Variation In Host Plant Quality Research Articles

Spatial habitat heterogeneity influences host-pathogen dynamics in a patchy population of Ranchman's tiger moth.

Host-pathogen dynamics are influenced by many factors that vary locally, but models of disease rarely consider dynamics across spatially heterogeneous environments. In addition, theory predicts that dispersal will influence host-pathogen dynamics of populations that are linked, although this has not been examined empirically in natural systems. We examined the spatial dynamics of a patchy population of tiger moths and its baculovirus pathogen, in which habitat type and weather influence dynamics. Theoretical models of host-baculovirus dynamics predict that such variation in dynamics between habitat types could be driven by a range of factors, of which we predict two are likely to be operating in this system: (1) differences in the environmental persistence of pathogens or (2) differences in host intrinsic rates of increase. We used time series models and monitored infection rates of hosts to characterize population and disease dynamics and distinguish between these possibilities. We also examined the role of host dispersal (connectivity) and weather as important contributors to dynamics, using time series models and experiments. We found that the population growth rate was higher, delayed density dependence was weaker, and long-period oscillations had lower amplitudes in high-quality habitat patches. The infection rate was higher on average in high-quality habitat, and this was likely to have been driven by higher mean population densities and no differences in pathogen persistence in different habitats (delayed density dependence). Time series modeling and experiments also showed an interactive effect of temperature and precipitation on moth population growth rates (likely caused by variation in host plant quality and quantity), and an effect of connectivity. Our results showed that spatial heterogeneity, connectivity, climate, and their interactions were important in driving host-baculovirus dynamics. In particular, our study found that connected patches and spatial heterogeneity generated differences in dynamics that only partially aligned with theoretical predictions.

Population-specific responses of an insect herbivore to variation in host-plant quality.

Anthropogenic climate change poses a substantial challenge to many organisms, to which they need to respond to avoid fitness reductions. Investigating responses to environmental change is particularly interesting in herbivores, as they are potentially affected by indirect effects mediated via variation in host‐plant quality. We here use the herbivorous insect Pieris napi to investigate geographic variation in the response to variation in food quality. We performed a common garden experiment using replicated populations from Germany and Italy, and manipulated host quality by growing host plants at different temperature and water regimes. We found that feeding on plants grown at a higher temperature generally diminished the performance of P. napi, evidenced by a prolonged development time and reduced larval growth rate, body mass, fat content, and phenoloxidase activity. Genotype by environment interactions (G × E) were present in several performance traits, indicating that Italian populations (1) respond more strongly to variation in host‐plant quality and (2) are more sensitive to poor food quality than German ones. This may reflect a cost of the rapid lifestyle found in Italian populations. Consequently, German populations may be more resilient against environmental perturbations and may perhaps even benefit from warmer temperatures, while Italian populations will likely suffer from the concomitantly reduced host‐plant quality. Our study thus exemplifies how investigating G × E may help to better understand the vulnerability of populations to climate change.

Aphids show interspecific and intraspecific variation in life history responses to host plant infection by the fungal pathogen Botrytis cinerea

AbstractThe life histories of insect herbivores are affected by variation in host plant quality, with poor quality typically being associated with reduced herbivore fecundity, size and longevity. Plant pathogens are ubiquitous in nature and can alter host plant quality as experienced by insect herbivores. We asked how host plant infection by the widespread and economically important fungal pathogen Botrytis cinerea affected the life history traits of two aphid species. We found that the life history traits of the black bean aphid Aphis fabae were negatively affected by being reared on infected host plants, showing reduced fecundity, population growth rate, size, off‐plant survival time and development rate. In contrast, we found that pea aphids Acyrthosiphon pisum benefitted from being reared on infected plants, and that the degree of benefit varied between pea aphid clonal lines. This work suggests that the ecological and economic consequences of plant pathogen infection on the dynamics of aphid pests could be difficult to predict.

Developmental stage-dependent response and preference for host plant quality in an insect herbivore

Larval-derived nutritional reserves are essential in shaping insects' adult fitness. Early larval instars of many Lepidopteran species are often sessile, and the conditions experienced by these larvae are often highly dependent on the mother's oviposition choice. Later larval stages are more mobile and therefore can choose their food whenever alternatives are available. We tested how feeding on a drought-exposed host plant impacts life history in an insect herbivore, and whether the observed responses depended on developmental stage. We used drought to alter host plant quality of the ribwort plantain, Plantago lanceolata, and assessed whether host plant preference of postdiapause larvae and adult females increased their own or their offspring's performance, respectively, in the Glanville fritillary butterfly, Melitaea cinxia. Larval response to drought-exposed host plants varied with developmental stage: early larval stages (prediapause) had decreased survival and body mass on drought-exposed plants, while later larval stages (postdiapause) developed faster, weighed more and had a higher growth rate on the drought-exposed plants. Postdiapause larvae also showed a preference for drought-exposed host plants, i.e. those that increased their performance, but only when fed on well-watered host plants. Adult females, on the other hand, showed an oviposition preference for well-watered plants, hence matching the performance of their prediapause but not their postdiapause offspring. Our results highlight how variation in environmental conditions generates stage-specific responses in insects. Individuals fine-tune their own or their offspring's diet by behavioural adjustments when variation in host plant quality is available.

A random survival forest illustrates the importance of natural enemies compared to host plant quality on leaf beetle survival rates

BackgroundWetlands are habitats where variation in soil moisture content and associated environmental conditions can strongly affect the survival of herbivorous insects by changing host plant quality and natural enemy densities. In this study, we combined natural enemy exclusion experiments with random survival forest analyses to study the importance of local variation in host plant quality and predation by natural enemies on the egg and larval survival of the leaf beetle Galerucella sagittariae along a soil moisture gradient.ResultsOur results showed that the exclusion of natural enemies substantially increased the survival probability of G. sagittariae eggs and larvae. Interestingly, the egg survival probability decreased with soil moisture content, while the larval survival probability instead increased with soil moisture content. For both the egg and larval survival, we found that host plant height, the number of eggs or larvae, and vegetation height explained more of the variation than the soil moisture gradient by itself. Moreover, host plant quality related variables, such as leaf nitrogen, carbon and phosphorus content did not influence the survival of G. sagittariae eggs and larvae.ConclusionOur results suggest that the soil moisture content is not an overarching factor that determines the interplay between factors related to host plant quality and factors relating to natural enemies on the survival of G. sagittariae in different microhabitats. Moreover, the natural enemy exclusion experiments and the random survival forest analysis suggest that natural enemies have a stronger indirect impact on the survival of G. sagittariae offspring than host plant quality.

Inclusion of host quality data improves predictions of herbivore phenology

AbstractUnderstanding the correspondence between ambient temperature and insect development is necessary to forecast insect phenology under novel environments. In the face of climate change, both conservation and pest control efforts require accurate phenological predictions. Here, we compare a suite of degree‐day models to assess their ability to predict the phenology of a common, oligophagous butterfly, the silver‐spotted skipper, Epargyreus clarus (Cramer) (Lepidoptera: Hesperiidae). To estimate model parameters, we used development time of eggs and larvae reared in the laboratory at six constant temperatures ranging from 8 to 38 °C and on two host plants of contrasting quality (kudzu and wisteria). We employed three approaches to determine the base temperature to calculate degree days: linear regression, modified reduced major axis regression, and application of a generic base temperature value of 10 °C, which is commonly used in the absence of laboratory data. To calculate the number of degree days required to complete a developmental stage, we used data from caterpillars feeding on high‐ and low‐quality hosts, both in the field and in the laboratory. To test model accuracy, we predicted development time of seven generations of larvae reared in the field on the same host plants across 3 years (2014–2016). To compare performance among models, we regressed predicted vs. observed development time, and found that r2 values were significantly larger when accounting for host plant quality. The accuracy of development time predictions varied across the season, with estimates of the first two generations being more accurate than estimates of the third generation, when ambient temperatures dropped outside the range in which development rate and temperature have a linear relationship. Overall, we show that accounting for variation in host plant quality when calculating development time in the field is more important than the choice of the base temperature for calculating degree days.

Temperature effects on a marine herbivore depend strongly on diet across multiple generations.

Increasing sea surface temperatures are predicted to alter marine plant-herbivore interactions and, thus, the structure and function of algal and seagrass communities. Given the fundamental role of host plant quality in determining herbivore fitness, predicting the effects of increased temperatures requires an understanding of how temperature may interact with diet quality. We used an herbivorous marine amphipod, Sunamphitoe parmerong, to test how temperature and diet interact to alter herbivore growth, feeding rates, survival, and fecundity in short- and long-term assays. In short-term thermal stress assays, S. parmerong was tolerant to the range of temperatures that it currently experiences in nature (20-26°C), with mortality at temperatures >27°C. In longer term experiments, two generations of S. parmerong were reared in nine combinations of temperature (ambient, +2, +4°C) and diet (two high- and one low-quality algal species) treatments. Temperature and diet interacted to determine total numbers of amphipods in the F1 generation and the potential F2 population size (sum of brooded eggs and newly hatched juveniles). The size and development rate of F1 individuals were affected by diet, but not temperature. Consumption rates per capita were highest at intermediate temperatures but could not explain the observed differences in survival. Our results show that predicting the effects of increasing temperature on marine herbivores will be complicated by variation in host plant quality, and that climate-driven changes to plant availability will affect herbivore performance, and thus the strength of plant-herbivore interactions.

Acquiring nutrients from tree leaves: effects of leaf maturity and development type on a generalist caterpillar.

The rapid growth and prolific reproduction of many insect herbivores depend on the efficiencies and rates with which they acquire nutrients from their host plants. However, little is known about how nutrient assimilation efficiencies are affected by leaf maturation or how they vary between plant species. Recent work showed that leaf maturation can greatly decrease the protein assimilation efficiency (PAE) of Lymantria dispar caterpillars on some tree species, but not on species in the willow family (Salicaceae). One trait of many species in the Salicaceae that potentially affects PAE is the continuous (or "indeterminate") development of leaves throughout the growing season. To improve our understanding of the temporal and developmental patterns of nutrient availability for tree-feeding insects, this study tested two hypotheses: nutrients (protein and carbohydrate) are more efficiently assimilated from immature than mature leaves, and, following leaf maturation, nutrients are more efficiently assimilated from indeterminate than determinate tree species. The nutritional physiology and growth of a generalist caterpillar (L. dispar) were measured on five determinate and five indeterminate tree species while their leaves were immature and again after they were mature. In support of the first hypothesis, caterpillars that fed on immature leaves had significantly higher PAE and carbohydrate assimilation efficiency (CAE), as well as higher protein assimilation rates and growth rates, than larvae that fed on mature leaves. Contrary to the second hypothesis, caterpillars that fed on mature indeterminate tree leaves did not have higher PAE than those that fed on mature determinate leaves, while CAE differed by only 3% between tree development types. Instead, "high-PAE" and "low-PAE" tree species were found across taxonomic and development categories. The results of this study emphasize the importance of physiological mechanisms, such as nutrient assimilation efficiency, to explain the large variation in host plant quality for insect herbivores.

Consumptive and nonconsumptive effect ratios depend on interaction between plant quality and hunting behavior of omnivorous predators

SummaryPredators not only consume prey but exert nonconsumptive effects in form of scaring, consequently disturbing feeding or reproduction. However, how alternative food sources and hunting mode interactively affect consumptive and nonconsumptive effects with implications for prey fitness have not been addressed, impending functional understanding of such tritrophic interactions. With a herbivorous beetle, two omnivorous predatory bugs (plant sap as alternative food, contrasting hunting modes), and four willow genotypes (contrasting suitability for beetle/omnivore), we investigated direct and indirect effects of plant quality on the beetles key reproductive traits (oviposition rate, clutch size). Using combinations of either or both omnivores on different plant genotypes, we calculated the contribution of consumptive (eggs predated) and nonconsumptive (fewer eggs laid) effect on beetle fitness, including a prey density‐independent measure (c:nc ratio). We found that larger clutches increase egg survival in presence of the omnivore not immediately consuming all eggs. However, rather than lowering mean, the beetles generally responded with a frequency shift toward smaller clutches. However, female beetles decreased mean and changed clutch size frequency with decreasing plant quality, therefore reducing intraspecific exploitative competition among larvae. More importantly, variation in host plant quality (to omnivore) led to nonconsumptive effects between one‐third and twice as strong as the consumptive effects. Increased egg consumption on plants less suitable to the omnivore may therefore be accompanied by less searching and disturbing the beetle, representing a “cost” to the indirect plant defense in the form of a lower nonconsumptive effect. Many predators are omnivores and altering c:nc ratios (with egg retention as the most direct link to prey fitness) via plant quality and hunting behavior should be fundamental to advance ecological theory and applications. Furthermore, exploring modulation of fitness traits by bottom‐up and top‐down effects will help to explain how and why species aggregate.

Feeding Experience Affects the Behavioral Response of Polyphagous Gypsy Moth Caterpillars to Herbivore-induced Poplar Volatiles.

Plant volatiles influence host selection of herbivorous insects. Since volatiles often vary in space and time, herbivores (especially polyphagous ones) may be able to use these compounds as cues to track variation in host plant quality based on their innate abilities and previous experience. We investigated the behavioral response of naïve (fed on artificial diet) and experienced (fed on poplar) gypsy moth (Lymantria dispar) caterpillars, a polyphagous species, towards constitutive and herbivore-induced black poplar (Populus nigra) volatiles at different stages of herbivore attack. In Y-tube olfactometer assays, both naïve and experienced caterpillars were attracted to constitutive volatiles and volatiles released after short-term herbivory (up to 6 hr). Naïve caterpillars also were attracted to volatiles released after longer-term herbivory (24–30 hr), but experienced caterpillars preferred the odor of undamaged foliage. A multivariate statistical analysis comparing the volatile emission of undamaged plants vs. plants after short and longer-term herbivory, suggested various compounds as being responsible for distinguishing between the odors of these plants. Ten compounds were selected for individual testing of caterpillar behavioral responses in a four-arm olfactometer. Naïve caterpillars spent more time in arms containing (Z)-3-hexenol and (Z)-3-hexenyl acetate than in solvent permeated arms, while avoiding benzyl cyanide and salicyl aldehyde. Experienced caterpillars avoided benzyl cyanide and preferred (Z)-3-hexenyl acetate and the homoterpene (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT) over solvent. Only responses to DMNT were significantly different when comparing experienced and naïve caterpillars. The results show that gypsy moth caterpillars display an innate behavioral response towards constitutive and herbivore-induced plant volatiles, but also that larval behavior is plastic and can be modulated by previous feeding experience.Electronic supplementary materialThe online version of this article (doi:10.1007/s10886-016-0698-7) contains supplementary material, which is available to authorized users.

Biochemical interactions for antibiosis mechanism of resistance to Chilo partellus (Swinhoe) in different maize types

Variation in performance and abundance of herbivorous insects is attributed to variation in host plant quality, being determined by nutritional composition, allelochemistry and specific anatomical features. Present studies were undertaken to gain an understanding on the role of different biochemical factors in plant defense to Chilo partellus in diverse maize types with variable morphological and favorable agronomic traits. All the white and yellow kernel maize genotypes tested had significantly lower deadhearts and more adverse effects on one or the other biological parameters of C. partellus, as compared to the resistant check, CML 334. The phenolic acids viz., ferulic acid and p-coumaric acid, and the nutritional biochemical factors such as protein, oil, chlorophyll-b, total chlorophyll and total carotenoid showed significant negative correlations, while total sugars and starch had positive correlation with pupal period of C. partellus. Oil content showed significant positive correlation with larval weight, and chlorophyll-a negative correlation with larval and pupal weights of C. partellus. Iron and zinc contents in maize seedlings showed significant positive correlation with pupal weight, whereas zinc content showed negative correlation with plant deadhearts, larval survival and adult emergence of C. partellus. Present studies show that different maize types expressed antibiosis mechanism of resistance to spotted stem borer, while the amount of phenolic acids and the nutritional biochemical constituents were highly variable in their seedlings, suggesting that the maize plant defense against C. partellus could be due to concentration of particular biochemical constituent and/or interaction with different biochemical compounds.

Cascading effects of host plant inbreeding on the larval growth, muscle molecular composition, and flight capacity of an adult herbivorous insect

Summary A primary function of adult winged insects is dispersal. Limiting larval dietary intake (partial starvation) has been shown to affect the flight muscle metabolism of adult moths reared on artificial diet, but a more ecologically relevant question is whether natural variation in host plant quality can lead to differences in the flight capacity of adult insects. Recent studies have shown that inbreeding compromises plant antiherbivore defences. We created inbred and outbred progeny from locally collected horsenettle (Solanum carolinense L.) and examined how host plant inbreeding affects the growth, development, and flight muscle physiology of tobacco hornworm (Manduca sexta L.), a specialist herbivore on Solanaceae. We tested the hypothesis that within population genetic variation in host plant quality, resulting from inbreeding, can create significant changes to the larval development and flight physiology of an adult insect. We found that Manduca larvae reared on inbred horsenettle plants grew faster and developed into larger pupae compared to larvae reared on outbred plants. Adult flight metabolic rate was greater in adults reared on inbred plants compared to outbred plants, and this elevation was independent of body mass when we excluded one plant family that produced small, low metabolic rate moths regardless of breeding regime. Differences in mass‐specific flight metabolism were associated with changes in alternative splicing of troponin t, a flight muscle protein that regulates muscle contraction. These results show that host plant inbreeding can create effects that cascade through larval and pupal development to affect dispersal‐related traits of the adult stage. Hence, plant inbreeding may also impact herbivore population dynamics, particularly their ability to spread away from, and possibly into, isolated patches of inbred plants creating increased herbivore pressure on these plant populations. More generally, our findings reveal that changes in population biology at one trophic level can affect the metabolic physiology and flight capacity of an animal at a higher trophic level.

Chemical and mechanical defenses vary among maternal lines and leaf ages in Verbascum thapsus L. (Scrophulariaceae) and reduce palatability to a generalist insect.

Intra-specific variation in host-plant quality affects herbivore foraging decisions and, in turn, herbivore foraging decisions mediate plant fitness. In particular, variation in defenses against herbivores, both among and within plants, shapes herbivore behavior. If variation in defenses is genetically based, it can respond to natural selection by herbivores. We quantified intra-specific variation in iridoid glycosides, trichome length, and leaf strength in common mullein (Verbascum thapsus L, Scrophulariaceae) among maternal lines within a population and among leaves within plants, and related this variation to feeding preferences of a generalist herbivore, Trichopulsia ni Hübner. We found significant variation in all three defenses among maternal lines, with T. ni preferring plants with lower investment in chemical, but not mechanical, defense. Within plants, old leaves had lower levels of all defenses than young leaves, and were strongly preferred by T. ni. Caterpillars also preferred leaves with trichomes removed to leaves with trichomes intact. Differences among maternal lines indicate that phenotypic variation in defenses likely has a genetic basis. Furthermore, these results reveal that the feeding behaviors of T. ni map onto variation in plant defense in a predictable way. This work highlights the importance of variation in host-plant quality in driving interactions between plants and their herbivores.

Effects of High-Quality Aromatic Rice Varieties on the Fitness of the Striped Stem Borer, Chilo suppressalis (Walker) in Central China

The striped stem borer (SSB), Chilo suppressalis (Walker) (Lepidoptera: Pyralidae), is a major pest of rice in China. Variation in host-plant quality may affect the body size of herbivorous insects, which in turn, can determine their life-history parameters such as survival, fecundity and fitness. In this study, we tested the effects of high-quality aromatic rice varieties on the fitness of SSB in China. Results showed that 1st instar larvae had higher penetrating rates and survival rates on the high-quality aromatic rice varieties (EX-1 and WX-988) than that on the non-aromatic rice varieties (EZ-5 and LYP-9). In addition, shorter developmental periods, greater female pupal weights and higher of other life-history parameters (hatching rate, pupation rate, eclosion rate and increase index) were found on the high-quality aromatic rice varieties, although only female pupal weight showed a significant difference between the two varieties. The highest dead heart rate was found on the aromatic rice variety of EX-1. These results indicate that SSB sustains a lower fitness cost when consuming the high-quality aromatic rice varieties (EX-1 and WX-988) than on the non-aromatic rice varieties (EZ-5 and LYP-9) in Central China.

Temporal variations in abiotic conditions exert little influence on diurnal activity of late-instar spruce budworm larvae on balsam fir

AbstractSpruce budworm, Choristoneurafumiferana (Clemens) (Lepidoptera: Tortricidae), larvae can cause extensive defoliation in balsam fir (Abies balsamea [Linnaeus] Miller) and exhibit high temporal and spatial variability in individual behaviour. We gathered field data to determine the influence of daily and hourly variation in abiotic conditions on daytime activity patterns of late-instar budworm larvae on balsam fir. In both years of our study, less than 10% of larvae were observed feeding during the daytime. Most larvae observed were either resting, spinning silk, or roaming, although the proportion of individuals engaged in each activity varied between years, with more larvae in the first year roaming (48%) and in the second year either resting (38%) or spinning silk (42%). Daily and hourly variation in abiotic conditions had a limited influence on activity patterns. Our results indicate that in nature, mid- to late-instar budworm larvae maintain fairly consistent activity patterns across a wide range of abiotic conditions. We suggest that site-specific conditions such as variation in host-plant quality and/or budworm population density may be more important than weather in determining the relative frequencies of different larval activities in space and time.

Impact of host plant quality on geometrid moth expansion on environmental and local population scales

The warming climate has enabled a rapid expansion of many forest pests. The adaptation potential of the invaders affects largely on how well the invasive species can spread to new areas and in what extent can they have an impact on the invaded ecosystem. To measure the adaptation potential of an invasive (winter moth) and a potentially invasive defoliating moth species (scarce umber moth), we examined life history parameters in two environments on a set of genetically diverse host trees and compared the traits with those of a resident moth species (autumnal moth). In addition, variations in life history parameters due to host genotype were calculated and compared. The bioassay was executed by rearing moth larvae on 11 half-sib families of the host tree in two large tree line gardens. The use of half-sib families allowed us to calculate the variances due to tree genotype and also to examine if the new arrivals can affect the selection pressure on the genetic population structure of the host. According to our results, the natural genetic variation in host plant quality and small environmental differences are not sufficiently effective to restrict the spread of the already established invasive species. The invading moth species may even be better at adapting to variation in food quality than the resident moth species. The overall effect of the natural variation in tree quality was similar for all three moth species. Therefore, the newcomers are likely to only inflict a quantitative rather than qualitative change in the selection pressure on the host.

Modelling the spatial population dynamics of the green oak leaf roller ( Tortrix viridana) using density dependent competitive interactions: Effects of herbivore mortality and varying host-plant quality

Cyclic population dynamics of forest insects with periods of more than two generations have been discussed in relation to a variety of extrinsic and intrinsic forces. In the present study, we employed the selection pressure of density dependent competitive interactions according to Witting's equations ( Witting, 2000) as driver for a discrete spatiotemporal model of the green oak leaf roller ( Tortrix viridana). The model was successfully parameterised to rebuild the cyclic population dynamics of an empirical data set of a 30-year leaf roller monitoring in Russia. Our analysis focussed on the role of herbivore mortality and host plant food quality, which have a significant effect on T. viridana population dynamics. An additional egg or larvae mortality lowers population density and can lead to selection pressures that favour individuals with higher growth rate. This increased population growth rate can not only compensate the additional mortality, but also can lead to higher average moth abundances in subsequent generations. Furthermore, we analysed the effect of inter- and intraspecific variation in host plant quality on herbivore population dynamics and the spatial distribution of abundance and defoliation patterns. We found significant effects of the qualitative composition of a trees neighbourhood on the herbivore population of the respective tree. Also, the patchy damage patterns observable in reality have been reproduced by the present model. The applicability of the model approach and the putative genetic processes underlying Witting's model are discussed.

A tree in the jaws of a moth – temporal variation in oak leaf quality and leaf‐chewer performance

The extent to which individual host trees maintain their relative quality over time may affect patterns of abundance, distribution and microevolution in herbivorous insects. In this study, we explore temporal consistency in the quality of oak Quercus robur foliage, using leaf‐chewing larvae of the moth Amphipyra pyramidea as our model herbivores. By utilising an artificial diet, we are able to isolate the impact of chemical contents from physical attributes, and thereby to ask to what extent purely chemical parameters create tree‐to‐tree differences in host quality, how consistent such differences are among trees between different parts of a single growth season, and to what extent individual moth larvae are able to compensate for chemical variation in food quality. We find that with physical traits controlled for, chemical traits suffice to create strong differences in larval growth rates between trees, and between larvae fed on young and mature foliage. Nevertheless, these initial differences are efficiently compensated for the fact that larvae with lower growth rates continue to grow for a longer time, and thereby end up at the same size as larvae with high growth rates. At the pupal stage, we could no longer detect differences between either larvae fed foliage from different trees or between larvae fed young versus mature foliage – despite notably little variation among individuals within each group. Such compensatory responses were also reflected in patterns of consistency. The intraclass correlation for larval weight was relatively high (ρ=0.45), but lower for development time (ρ=0.26), and non‐existent for pupal weight (ρ=0.00). These results suggest that in terms of pupal mass, A. pyramidea is able to compensate more or less completely for differences in resource quality, that patterns of consistency vary with the specific trait examined, and that the net effect of spatiotemporal variation in host plant quality on herbivore fitness should be dissected by experiments aimed at different life history traits. If slow growth comes with high mortality, spatiotemporal patterns in resource quality may have a major impact on herbivore fitness; if not, the patterns may be nullified by efficient compensatory mechanisms.

HOST PLANT QUALITY AND LOCAL ADAPTATION DETERMINE THE DISTRIBUTION OF A GALL-FORMING HERBIVORE

Herein we report results of transplant experiments that link variation in host plant quality to herbivore fitness at the local scale (among adjacent plants) with the process of local (demic) adaptation at the landscape scale to explain the observed distribution of the specialist gall former Belonocnema treatae (Hymenoptera: Cynipidae) within populations of its host plant, Quercus fusiformis. Field surveys show that leaf gall densities vary by orders of magnitude among adjacent trees and that high-gall-density trees are both rare (< 5%) and patchily distributed. B. treatae from each of five high-gall-density trees were reared on (1) the four nearest low-gall-density trees, (2) the four alternative high-gall-density trees, and (3) their natal trees (control). Each treatment (source X rearing site) was replicated three times. Nine components of performance that sequentially contribute to fitness were evaluated with over 21000 galls censused across the 25 experimental trees. When reared on their natal trees and compared with low-gall-density neighbors, transplanted gall formers had higher gall initiation success (P < 0.05), produced more (P < 0.001) and larger galls (P < 0.001), and produced a higher proportion of galls that exceeded the threshold size for natural enemy avoidance (P < 0.05). Comparison of gall-former performance on natal vs. alternative high-gall-density trees demonstrated significant (P < 0.001) differences in six performance measures with five differing in the direction predicted by the hypothesis of local adaptation. Overall, these linked experiments document direct and indirect effects of host plant variation on gall-former performance and demonstrate convincingly that (1) high-gall-density trees equate to high-quality trees that are surrounded by trees of relatively lower quality to the herbivore and (2) gall-former populations have become locally adapted to individual trees.

Nitrogen homeostasis in a willow leaf beetle, Plagiodera versicolora, is independent of host plant quality

AbstractWhile foliar nitrogen (N) content of host plants depends on environmental conditions, N content of herbivorous insects may remain relatively constant due to homeostasis. However, it is unknown to what extent insects can maintain their body elemental composition against natural variation in host plant quality. The present study examined the performance and N content of a willow leaf beetle, Plagiodera versicolora Laicharting (Coleoptera: Chrysomelidae), when fed leaves of host willow, Salix eriocarpa Franchet et Savatier (Salicaceae), with varying nutritional status.Water content, toughness, and N content of willow leaves varied seasonally, and they affected performance of the leaf beetle. The leaf beetle achieved high performance when fed young leaves. On the other hand, the N content of the leaf beetle changed little, and it was independent of that of willow leaves, indicating strong N homeostasis of the leaf beetle. We discussed the function of N homeostasis in herbivorous insects in tritrophic level interactions.