained were 1.60, 1.52 and 1.44 respectively. A lower mortality was recorded for the larvae reared on holm oak. Growth and feeding indices were determined for the larvae from the third up to the last instar. The highest leaf consumption was detected for the fifth instar larvae reared on holm oak. For the caterpillars which completed five instars before pupating, the relative consumption rate (RCR) decreased from the third up to the fifth instar: from 4.8 to 1.7 (cork oak), from 7.4 to 3.3 (holm oak) and from 14.3 to 2.1 (downy oak). The relative growth rate (RGR) was highest during the fourth stadium (0.24, 0.27 and 0.33 for larvae reared with cork oak, holm oak and downy oak leaves respectively) and decreased in the fifth instar (0.09, 0.14 and 0.14 for larvae reared with cork oak, holm oak and downy oak leaves respectively), probably because of greater expense of energy due to the approach of maturity. Feeding and growth indices could be useful to define a defoliation prediction model.     

The influence of greenhouse chrysanthemum on the interaction between the beet armyworm, Spodoptera exigua, and the baculovirus SeMNPV: parameter quantification for a process-based simulation model

During the building of a process-based simulation model for the epidemiology of the multicapsid nucleopolyhedrovirus of S. exigua (SeMNPV) in populations of Spodoptera exigua (Hübner) in greenhouse chrysanthemum, it was found that the effect of host plants had been under-rated. 'Missing links' included (i) the 'natural' background mortality of larvae of S. exigua in practical cropping conditions; (ii) the developmental rate of larvae of S. exigua on plant substrate in a glasshouse as compared to artificial medium in the laboratory; (iii) the validity of the results of dose-mortality and time-mortality bioassays conducted on artificial medium as compared to natural plant substrate; (iv) the distribution of inoculum released from deceased caterpillars over chrysanthemum leaves; and (v) the leaf visit rate of healthy caterpillars (as it affects horizontal transmission). Experiments were carried out to quantify these processes. Developmental rates of S. exigua larvae on greenhouse chrysanthemum were 36% lower than on an artificial diet. The fraction survival during the first, second, third and fourth instar S. exigua larvae in greenhouse chrysanthemum was 0.60, 0.80, 0.88 and 0.95, respectively. Forty percent of the first instar larvae reached the fifth larval stage. Second instar S. exigua larvae reared on chrysanthemum were significantly more susceptible to SeMNPV than larvae reared on an artificial diet. The food source had no effect on the time to kill S. exigua larvae. Cadavers of second, third and fourth instar S. exigua larvae contaminated on average 1.4, 2.5 and 3.3 chrysanthemum leaves. Second to fourth instar S. exigua larvae visited 2–3 leaves per day and spent 15–55% of the time on the underside of leaves. The above information is of critical importance for a trustworthy simulation of the epidemiology of SeMNPV in chrysanthemum.     

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.     

Group size effects on survivorship and adult development in the gregarious larvae of Euselasia chrysippe (Lepidoptera, Riodinidae)

Caterpillars living in aggregations may derive several benefits that outweigh the costs, including better survivorship and improved growth rates. I tested whether larval group size had an effect on these two vital rates in Euselasia chrysippe. These caterpillars feed gregariously during all instars and move in processionary form over the host plant and even pupate together. There was a positive relationship between group size and larval survivorship in the field, although genetic variability was not taken into account in this experiment. Under laboratory conditions, there was also a positive relationship between group size, and larval growth rate and adult weight. This supports the hypothesis that aggregations facilitate feeding and larval growth. Single sixth instar larvae in the laboratory also had a lower survivorship than larvae in groups. These results provide further evidence of the benefits of group living for gregarious caterpillars.     

The lethal plant defense paradox remains: inducible host-plant aristolochic acids and the growth and defense of the pipevine swallowtail

Toxic plants with sequestering specialists are presented with a problem because plant derived toxins protect herbivores against natural enemies. It has been suggested that early induction of toxins and later relaxation of these defenses may help the plant resolve this problem because neonate caterpillars incur the physiological cost of dealing with toxins in early life, but are denied toxins when they are able to sequester them efficiently. In California, the pipevine swallowtail, Battus philenor L. (Lepidoptera: Papilionidae), feed exclusively on Aristolochia californica Torrey (Aristolochiaceae), an endemic vine that contains toxic alkaloids called aristolochic acids that caterpillars sequester to provide chemical defense in immature and adult stages. In a field experiment, the concentration of aristolochic acids doubled in the plant following leaf damage and returned to constitutive levels after six days. Neonate pipevine swallowtail caterpillars showed no aversion to high levels of aristolochic acid in a preference test. Caterpillars reared on leaves with supplemented aristolochic acid showed no physiological cost or increased mortality compared to caterpillars reared on un-supplemented leaves. Searching efficiency and capture rate of lacewing larvae (Chrysoperla), a common predator of first instar caterpillars, was compromised significantly after feeding on caterpillars reared on leaves with supplemented concentrations of aristolochic acid compared to caterpillars feeding on control plants. Additionally, mortality of lacewings increased when they were provided with a diet of B. philenor caterpillars reared on supplemented leaves compared to caterpillars reared on control leaves. Thus, the induction of aristolochic acids in the plant following leaf damage does not resolve the problem confronted by the plant and may confer benefits to this sequestering specialist.     

Intestinal bacteria affect growth of Bacillus thuringiensis in larvae of the oriental tea tortrix, Homona magnanima diakonoff (Lepidoptera: tortricidae)

Spores and parasporal crystals of a Bacillus thuringiensis serovar aizawai were fed to fifth instar larvae of the oriental tea tortrix, Homona magnanima, that had been reared aseptically or that had been reared normally. Viable cell numbers of B. thuringiensis and other bacteria in H. magnanima larvae were estimated by homogenization of samples and dilution plating on peptone-polymyxin agar medium for B. thuringiensis cells and on nutrient agar medium for the other bacterial cells. B. thuringiensis did not grow in the larval cadavers of normally reared H. magnanima while bacteria other than B. thuringiensis grew rapidly. In contrast, B. thuringiensis within the larval cadavers of aseptically reared H. magnanima grew and increased 20 times. The bacteria other than B. thuringiensis from the sample homogenates of normally reared larvae that were fed on B. thuringiensis-treated diets had the same characteristics as the bacteria isolated from the guts of healthy H. magnanima larvae, which were putatively identified as Streptococcus spp. and Staphylococcus spp., typical intestinal bacteria of insects. The results strongly suggest that intestinal bacteria influence the growth of B. thuringiensis in the larvae.     

Diet factors responsible for the change of the glucose oxidase activity in labial salivary glands of Helicoverpa armigera

We investigated the change of the glucose oxidase (GOX) activity in labial salivary glands of Helicoverpa armigera larvae fed with the artificial diet or host plant tobacco and the major factors responsible for such a change. Throughout larval development, the labial salivary GOX activities in caterpillars reared on the artificial diet were remarkably higher than those fed with the plant. After fifth-instar plant-fed caterpillars were transferred to the artificial diet, their labial salivary GOX activity increased quickly, which was closely correlated with the time spent feeding on the artificial diet. The total sugar content of the artificial diet was 68 times higher than that of the tobacco leaves. We hypothesized that sugars and secondary metabolites are the possible causes of induction of GOX activity. When fifth-instar caterpillars were fed with tobacco leaves coated with glucose or sucrose, their labial salivary GOX activity was significantly higher than those fed with leaves without sugar coating. Following native PAGE, 1 single band of the labial salivary GOX was observed in all the caterpillars fed with different diets, implying that only the activity of the isoenzyme was changed in response to different diets. Furthermore, the labial salivary GOX activity was determined after caterpillars were fed with artificial diets containing chlorogenic acid, rutin, and quercetin. The results showed that all these phenolic compounds had no effect on the GOX activity. We conclude that sugar in diets was a major factor influencing the labial salivary GOX activity of the larvae. Arch. Insect Biochem. Physiol. 2008.     

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.     

Effects of larval diet on myrmecophilous qualities of Polyommatus icarus caterpillars (Lepidoptera: Lycaenidae)

Summary Fourth instar larvae of Polyommatus icarus maintain myrmecophilous associations with ants. In laboratory experiments, the attractiveness of larvae reared on foliage of the tree Robinia pseudacacia was significantly reduced compared with caterpillars fed with herbaceous Fabaceae. The ability to secrete carbohydraterich liquids from the dorsal nectary organ was also strongly reduced, while the function of the tentacle organs remained unaffected. The reductions in attractiveness and secretion abilities are caused by the food quality itself and not by secondary effects such as reduced larval size. The ecological significance of the results are discussed with respect to facultatively and obligately myrmecophilous lycacnids.     

Biochemical ecology of the forest tent caterpillar: responses to dietary protein and phenolic glycosides

Summary Interactions between quaking aspen (Populus tremuloides) and the forest tent caterpillar (Malacosoma disstria) are likely to be influenced by leaf protein and phenolic glycoside levels, and insect detoxication activity. We investigated the direct and interactive effects of dietary protein and phenolic glycosides on larval performance and midgut enzyme activity of forest tent caterpillars. We conducted bioassays with six artificial diets, using both first and fourth stadium larvae. Four of the diets comprised a 2×2 factorial design-two levels of protein, each with and without phenolic glycosides. Additionally, we assayed high protein diets containing S,S,S-tributylphosphorotrithioate (DEF, an esterase inhibitor) and DEF plus phenolic glycosides. Enzyme solutions were prepared from midguts of sixth instars and assayed for -glucosidase, esterase and glutathione transferase activities. First instar mortality and development times were higher for larvae on diets low in protein or containing phenolic glycosides. Effects of phenolic glycosides were especially pronounced at low protein levels and when administered with DEF. Fourth instar development times were prolonged, and growth rates reduced, in response to consumption of low protein diets. Effects of phenolic glycosides on growth were less pronounced, although the effect for larvae on the low protein diet was nearly significant. Activity of each of the enzyme systems was reduced in larvae reared on low protein diets, and esterase activity was induced in larvae fed phenolic glycosides. Our results suggest that larval performance may be strongly affected by levels of protein and phenolic glycosides commonly occurring in aspen foliage, and that these factors may play a role in differential defoliation of aspen by forest tent caterpillars.     

Digestive duet: midgut digestive proteinases of Manduca sexta ingesting Nicotiana attenuata with manipulated trypsin proteinase inhibitor expression

BACKGROUND: The defensive effect of endogenous trypsin proteinase inhibitors (NaTPIs) on the herbivore Manduca sexta was demonstrated by genetically altering NaTPI production in M. sexta's host plant, Nicotiana attenuata. To understand how this defense works, we studied the effects of NaTPI on M. sexta gut proteinase activity levels in different larval instars of caterpillars feeding freely on untransformed and transformed plants. METHODOLOGY/ PRINCIPAL FINDINGS: Second and third instars larvae that fed on NaTPI-producing (WT) genotypes were lighter and had less gut proteinase activity compared to those that fed on genotypes with either little or no NaTPI activity. Unexpectedly, NaTPI activity in vitro assays not only inhibited the trypsin sensitive fraction of gut proteinase activity but also halved the NaTPI-insensitive fraction in third-instar larvae. Unable to degrade NaTPI, larvae apparently lacked the means to adapt to NaTPI in their diet. However, caterpillars recovered at least part of their gut proteinase activity when they were transferred from NaTPI-producing host plants to NaTPI-free host plants. In addition extracts of basal leaves inhibited more gut proteinase activity than did extracts of middle stem leaves with the same protein content. CONCLUSIONS/ SIGNIFICANCE: Although larvae can minimize the effects of high NaTPI levels by feeding on leaves with high protein and low NaTPI activity, the host plant's endogenous NaTPIs remain an effective defense against M. sexta, inhibiting gut proteinase and affecting larval performance.     

Effects on oviposition behaviour and larval development of Spodoptera littoralis by herbivore-induced changes in cotton plants

Herbivore feeding induces chemical defence responses in plants. In this study we investigate how herbivore-induced changes in cotton plants, Gossypium hirsutum L., influence the oviposition behaviour and larval development of Spodoptera littoralis Boisd. (Lepidoptera: Noctuidae). In two-choice experiments female moths preferred to oviposit on small plants (3–4 leaves) that had been fed on by 3rd to 4th instar larvae (72%) over non-damaged control plants. However, when using larger plants (8 to 10 true leaves) the preference was reversed, with only 31% of the eggs deposited on the induced plants. The same trend was found with plants that had been given a similar level of damage by 6th instar larvae. However, the difference between the treatments was in both cases smaller with only 60% of the eggs deposited on the preferred plant treatment. If cotton plants subjected to artificial damage were compared with undamaged plants, none of the treatments were preferred for oviposition. No significant difference was found in larval weight, pupal weight, survival, or development time, between larvae reared their entire development on leaves from induced or from non-induced plants.     

Influence of food and larval age on the defensive chemistry of Saturnia pyri

Scolus secretions and hemolymph of caterpillars of Saturnia pyri fed with two different foodplants (Crataegis monogyna, Prunus spinosa) were chemically analyzed and their chemical similarities determined. The secondary-compound patterns obtained for the two body fluids showed no significant differences when compared between the two groups of alternatively fed last-instar larvae. Thus, the composition of these fluids of full-grown caterpillars is not influenced by the larval diet. However, younger larvae on P. spinosa revealed a diversity of compounds differing significantly from that of larger caterpillars fed with either C. monogyna (both body fluids) or P. spinosa (hemolymph only). This indicates that, on the one hand, the hemolymph composition is adapted to the changing physiological requirements of the given instars whereas, on the other hand, the defensive mixtures remain unaltered in the late larval instars due to a constant spectrum of potential enemies.     

Crocidolomia pavonana larval foraging: behavior and feeding site preferences on cabbage, Brassica oleracea

Experiments were conducted to determine feeding site preferences of Crocidolomia pavonana (Fabricius) (Lepidoptera: Crambidae) larvae within cabbage plants, Brassica oleracea L. var. capitata cv. Warrior (Brassicaceae), and to determine whether induced plant responses to herbivory affect the behavior of larvae. In the first experiment, intra-plant damage and larval distribution were recorded to account for the spreading pattern of damage and larval feeding behavior on a plant; larvae initially fed on the base of leaves and moved progressively to the bud, leaf tips were avoided. In the second experiment, larval performance (the duration of the first instar, survival to the second instar, and weight of second instars) was assessed when larvae fed on the bud, the base, and the tip of the youngest fully expanded leaf on a plant. Crocidolomia pavonana larvae performed best when they fed on bud leaf tissue and most poorly when they fed on leaf tissue at the base of leaves. In the third experiment, expression of induced resistance was tested on each of the three plant parts using a first-instar bioassay. Negative impacts on larval growth and development were not detected when larvae fed on the bud or base tissue when plants were damaged prior to the assay. However, negative effects were detected in larvae feeding on tip leaf tissue when the base of the leaf was damaged prior to the assay or if the bud tissue was damaged simultaneously with the assay. The findings indicate that resource heterogeneity for C. pavonana within-cabbage plants is determined by both the initial quality of food at a location and by subsequent induced changes as a result of larval feeding; both contribute to the feeding pattern observed in these gregarious larvae.     

Host plant exploitation within a population of a generalist herbivore, Choristoneura rosaceana

Feeding performance of larvae of the obliquebanded leafroller, Choristoneura rosaceana (Harris), was estimated on apple and three alternative hosts in the Okanagan Valley. The obliquebanded leafroller is native to the study area and apple was introduced in the valley at the beginning of the century. Components of insect fitness were measured to help formulate predictions about the evolution of host preference in this generalist herbivore. In the summer, females attained a higher pupal weight on apple than on trembling aspen, wild rose and snowberry. Males performed best on apple and trembling aspen. In the field, females arising from caterpillars fed on apple leaves had a 26% greater mean fecundity than if fed on alternative hosts. The diet also affected development time; females initiated pupation later on trembling aspen than on the other hosts. Finally, no differences were found in larval survival nor in the proportion of larvae diapausing when fed on different hosts. These components of fitness indicate apple as the most suitable host in the summer.In the fall, diet influenced larval propensity to initiate diapause in the second or third instar, the time elapsed from hatching to diapause initiation and the weight of the instars initiating diapause. However, no difference in overwintering survival among larvae fed on different hosts was detected. The rank order of water and nitrogen contents of leaves of the host plants did not match the rank order of larval feeding performance in either season. A high suitability of apple and its abundance in the Okanagan valley are factors that would favor the evolution of an increased utilization of this host by the obliquebanded leafroller.     

Developmental variation in the forest tent caterpillar: life history consequences of a threshold size for pupation

In insects, size and age at adult emergence depend on larval growth that occurs in discrete steps or instars. Understanding the mechanisms controlling stepwise larval growth and the onset of metamorphosis is essential to the study of insect life history. We examined the patterns of growth of forest tent caterpillars Malacosoma disstria to quantify variation in the number of instars that larvae undergo before pupation, to identify the mechanisms underlying variation in larval development, and to evaluate the life history consequences of this variation. All caterpillars were reared under the same conditions; at each molt, the date, the head capsule width and the mass of the freshly molted insect were recorded. Logistic regression analysis showed that a threshold size (measured either as mass or head capsule width) must be reached at the beginning of a stadium for pupation to occur at the next molt. This threshold size was higher for females than for males, and as a result, females attained a higher pupal mass than males. To achieve this larger size, females often required more instars than males, despite a higher growth ratio (size increase within an instar). Within each sex, slow growing individuals exhibited more larval instars and longer larval development time, but attained the same pupal mass as faster growing individuals. The combination of a threshold size for pupation, discrete growth steps and variation in the number of these steps can thus complicate relationships between growth rate, pupal mass and larval development time. In our study, growth ratio and number of instars were correlated with development time but not with pupal mass, and no relationship was observed between development time and pupal mass. These findings imply that, in species with variable instar number, one cannot extrapolate overall larval growth from growth during a single instar. Given the constraints of discrete larval growth, variation in instar number provides greater flexibility for insects to compensate for poor growing conditions. In this case, inferior larval growth conditions don't necessarily lead to smaller adult size.