Nutritional indices in the gypsy moth (Lymantria dispar (L.)) under field conditions and host switching situations

A large proportion of gypsy moths (Lymantria dispar (L.)) are likely to experience multiple species diets in the field due to natural wandering and host switching which occurs with these insects. Nutritional indices in fourth and fifth instar gypsy moth larvae were studied in the field for insects that were switched to a second host species when they were fourth instars. The tree species used as hosts were northern pin oak (Quercus ellipsoidalis E. J. Hill), white oak (Q. alba L.), big-tooth aspen (Populus grandidentata Michx.), and trembling aspen (P. tremuloides Michx.). Conclusions of this study include: 1) Insects which fed before the host switch on northern pin oak performed better after the host switch than did insects with other types of early dietary experience. While the northern pin oak-started insects had very low relative food consumption rates on their second host species immediately after the switch, one instar later they had the highest ranked consumption rates. During both instars they had the second highest efficiencies of converting ingested and digested food to body mass. High food consumption rates and relatively high efficiency of food conversion helped these insects to obtain the highest ranked mean relative growth rates in the fifth instar compared to the relative growth rates obtained by insects from any of the other first host species. 2) Among the four host species examined, a second host of trembling aspen was most advantageous for the insects. Feeding on this species after the switch led to higher larval weights and higher relative growth rates for insects than did any of the other second host species. The insects on trembling aspen attained excellent growth despite only mediocre to low food conversion efficiencies. The low efficiencies were offset by high relative food consumption rates. 3) Low food consumption rates often tend to be paired with high efficiency of conversion and vice versa. 4) There is no discernable tendency for the first plant species eaten to cause long-term inductions which affect the ability of gypsy moths to utilize subsequent host plants. Insects did not tend to consume more, grow faster, or be more efficient if their second host plant was either the same as their rearing plant or congeneric to it. Methods are delineated which allow values of nutritional indices to be obtained for insects on intact host plants under field conditions. These methods are useful for the purpose of answering questions about the relative effects that different diet treatments have on insect response.     

Increased mortality of gypsy moth Lymantria dispar (L.) (Lepidoptera: Lymantriidae) exposed to gypsy moth nuclear polyhedrosis virus in combination with the phenolic gycoside salicin

Second instar gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae), larvae suffered significantly greater mortality from aerially applied gypsy moth nuclear polyhedrosis virus (Gypchek) when the virus was consumed on quaking aspen, Populus tremuloides Michx., versus red oak, Quercus spp. L., foliage. Laboratory assays in which various doses of Gypchek and salicin (a phenolic glycoside present in aspen foliage) were tested in combination demonstrated that salicin significantly increased total larval mortality and lowered the LD50 estimates (dose of Gypchek that resulted in 50% population mortality) for the virus, although not significantly. While salicin did not impact larval survival in the absence of Gypcek, it did act to significantly deter feeding when it was present in high concentrations (up to 5.0%) within the treatment formulations. The enhanced activity of Gypchek in the presence of salicin is similar to prior reports of enhanced activity of the bacterial pathogen Bacillus thuringiensis when consumed concurrently with phenolic glycosides commonly present in aspen foliage. The enhancement of viral activity is in contrast to the inhibitory effects on the virus reported for another common group of phenolic compounds, tannins.     

Adaptation of the gypsy moth to an unsuitable host plant

The pattern of adaptation with regard to life history traits and traits thought to be important in feeding habits of caterpillars in two populations of the gypsy moth (Lymantria dispar L.; Lepidoptera: Lymantriidae) originating from the locust tree (Robinia pseudoacacia; Fabaceae) and oak (Quercus petrea; Fagaceae) forests were investigated in the laboratory. The Robinia population has experienced unsuitable locust tree leaves as an exclusive food resource for more than 40 years. Since Quercus species are the principal host plants of the gypsy moth, the specific objectives of this study have been to measure the extent of differentiation between ancestral and derived populations in several life history traits (egg-to-adult viability, duration of larval and pupal stages, and pupal weight) and nutritional indices – relative growth rate (RGR), relative consumption rate (RCR), assimilation efficiency (AD), gross growth efficiency (ECI), and net growth efficiency (ECD). Significant differences between the Quercus and Robinia populations were detected in pupal duration, RGR, RCR, and AD. The presence of a significant population × host interaction in traits such as preadult viability, duration of pupal stage, RGR, and ECI suggests that adaptation of the gypsy moth to the unsuitable host might be ongoing. Using a full-sib design, we screened for genetic variation in life history traits within both populations, and examined the genetic correlations of performance across oak and locust leaves within both populations. The genetic variances for analyzed life history traits were lower under conditions that are commonly encountered in nature. Our data show that positive cross-host genetic correlations preponderate within both populations.     

Effect of dietary nitrogen on gypsy moth larval nutritional indices, development and lipid content

Nutritional indices, development rates, percent dry weights and total lipids were determined in gypsy moth larvae (Lymantria dispar L.) reared on a high wheat germ (HWG) diet or diets prepared from lyophilized, ball-milled oak or pine foliage as the only source of dietary nitrogen (N). With regard to both total and proteinaceous N content, HWG diet>oak diet>pine diet. All nutritional indices measured were significantly lower in second instars fed pine diet vs. oak diet. Protein supplementation of pine diet with either casein or ovalbumin to bring total N up to the level present in oak diet resulted in small increased in approximate digestibility (AD) and effciency of conversion of ingested food (ECI), but relative growth rate (RGR) remained unaffected. The low RGR of larvae fed pine diet (unsupplemented or protein supplemented), as compared to those fed HWG or oak diet, was accompanied by significantly lower larval percent dry weight and percent total lipid. In contrast, RGR, larval percent dry weight and total lipid values were comparable in second instars fed HWG or oak diet. Insects reared from the first through the final instar on oak diet exhibited lower pupal weights compared to those reared on HWG. Casein addition to oak diet generally resulted in even more extended larval development times and further reduced pupal weights, but wheat germ addition to oak diet did not alter development rates and caused an increase in pupal weights.     

Comparison of allozyme,RFLP, and RAPD markers for revealing genetic variation within and between trembling aspen and bigtooth aspen

We examined genetic variation in allozyme loci, nuclear DNA restriction fragment length polymorphisms (RFLPs), and random amplified polymorphic DNAs (RAPDs) in 130 trembling aspen (Populus tremuloides) and 105 bigtooth aspen (P. grandidentata) trees. In trembling aspen 10 out of 13 allozyme loci assayed (77%) were polymorphic (P), with 2.8 alleles per locus (A) and an expected heterozygosity (H_e) of 0.25. In contrast, bigtooth aspen had a much lower allozyme genetic variability (P=29%; A=1.4; H_e=0.08). The two species could be distinguished by mutually exclusive alleles at Idh-1, and bigtooth aspen has what appears to be a duplicate 6PG locus not present in trembling aspen. We used 138 random aspen genomic probes to reveal RFLPs in HindIII digests of aspen DNA. The majority of the probes were from sequences of low copy number. RFLP results were consistent with those of the allozyme analyses, with trembling aspen displaying higher genetic variation than bigtooth aspen (P=71%, A=2.7, and H_e=0.25 for trembling aspen; P=65%, A=1.8, and H_e=0.13 for bigtooth aspen). The two species could be distinguished by RFLPs revealed by 21 probes (15% of total probes assayed). RAPD patterns in both species were studied using four arbitrary decamer primers that revealed a total of 61 different amplified DNA fragments in trembling aspen and 56 in bigtooth aspen. Assuming a Hardy-Weinberg equilibrium, estimates of P=100%, A=2, and H_e=0.30 in trembling aspen and P=88%, A=1.9, and H_e=0.31 in bigtooth aspen were obtained from the RAPD data. Five amplified DNA fragments were species diagnostic. All individuals within both species, except for 2 that likely belong to the same clone, could be distinguished by comparing their RAPD patterns. These results indicate that (1) RFLPs and allozymes reveal comparable patterns of genetic variation in the two species, (2) trembling aspen is more genetically variable than bigtooth aspen at both the allozyme and DNA levels, (3) one can generate more polymorphic and species-specific loci with DNA markers than with allozymes in aspen, and (4) RAPDs provide a very powerful tool for fingerprinting aspen individuals.     

Behavioural and physiological plasticity of gypsy moth larvae to host plant switching

Larvae of the gypsy moth, Lymantria dispar L. (Lepidoptera: Lymantriidae), a generalist species, frequently encounter spatial and temporal variations in diet quality. Such variation favoured the evolution of high behavioural and physiological plasticity which, depending on forest stand composition, enables more or less successful exploitation of the environment. Even in mixed oak stands, a suitable habitat, interspecific and intraspecific host quality variation may provoke significant variation in gypsy moth performance and, consequently, defoliation severity. To elucidate the insufficiently explored relationship between gypsy moth and oaks (Fagaceae), we carried out reciprocal switches between Turkey oaks (Quercus cerris L.) and less nutritious Hungarian oaks (Quercus frainetto Ten.) (TH and HT groups), under controlled laboratory conditions, and compared larval performance between the switched larvae and larvae continuously fed on either Turkey oak (TT) or Hungarian oak (HH). We found that larval traits were most strongly affected by among‐tree variation in oak quality and identity of the host consumed during the fourth instar. Switching from Turkey to Hungarian oak (TH) led to a longer period of feeding, decrease of mass gain, growth, and consumption rate, lower efficiency of food use and nutrient conversion, and increase of protease and amylase activities. Larvae exposed to the reverse switch (HT) attained values of these traits characteristic for TT larvae. It appeared that the lower growth in the TH group than in the TT group was caused by both behavioural (consumption, pre‐ingestive) and metabolic (post‐digestive) effects from consuming oaks. Multivariate analyses of growth, consumption, and efficiency of food use revealed that early diet experience influenced the sensitivity of the most examined traits to less suitable Hungarian oaks, suggesting the development of behavioural and physiological adjustments. Our results indicate that lower risks of defoliation by gypsy moth might be expected in mixed stands with a higher proportion of Hungarian oak.     

Effects of elevated CO2 on feeding preference and performance of the gypsy moth (Lymantria dispar) larvae

Multiple‐choice assays were conducted in the laboratory to examine the effect of CO₂‐induced changes in leaf quality on growth, nutritional indices and preferences of the gypsy moth (Lymantria dispar) larvae. The tested tree species, one‐year‐old aspen (Populus pseudo‐simonii Kitag.), two‐year‐old birch (Betula platyphylla) and three‐year‐old oak (Quercus mongolica Fisch.) were transplanted to open‐top chambers at ambient or elevated CO₂ (650 ppm) concentrations in May 2005. The present study was conducted in 2006. Leaves from the upper and lower crowns of each tree species were sampled and analysed, respectively. On both measurement dates (24 July 2006 and 16 August 2006), there were significant CO₂ effects on levels of condensed tannin, starch, nitrogen and C : N ratio. But only on the latter date, were the concentrations of total phenolics, soluble sugar, carbon and total non‐structural carbohydrates significantly affected by elevated CO₂. Leaf dry weight content and specific leaf weight were almost completely unaffected by CO₂ enrichment. Gypsy moth larvae exhibited a clear selectivity for tested leaf types (leaves from the upper and lower crowns of each tree species) even in their early instar stage, with the upper leaves of P. pseudo‐simonii being the most preferable and the lower leaves of Q. mongolica being the least preferred. The changes in leaf quality significantly reduced larval growth and altered partial indices of insect performance (e.g., relative growth rate and relative consumption rate). However, at least in this short‐term choice feeding assay (13 days), the CO₂‐induced changes in leaf quality had no significant effects on food preference behaviour of the gypsy moth larvae, neither within the limited range of host plants nor within the leaves at different canopy heights of the same tree species.     

Elevated atmospheric CO2: effects on phytochemistry, insect performance and insect-parasitoid interactions

This study was conducted to examine the effects of CO₂-mediated changes in tree chemistry on the performance of the gypsy moth ((Lymantria dispar L.) and the parasitold Cotesia melanoscela (Ratz.). We used carbon-nutrient balance theory to develop hypotheses regarding changes in tree chemistry and the performance of both insects under elevated CO₂. As predicted, levels of foliar nitrogen declined and concentrations of carbon-based compounds (e.g. starch and phenolics) increased under elevated CO₂. Gypsy moth performance (e.g. growth, development) was altered by CO₂-mediated changes in foliar chemistry, but the magnitude was small and varied across tree species. Larvae feeding on high CO₂ aspen exhibited the largest reduction in performance, relative to larvae feeding on birch, oak, or maple. Parasitism by C. melanoscela significantly prolonged gypsy moth development and reduced growth rates. Overall, the effect of parasitism on gypsy moth performance did not differ between CO₂ treatments. Altered gypsy moth performance on high CO₂ foliage in turn affected parasitoid performance, but the response was variable: parasitoid mortality increased and adult female size declined slightly under high CO₂, while development time and adult male size were unaffected. Our results suggest that CO₂-induced changes in plant chemistry were buffered to the extent that effects on third trophic level interactions were weak to non-existent for the system examined in this study.     

Effects of a kaolin-based particle film on oviposition and feeding of gypsy moth (Lep., Lymantriidae) and forest tent caterpillar (Lep., Lasiocampidae) in the laboratory

Abstract:  In the laboratory we investigated a kaolin-based crop protectant, Surround^® WP, that was mixed in a spray carrier (94% water, 2% methanol, 4% Triton X100) and applied as particle film against gypsy moth Lymantria dispar (L.) and forest tent caterpillar Malacosoma disstria Hubner. Paired-choice feeding tests showed that gypsy moth larvae were significantly (P < 0.05) less disposed to feed on red oak Quercus rubrus L. treated with high concentrations (60 and 120 g/l carrier) of kaolin than on untreated foliage. There was no significant difference (P > 0.05) between gypsy moth's consumption of kaolin-treated and untreated red oak when given a single food choice, but the adults were less inclined to oviposit on the kaolin-treated surfaces than on controls. However, no kaolin treatment completely deterred gypsy moth feeding or oviposition. Kaolin particle film did not influence forest tent caterpillar feeding and there were insufficient data to evaluate the effect of kaolin on the species' oviposition. The spray carrier did not increase the efficacy of the kaolin, but as an independent, albeit impractical treatment it constrained feeding by both insect species but did not affect gypsy moth oviposition. These initial laboratory findings suggest that applying kaolin-based particle film to forests would present significant challenges; but if these could be surmounted, the technique might assist in the management of gypsy moth but not likely in that of forest tent caterpillar.     

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.     

Facilitation between invasive herbivores: hemlock woolly adelgid increases gypsy moth preference for and performance on eastern hemlock

1. Interactions between invertebrate herbivores with different feeding modes are common on long-lived woody plants. In cases where one herbivore facilitates the success of another, the consequences for their shared host plant may be severe. Eastern hemlock (Tsuga canadensis), a canopy-dominant conifer native to the eastern U.S., is currently threatened with extirpation by the invasive stylet-feeding hemlock woolly adelgid (Adelges tsugae). The effect of adelgid on invasive hemlock-feeding folivores remains unknown. 2. This study evaluated the impact of feeding by hemlock woolly adelgid on gypsy moth (Lymantria dispar) larval preference for, and performance on, eastern hemlock. To assess preference, 245 field-grown hemlocks were surveyed for gypsy moth herbivory damage and laboratory paired-choice bioassays were conducted. To assess performance, gypsy moth larvae were reared to pupation on adelgid-infested or uninfested hemlock foliage, and pupal weight, proportional weight gain, and larval period were analysed. 3. Adelgid-infested hemlocks experienced more gypsy moth herbivory than did uninfested control trees, and laboratory tests confirmed that gypsy moth larvae preferentially feed on adelgid-infested hemlock foliage. Gypsy moth larvae reared to pupation on adelgid-infested foliage gained more weight than larvae reared on uninfested control foliage. 4. These results suggest that the synergistic effect of adelgid and gypsy moth poses an additional threat to eastern hemlock that may increase extirpation risk and ecological impact throughout most of its range.     

Hostplant,larval age,and feeding behavior influence midgut pH in the gypsy moth (Lymantria dispar)

Summary The midgut pH of late instar gypsy moth (Lymantria dispar L.) larvae is strongly alkaline, and varies with diet, larval stadium, and time since feeding. Midgut pH rises with time since feeding, and does so more quickly, reaching greater maximum values, on some diets than others. Leaf tissues of 23 tree species resist increases in alkalinity differentially; this trait and differing initial leaf pH may explain the impact of diet on gut pH. Third instar larvae may have gut conditions favorable for tannin-protein binding shortly after ingesting certain foods, but with time midgut alkalinity becomes great enough to dissociate tannin-protein complexes. Older instars rarely exhibit gut pHs low enough to permit tannin activity. Alkaline gut conditions may explain the gypsy moth's ability to feed on many tanniniferous plant species, especially in later instars. Consequences for pathogen effectiveness are discussed.     

Estimating the susceptibility of tree species to attack by the gypsy moth, Lymantria dispar

ABSTRACT.

• 1 Numbers of gypsy moth larvae feeding on each of 922 randomly sampled trees in a Quercus—Acer—Fraxinus forest in southwestern Quebec, Canada were counted in 1979 and in 1980 to quantify the larval feeding preferences as observed in the field for eighteen deciduous and one coniferous tree species at the northern range limit of the gypsy moth.
• 2 Both the diameter at breast height (dbh) and the estimated foliage biomass of the sampled trees were used to calculate the relative proportions of foliage represented by each of the nineteen tree species in the forest canopy. With these data on availability and utilization of the tree species by the gypsy moth larvae an Ivlev-type electivity index was used to quantify the larval feeding preferences. These preferences observed in the field define the susceptibility of a tree species to attack by the gypsy moth.
• 3 The feeding preferences calculated using estimated foliage biomass were comparable to the simpler calculation based on dbh (Spearman's rho = 0.79; P= 0.0001). The dbh-based feeding preferences remained almost unchanged in 1979 and 1980 (Spearman's rho = 0.83; P= 0.0001).
• 4 The composite 1979—80, dbh-based feeding preferences show Quercus rubra, Populus grandidentata, Ostrya virginiana, Amelanchier spp. and Acer saccharum were preferentially attacked by gypsy moth. Prunus serotina, Betula lutea, Acer rubrum, A. pensylvanicum, Fraxinus americana, Ulmus rubra, P. pensylvanicum and B. papyrifera were avoided. All nineteen tree species were, however, utilized to at least some degree by gypsy moth larvae.
• 5 These results quantitatively affirm and clarify earlier reports of gypsy moth feeding preferences in North America and Eurasia. The advantages and limitations of using an electivity index to estimate the susceptibility of different tree species to attack by folivores like the gypsy moth are discussed.

     

Initiation of maternal effects in Lymantria dispar: Genetic and ecological components of egg provisioning

Resources supplied by mothers to offspring through the egg are known to significantly influence offspring life history traits in the gypsy moth, Lymantria dispar. The purpose of this research was to determine the relative contribution of genetics (based on familial contribution) and the nutritional environment of the parents to the mean and variance of resources supplied to the eggs. Vitellogin, the dominant egg storage protein in the gypsy moth, was selected as the focus of the study. The amount of vitellogin in individual eggs from 48 mothers reared on one of four host species, quaking aspen, chestnut oak, red oak, or pitch pine was quantified with an immunoassay. Results of a nested analysis of variance show that both genetics and parental nutritional experience make significant contributions to egg vitellogin levels. When parents were reared on quaking aspen, vitellogin levels were highest and the expression of familial variation was greatest. This study shows that polyphagy can amplify phenotypic variance in reproductive traits through the interaction between genotype and nutritional environment. To the extent that egg resources influence offspring vigor, the fitness of offspring can include a time-lagged component which arises from the interaction between the parental genotype and the parental environment. The time-lagged expression of such a maternal trait is capable of influencing the rate and direction of character evolution and the stability of local population dynamics.     

Endogenous and induced monooxygenase activity in gypsy moth larvae feeding on natural and artificial diets

NADPH oxidase activity was measured in third to sixth instar gypsy moth larvae fed oak or pine foliage. Activity levels ranged from 400 to 1,900 pmol NADPH oxidized/min/mg microsomal protein, but enzyme activity was not correlated with host plant ingested. Similarly, activity levels in larvae fed diets containing inducers, such as the terpenoid α-pinene or pentamethylbenzene, ranged from 700 to 1,500 pmol NADPH oxidized/min/mg protein, levels that were comparable to those measured for larvae fed control diets. O-demethylase activity in older instar gypsy moth larvae fed pine averaged 109 pmol p-nitrophenol/min/mg protein, and activity levels in those fed diet containing α-pinene ranged from 22 to 55 pmol/min/mg protein. Although statistically significant, these induced O-demethylase levels are well below those observed for Heliothis zea larvae. Our findings indicate that monooxygenases play a minor, if any, role in the ability of later instar gypsy moth larvae to develop successfully on pine foliage.     

Variation in the ability of larvae of phytophagous insects to develop on evolutionarily unfamiliar plants: a study with gypsy moth Lymantria dispar and Eucalyptus

• 1 By examining variation in the abilities of polyphagous insects to develop on host plants with secondary metabolites that they have never encountered previously, we may be able to gain some insights into the nature of evolution of biochemical mechanisms to process plant secondary metabolites by phytophagous insects.
• 2 The present study aimed to examine variation in the ability of gypsy moth larvae Lymantria dispar (Lymantriidae) to complete development on different species of the plant genus Eucalyptus (Myrtaceae). Leaves of at least some Eucalyptus species contain formylated phloroglucinol derivatives. These are secondary metabolites that are evolutionarily unfamiliar to the gypsy moth.
• 3 Larvae of gypsy moth showed extremely variable responses in larval performance between Eucalyptus species, between individual trees within host plant species, between moth populations, and between individuals within moth populations.
• 4 Larval survivorship was in the range 0–94%, depending on the host. Failure of at least some larvae to complete development on some Eucalyptus species indicates that gypsy moth larvae have a limited ability to process secondary metabolites in eucalypt leaves.
• 5 At least some individuals, however, appear to already possess biochemical mechanisms that process the secondary metabolites in leaves of Eucalyptus species, and therefore the abilities of larvae to complete development on phylogenetically and chemically unfamiliar hosts are already present before the gypsy moth encounters these potential hosts.

     

Behavioural response to an unsuitable host plant in the gypsy moth (Lymantria dispar L.)

To assess local differentiation in host preference, a two-choice test was performed on first-instar gypsy moth larvae originating from an oak and locust-tree forest. More than 40 generations feeding on locust-tree leaves, rich in alkaloids, led to non-efficient discrimination of host leaves in larvae from a locust-tree forest. Possible causes of observed population differences are discussed in the present paper.     

Host‐associated divergence in the activity of digestive enzymes in two populations of the gypsy moth Lymantria dispar (Lepidoptera: Erebidae)

The gypsy moth is a generalist insect pest with an extremely wide host range. Adaptive responses of digestive enzymes are important for the successful utilization of plant hosts that differ in the contents and ratios of constituent nutrients and allelochemicals. In the present study, we examined the responses of α‐amylase, trypsin, and leucine aminopeptidase to two tree hosts (suitable oak, Quercus cerris, and unsuitable locust tree, Robinia pseudoacacia) in the fourth, fifth, and sixth instars of gypsy moth larvae originating from oak and locust tree forest populations (hereafter assigned as Quercus and Robinia populations, respectively). Gypsy moths from the Robinia forest had been adapting to this unsuitable host for more than 40 generations. To test for population‐level host plant specialization, we applied a two‐population × two‐host experimental design. We compared the levels, developmental patterns, and plasticities of the activities of enzymes. The locust tree diet increased enzyme activity in the fourth instar and reduced activity in advanced instars of the Quercus larvae in comparison to the oak diet. These larvae also exhibited opposite developmental trajectories on the two hosts, i.e. activity increased on the oak diet and decreased on the locust tree diet with the progress of instar. Larvae of the Robinia population were characterized by reduced plasticity of enzyme activity and its developmental trajectories. In addition, elevated trypsin activity in response to an unsuitable host was observed in all instar larvae of the Robinia population, which demonstrated that Robinia larvae had an improved digestive performance than did Quercus larvae.     

Leaf age effects of elevated CO2-grown white oak leaves on spring-feeding lepidopterans

Folivorous insect responses to elevated CO₂-grown tree species may be complicated by phytochemical changes as leaves age. For example, young expanding leaves in tree species may be less affected by enriched CO₂-alterations in leaf phytochemistry than older mature leaves due to shorter exposure times to elevated CO₂ atmospheres. This, in turn, could result in different effects on early vs. late instar larvae of herbivorous insects. To address this, seedlings of white oak (Quercus alba L.), grown in open-top chambers under ambient and elevated CO₂, were fed to two important early spring feeding herbivores; gypsy moth (Lymantria dispar L.), and forest tent caterpillar (Malacosoma disstria Hübner). Young, expanding leaves were presented to early instar larvae, and older fully expanded or mature leaves to late instar larvae. Young leaves had significantly lower leaf nitrogen content and significantly higher total nonstructural carbohydrate:nitrogen ratio as plant CO₂ concentration rose, while nonstructural carbohydrates and total carbon-based phenolics were unaffected by plant CO₂ treatment. These phytochemical changes contributed to a significant reduction in the growth rate of early instar gypsy moth larvae, while growth rates of forest tent caterpillar were unaffected. The differences in insect responses were attributed to an increase in the nitrogen utilization efficiency (NUE) of early instar forest tent caterpillar larvae feeding on elevated CO₂-grown leaves, while early instar gypsy moth larval NUE remained unchanged among the treatments. Later instar larvae of both insect species experienced larger reductions in foliage quality on elevated CO₂-grown leaves than earlier instars, as the carbohydrate:nitrogen ratio of leaves substantially increased. Despite this, neither insect species exhibited changes in growth or consumption rates between CO₂ treatments in the later instar. An increase in NUE was apparently responsible for offsetting reduced foliar nitrogen for the late instar larvae of both species.     

Suitability,digestibility and assimilation of various host plants of the gypsy moth Lymantria dispar L.

Summary The development and survival of gypsy moth (Lymantria dispar) larvae is strongly influenced by the host plant upon which they feed. The most rapid development and largest pupae were produced from grey birch fed larvae. Beech and maple-fed larvae produced the smallest pupae while maple-fed larvae exhibited prolonged development. White and red oak-fed larvae exhibited development and pupal weights intermediate between the above two groups. The approximate digestibility (AD) and efficiencies of conversion of food (ECD and ECI) were generally highest among grey birchfed individuals. The utilization of the relatively closely related oak species, as reflected in AD and ECD values, differed.Leaves were examined for 14 elements. The content of each element varied among host plant species and over time. For example, nitrogen levels were highest in grey-birch and dropped over time in all host plants.Lepidoptera: LymantriidaePaper No. 2277 Massachusetts Agricultural Experiment Station, University of Massachusetts at Amherst, MA U.S.A. This research supported (in part) from Experiment Station Project No. 437