Food quality, competition, and parasitism influence feeding preference in a neotropical lepidopteran

We surveyed Lepidoptera found on 11 species of Inga (Fabaceae:Mimosoideae) co-existing on Barro Colorado Island, Panama, to evaluate factors influencing diet choice. Of the 47 species of caterpillars (747 individuals) recorded, each fed on a distinct set of Inga. In the field, 96% of the individuals were found on young leaves. Growth rates of caterpillars that were fed leaves in the laboratory were 60% higher on young leaves compared to mature leaves. When caterpillars were fed leaves of nonhost Inga, they grew more slowly. These data provide support for a link between preference and performance. However, among hosts on which larvae normally occurred, faster growth rates were not associated with greater host electivity (the proportion of larvae found on each host species in the field, corrected for host abundance). Growth rates on normal hosts were positively correlated with leaf expansion rates of the host, and fast expansion was associated with leaves with higher nutritional content. Detailed studies on a gelechiid leaf roller, the species with the largest diet breadth, allowed us to assess the importance of factors other than growth that could influence diet electivity. This species showed a 1.7-fold difference in growth rate among Inga hosts and faster growth on species with fast-expanding leaves. However, there was no correlation between caterpillar growth rate and abundance on different host species. Instead, abundance of the gelechiid on each Inga species was significantly correlated with the temporal predictability of food (synchrony of leaf flushing) and was negatively correlated with competition (amount of leaf area removed by species other than the gelechiid). Although rates of parasitism were high (23-43%), there were no differences among hosts. Parasitism was also not related to measures of escape, such as growth rates of caterpillars, leaf expansion rates, and synchrony of leaf production. Together, food availability, parasitism, and competition explained 84% of the variation in host preference by the gelechiid. We suggest that these ecological interactions may be particularly important in determining diet choice initially and that preferences may be reinforced by subsequent divergence in host chemistry and/or the herbivore's ability to tolerate the secondary metabolites.     

Species-specific responses of herbivores to within-plant and environmentally mediated between-plant variability in plant chemistry

Allocation of resources to growth and defense against herbivores crucially affects plant competitiveness and survival, resulting in a specific distribution of assimilates and defense compounds within plant individuals. Additionally, plants rarely experience stable environmental conditions, and adaptations to abiotic and biotic stresses may involve shifts in resistance to herbivores. We studied the allocation of phytochemicals in Brassica oleracea (Brussels sprouts) due to leaf age, drought stress and herbivore damage and assessed effects on two lepidopteran herbivores differing in diet breadth: the generalist Spodoptera littoralis and the specialist Pieris brassicae. Glucosinolates as secondary defense compounds and total nitrogen and carbon were quantified and linked to plant palatability, i.e., herbivore feeding preference. Herbivore responses were highly species-specific and partially related to changes in phytochemicals. Spodoptera littoralis preferred middle-aged leaves with intermediate levels of glucosinolates and nitrogen over young, glucosinolate and nitrogen rich leaves, as well as over old leaves, poor in glucosinolates and nitrogen. In contrast, P. brassicae preferred young leaves. Both species preferred severely drought-stressed plants to the well-watered control, although analyzed glucosinolate concentrations did not differ. Both S. littoralis and P. brassicae feeding induced an increase of indole glucosinolate levels, which may explain a reduced consumption of damaged plants detected for S. littoralis but not for P. brassicae. By revealing distinct, sometimes contrasting responses of two insect herbivores to within-plant and stress-mediated intraspecific variation in phytochemistry of B. oleracea, this study emphasizes the need to consider specific herbivore responses to understand and predict the interactions between herbivores and variable plants.     

Mother really knows best: host choice of adult phytophagous insect females reflects a within-host variation in suitability as larval food

Non-random distribution patterns of specialized phytophagous insects on their hosts may depend on intraspecific differences in plant tissue quality, including nutrients and secondary compounds. Secondary compounds are involved in plant resistance, but are also important for the recognition and acceptability of plants as resources by specialized insects. If individuals within a plant species vary in their content of such secondary substances, there may also be qualitative differences between them. In such cases, natural selection will favor insects with the ability to distinguish and prefer the more suitable plants. In Sweden, the leaf beetle Gonioctena linnaeana Schrank (Coleoptera, Chrysomelidae) is highly specialized on one host, the native willow Salix triandra L (Salicaceae). Field observations reveal that some host plants in a population harbor many feeding larvae, causing severe defoliation, whereas neighboring plants may have few or no feeding larvae. Our hypothesis is that the distribution pattern of G. linnaeana larvae in this population results from qualitative differences between individual host plants in combination with the ability of G. linnaeana females to distinguish between plants that are suitable and not suitable for offspring performance. We examine whether larval survival differs depending on diet and whether the content of secondary chemical compounds explains female preference. Based on the higher survival rate of larvae reared on leaves from preferred hosts, we conclude that G. linnaeana females have evolved a behavior that maximizes offspring performance and thus positively affects female fitness. A chemical survey of the plants indicates that luteolin-7-glucoside and an unidentified flavonoid are important for separating the preferred from the non-preferred plants.     

Sources of variation in herbivore preference: among-individual and past diet effects on amphipod host choice

Understanding which factors affect the feeding preferences of herbivores is essential for predicting the effects of herbivores on plant assemblages and the evolution of plant–herbivore interactions. Most studies of marine herbivory have focussed on the plant traits that determine preferences (especially secondary metabolites), while few studies have considered how preferences may vary among individual herbivores due to genetic or environmental sources of variation. Such intraspecific variation is essential for evolutionary change in preference behaviour and may alter the outcome of plant–herbivore interactions. In an abundant marine herbivore, we determined the relative importance of among-individual and environmental effects on preferences for three host algae of varying quality. Repeated preference assays were conducted with the amphipod Peramphithoe parmerong and three of its brown algal hosts: Sargassum linearifolium, S. vestitum and Padina crassa. We found no evidence that preference varied among individuals, thus constraining the ability of natural selection to promote increased specialisation on high-quality S. linearifolium. Most of the variation in preference occurred within individuals, with amphipod preferences strongly influenced by past diet. The increased tendency for amphipods to select alternate hosts to that on which they had been recently feeding indicates that amphipods are actively seeking mixed diets. Such a feeding strategy provides an explanation for the persistence of this herbivore on hosts in the field that support poor growth and survival if consumed alone. The effects of past diet indicate that herbivore preferences are a function of herbivore history in addition to plant traits and are likely to vary with the availability of algae in space and time.     

Associations of plant fitness, leaf chemistry, and damage suggest selection mosaic in plant-herbivore interactions

The geographic mosaic theory of coevolution states that variation in species interactions forms the raw material for coevolutionary processes, which take place over large geographic scales. One key assumption underlying the process of coevolution in plant-herbivore interactions is that herbivores exert selection on their host plants and that this selection varies among plant populations. We examined spatial variation in the existence and strength of phenotypic selection on host plant resistance exerted by specialist herbivores in 17 archipelago populations of the perennial herb Vincetoxicum hirundinaria (Asclepiadaceae). In these highly fragmented populations, V. hirundinaria is consumed by the larvae of two specialist herbivores: the folivorous moth Abrostola asclepiadis and the seed predator Euphranta connexa. Selection imposed on host plants by these herbivores was examined by analyzing the associations between levels of herbivory, plant fitness, and contents of a number of leaf chemicals reflecting plant resistance to and quality for the herbivores. We found extensive spatial variation in the levels of herbivory and in plant fitness. More importantly, the impact of both leaf herbivory and seed predation on plant fitness varied among plant populations, indicating spatial variation in phenotypic selection. In addition, leaf chemistry varied widely among plant populations, reflecting spatial variation in plant quality as food for the herbivores. However, leaf compounds influenced folivory similarly in all the studied plant populations, and interestingly, some of the compounds were associated with the intensity of seed predation. Finally, some of the leaf compounds were associated with plant fitness, and the strength and direction of these associations varied among plant populations. The observed spatial variation in the strength of the interactions between V. hirundinaria and its specialist herbivores suggests a geographic selection mosaic. Because the occurrence and strength of spatial variation varied between the two specialist herbivores, our results highlight the importance of considering multiple enemies when trying to understand evolution of interactions between plants and their herbivores.     

Combining optimal defense theory and the evolutionary dilemma model to refine predictions regarding plant invasion

Optimal defense theory posits that plants with limited resources deploy chemical defenses based on the fitness value of different tissues and their probability of attack. However, what constitutes optimal defense depends on the identity of the herbivores involved in the interaction. Generalists, which are not tightly coevolved with their many host plants, are typically deterred by chemical defenses, while coevolved specialists are often attracted to these same chemicals. This imposes an "evolutionary dilemma" in which generalists and specialists exert opposing selection on plant investment in defense, thereby stabilizing defenses at intermediate levels. We used the natural shift in herbivore community composition that typifies many plant invasions to test a novel, combined prediction of optimal defense theory and the evolutionary dilemma model: that the within-plant distribution of defenses reflects both the value of different tissues (i.e., young vs. old leaves) and the relative importance of specialist and generalist herbivores in the community. Using populations of Verbascum thapsus exposed to ambient herbivory in its native range (where specialist and generalist chewing herbivores are prevalent) and its introduced range (where only generalist chewing herbivores are prevalent), we illustrate significant differences in the way iridoid glycosides are distributed among young and old leaves. Importantly, high-quality young leaves are 6.5x more highly defended than old leaves in the introduced range, but only 2x more highly defended in the native range. Additionally, defense levels are tracked by patterns of chewing damage, with damage restricted mostly to low-quality old leaves in the introduced range, but not the native range. Given that whole-plant investment in defense does not differ between ranges, introduced mullein may achieve increased fitness simply by optimizing its within-plant distribution of defense in the absence of certain specialist herbivores.     

Integrating quality and quantity of mutualistic service to contrast ant species protecting Ferocactus wislizeni

Generalized, facultative mutualisms are often characterized by great variation in the benefits provided by different partner species. This variation may be due to differences among species in the quality and quantity of their interactions, as well as their phenology. Many plant species produce extrafloral nectar, a carbohydrate-rich resource, to attract ant species that can act as "bodyguards" against a plant's natural enemies. Here, we explore differences in the quality and quantity of protective service that ants can provide a plant by contrasting the four most common ant visitors to Ferocactus wislizeni, an extrafloral nectary-bearing cactus in southern Arizona. The four species differ in abundance when tending plants, and in the frequency at which they visit plants. By adding surrogate herbivores (Manduca sexta caterpillars) to plants, we demonstrate that all four species recruit to and attack potential herbivores. However, their per capita effectiveness in deterring herbivores (measured as the inverse of the number of workers needed to remove half of the experimentally added caterpillars) differs. Using these among-species differences in quality (per capita effectiveness) and quantity (number of workers that visit a plant and frequency of visitation), we accurately predicted the variation in fruit production among plants with different histories of ant tending. We found that plant benefits (herbivore removal and maturation of buds and fruits) typically saturated at high levels of ant protection, although plants could be "well defended" via different combinations of interaction frequency, numbers of ant workers per interaction, and per capita effects. Our study documents variation among prospective mutualists, distinguishes the components of this variation, and integrates these components into a predictive measure of protection benefit to the plant. The method we used to average saturating benefits over time could prove useful for quantifying overall service in other mutualisms.     

Plant secondary chemistry mediates the performance of a nutritional symbiont associated with a tree-killing herbivore

Many herbivores consume microbial food sources in addition to plant tissues for nutrition. Despite the ubiquity of herbivore-microbe feeding associations, few studies examine how host plant phenotypes affect microbial symbionts of herbivores. We tested the hypothesis that chemical polymorphism in a plant population mediates the performance of nutritional microbial symbionts. We surveyed the composition of ponderosa pine resin in northern Arizona, USA, for variation in six monoterpenes, and we approximated four chemical phenotypes. We reared populations of an herbivorous tree-killing beetle (Dendroctonus brevicomis) in ponderosa pine host material, controlling for three monoterpene compositions representing an alpha-pinene to delta-3-carene gradient. Beetles were reared in host material where the dominant monoterpene was alpha-pinene, delta-3-carene, or a phenotype that was intermediate between the two. We isolated nutritional fungal symbionts (Entomocorticium sp. B) from beetle populations reared in each phenotype and performed reciprocal growth experiments in media amended to represent four "average" monoterpene compositions. This allowed us to test the effects of natal host phenotype, chemical polymorphism, and the interaction between natal host phenotype and chemical polymorphism on a nutritional symbiont. Three important findings emerged: (1) fungal isolates grew 25-32% faster when acquired from beetles reared in the intermediate phenotype; (2) the mean growth rate of nutritional fungi varied up to 44% depending on which monoterpene composition media was amended with; and (3) fungal isolates uniformly performed best in the intermediate phenotype regardless of the chemical composition of their natal host. The performance of nutritional fungi related to both the chemical "history" of their associated herbivore and the chemical phenotypes they are exposed to. However, all fungal isolates appeared adapted to a common chemical phenotype. These experiments argue in favor of the hypothesis that chemical polymorphism in plant populations mediates growth of nutritional symbionts of herbivores. Intraspecific chemical polymorphism in plants contributes indirectly to the regulation of herbivore populations, and our experiments demonstrate that the ecological effects of plant secondary chemistry extend beyond the trophic scale of the herbivore-plant interaction.     

Systemically-induced response of cabbage plants against a specialist herbivore, Pieris brassicae

Summary. Plant responses to herbivory might directly affect the herbivore (“direct” defences) or might benefit the plant by promoting the effectiveness of natural antagonists of the herbivores (“indirect” defences). Brussels sprouts attacked by Pieris brassicae larvae release volatiles that attract a natural antagonist of the herbivores, the parasitoid Cotesia glomerata, to the damaged plant. In a previous study, we observed that feeding by caterpillars on the lower leaves of the plant triggers the systemic release of volatiles detectable by the parasitoids from upper leaves of the same plant.?The role of these systemically induced volatiles as indirect defence and the dynamics of their emission were investigated in wind-tunnel dual choice tests with C. glomerata. The systemically induced emission of volatiles varied depending on leaf age and on plant age. Systemic induction affected parasitoid effectiveness, as induced plants could be more easily located by parasitoids than non-induced ones.?The role of the systemic induction as a direct defence was investigated through behavioural and feeding tests with P. brassicae. In dual choice assays, 1st instar larvae preferred to feed and fed more on systemically induced than on non-induced leaves. In single choice assays, the leaf area consumed by caterpillars was larger on systemically induced leaves than on non-induced control leaves. However, caterpillars fed on systemically induced leaves attained the same weight as those feeding on non-induced controls. In addition, P. brassicae pupae whose larvae were fed on systemically induced leaves had longer developmental times than those of larvae fed on non-induced leaves. Adult oviposition behavior was not influenced by systemic induction.?We conclude that systemically induced responses in cabbage might reduce P. brassicae fitness both directly, by affecting their development and feeding behavior and indirectly by making caterpillars and pupae more vulnerable to attack by carnivores. The occurrence of a possible relationship between direct and indirect defence is discussed. Received 24 January 2001; accepted 3 May 2001.     

Population and leaf-level variation of iridoid glycosides in the invasive weed Verbascum thapsus L. (common mullein): implications for herbivory by generalist insects

Plant–insect interactions, which are strongly mediated by chemical defenses, have the potential to shape invasion dynamics. Despite this, few studies have quantified natural variation in key defensive compounds of invasive plant populations, or how those defenses relate to levels of herbivory. Here, we evaluated variation in the iridoid glycosides aucubin and catalpol in rosette plants of naturally occurring, introduced populations of the North American invader, Verbascum thapsus L. (common mullein; Scrophulariaceae). We examined two scales that are likely to structure interactions with insect herbivores—among populations and within plant tissues (i.e., between young and old leaves). We additionally estimated the severity of damage incurred at these scales due to insect chewing herbivores (predominantly grasshoppers and caterpillars), and evaluated the relationship between iridoid glycoside content and leaf damage. We found significant variation in iridoid glycoside concentrations among populations and between young and old leaves, with levels of herbivory strongly tracking leaf-level investment in defense. Specifically, across populations, young leaves were highly defended by iridoids (averaging 6.5× the concentration present in old leaves, and containing higher proportions of the potentially more toxic iridoid, catalpol) and suffered only minimal damage from generalist herbivores. In contrast, old leaves were significantly less defended and accordingly more substantially utilized. These findings reveal that quantitative variation in iridoid glycosides is a key feature explaining patterns of herbivory in an introduced plant. In particular, these data support the hypothesis that defenses limit the ability of generalists to feed on mullein’s well-defended young leaves, resulting in minimal losses of high-quality tissue, and increasing performance of this introduced species.     

The scent of alarm: ontogenetic and genetic variation in the osmeterial gland chemistry of Papilio glaucus (Papilionidae) caterpillars

Within a species, ontogenetic and genetic variation in defensive chemistry can provide the basis for natural selection from different predator types. The osmeterial chemistry of fifth (last) instar Papilio glaucus caterpillars is known to differ qualitatively from the composition of early instar caterpillars. However, the osmeterial chemistry of early instar caterpillars has not been thoroughly characterized and may change as the caterpillars undergo their first three molts. We have used GC/MS to identify a suite of about 50 different terpene compounds in the osmeterial secretions of P. glaucus caterpillars, and found the relative amounts of these compounds changed significantly with each molt. These quantitative changes preceded the more dramatic qualitative switch to the production of 2-methylbutyric and isobutyric acids after the molt to the fifth instar. We also examined the effects of diet and genetic background on the relative quantities of 15 terpenes present in the secretions of third instar caterpillars. Parentage was found to affect the percentages of many more of the individual components than did diet, although both exerted an effect. The ontogenetic and genetic variations in the composition of the osmeterial secretions appear to have an effect on would-be predators. In the laboratory, terpene secretion was found to discourage attack by ants, whereas the switch from terpene to acid production rendered the caterpillars less palatable to a larger predator, the green anole. In the field, the presence of functional osmeteria did not seem to dramatically increase survival in a field study, and only a small, non-significant advantage was seen. Similarly, field data was suggestive that parentage might affect the likelihood of survival in a natural setting, but the stage of the caterpillar and the field site significantly affected survivorship. Further studies with greater replicates will be needed to determine whether and to what extent chemical differences in osmeterial components as well as behavior contribute to differences in outcomes in the field.     

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.     

Fitness benefits of size-dependent diet switching in a marine herbivore

Although diet is one of the most important parameters affecting the fitness of terrestrial and marine herbivores, host plant choice and subsequent fitness on that host are not always correlated. This study investigated the effect of diet on fitness of the sea urchin Holopneustes purpurascens, which show an ontogenetic change in host plant use subsequent to recruitment. To test whether fitness on host plant mirrored host plant choice, small and large individuals were collected from both hosts and fed either host plant (Ecklonia radiata or Delisea pulchra). Small urchins survived better than large individuals. Those fed E. radiata produced fewer test lesions, grew faster and were more fecund than those fed D. pulchra, irrespective of size. This pattern was enhanced when the host plant the urchin previously inhabited was assessed. Our results show that diet is driving the previously recorded ontogenetic change in host plant use associated with increasing size in H. purpurascens, where medium-sized individuals switch from D. pulchra to E. radiata.     

Indirect benefits of symbiotic coccoids for an ant-defended myrmecophytic tree

The net benefits of mutualism depend directly on the costs and effectiveness of mutualistic services and indirectly on the interactions that affect those services. We examined interactions among Cordia alliodora myrmecophytic trees, their symbiotic ants Azteca pittieri, coccoid hemipterans, and foliar herbivores in two Neotropical dry forests. The tree makes two investments in symbiotic ants: it supplies nesting space, as domatia, and it provides phloem to coccoids, which then produce honeydew that is consumed by ants. Although higher densities of coccoids should have higher direct costs for trees, we asked whether higher densities of coccoids can also have higher indirect benefits for trees by increasing the effectiveness of ant defense against foliar herbivores. We found that trees benefited from ant defense against herbivores. Ants defended trees effectively only when colonies reached high densities within trees, and ant and coccoid densities within trees were strongly positively correlated. The benefits of reduced foliar herbivory by larger ant colonies were therefore indirectly controlled by the number of coccoids. Coccoid honeydew supply also affected per capita ant aggression against tree herbivores. Ants experimentally fed a carbohydrate-rich diet, analogous to sugar obtained from coccoids, were more aggressive against caterpillars per capita than ants fed a carbohydrate-poor diet. Ant defense was more effective on more valuable and vulnerable young leaves than on older leaves. Young domatia, associated with young leaves, contained higher coccoid densities than older domatia, which suggests that coccoids may also drive spatially favorable ant defense of the tree. If higher investments by one mutualistic partner are tied to higher benefits received from the other, there may be positive feedback between partners that will stabilize the mutualism. These results suggest that higher investment by trees in coccoids leads to more effective defense by ants against the tree's foliar herbivores.     

Life history traits predict relative abundance in an assemblage of forest caterpillars

Species in a given trophic level occur in vastly unequal abundance, a pattern commonly documented but poorly explained for most taxa. Theoretical predictions of species density such as those arising from the metabolic theory of ecology hold well at large spatial and temporal scales but are not supported in many communities sampled at a relatively small scale. At these scales ecological factors may be more important than the inherent limits to energy use set by allometric scaling of mass. These factors include the amount of resources available, and the ability of individuals to convert these resources successfully into population growth. While previous studies have demonstrated the limits of macroecological theory in explaining local abundance, few studies have tested alternative generalized mechanisms determining abundance at the community scale. Using an assemblage of forest moth species found co-occurring as caterpillars on a single host plant species, we tested whether species abundance on that plant could be explained by mass allometry, intrinsic population growth, diet breadth, or some combination of these traits. We parameterized life history traits of the caterpillars in association with the host plant in both field and laboratory settings, so that the population growth estimate was specific to the plant on which abundance was measured. Using a generalized least-squares regression method incorporating phylogenetic relatedness, we found no relationship between abundance and mass but found that abundance was best explained by both intrinsic population growth rate and diet breadth. Species population growth potential was most affected by survivorship and larval development time on the host plant. Metabolic constraints may determine upper limits to local abundance levels for species, but local community abundance is strongly predicted by the potential for population increase and the resources available to that species in the environment.     

Genetic variation in defense chemistry in wild cabbages affects herbivores and their endoparasitoids

Populations of wild Brassica oleracea L. grow naturally along the Atlantic coastlines of the United Kingdom and France. Over a very small spatial scale (i.e., <15 km) these populations differ in the expression of the defensive compounds, glucosinolates (GS). Thus far, very few studies have examined interactions between genetically distinct populations of a wild plant species and associated consumers in a multitrophic framework. Here, we compared the development of a specialist (Pieris rapae) and a generalist (Mamestra brassicae) insect herbivore and their endoparasitoids (Cotesia rubecula and Microplitis mediator, respectively) on three wild populations and one cultivar of B. oleracea under controlled greenhouse conditions. Herbivore performance was differentially affected by the plant population on which they were reared. Plant population influenced only development time and pupal mass in P. rapae, whereas plant population also had a dramatic effect on survival of M. brassicae. Prolonged development time in P. rapae corresponded with high levels of the indole GS, neoglucobrassicin, whereas reduced survival in M. brassicae coincided with high levels of the aliphatic GS, gluconapin and sinigrin. The difference between the two species can be explained by the fact that the specialist P. rapae is adapted to feed on plants containing GS and has evolved an effective detoxification system against aliphatic GS. The different B. oleracea populations also affected development of the endoparasitoids. Differences in food-plant quality for the hosts were reflected in adult size in C. rubecula and survival in M. mediator, and further showed that parasitoid performance is also affected by herbivore diet.     

Impact of an invasive oak gall wasp on a native butterfly: a test of plant-mediated competition

Phytophagous insects commonly interact through shared host plants. These interactions, however, do not occur in accordance with traditional paradigms of competition, and competition in phytophagous insects is still being defined. It remains unclear, for example, if particular guilds of insects are superior competitors or important players in structuring insect communities. Gall-forming insects are likely candidates for such superior competitors because of their ability to manipulate host plants, but their role as competitors is understudied. We investigate the effect of invasive populations of an oak gall wasp, Neuroterus saltatorius, on a native specialist butterfly, Erynnis propertius, as mediated by their shared host plant, Quercus garryana. This gall wasp occurs at high densities in its introduced range, where we stocked enclosures with caterpillars on trees that varied in gall wasp density. Biomass production of butterflies was lower in enclosures on high-density than on low-density trees because overwintering caterpillars were smaller, and fewer of them eclosed into adults the following spring. To see if the gall wasp induced changes in foliar quality, we measured host plant quality before and after gall induction on 30 trees each at two sites. We found a positive relationship between gall wasp density and the percentage change in foliar C:N, a negative relationship between gall wasp density and the percentage change in foliar water at one site, and no relationship between the percentage change in protein-binding capacity (i.e., phenolics) and gall-wasp density. Additionally, there was a negative relationship between foliar quality and butterfly performance. Our results provide evidence for a plant-mediated impact of an invasive oak gall wasp on a native butterfly and suggest that gall wasps could act as superior competitors, especially when they occur at high densities.     

Indirect Effects of Pandemic Deer Overabundance Inferred from Caterpillar‐Host Relations

Externally feeding phytophagous insect larvae (i.e., caterpillars, here, larval Lepidoptera and sawflies, Hymenoptera: Symphyta) are important canopy herbivores and prey resources in temperate deciduous forests. However, composition of forest trees has changed dramatically in the eastern United States since 1900. In particular, browsing by high densities of white‐tailed deer (Odocoileus virginianus) has resulted in forests dominated by browse‐tolerant species, such as black cherry (Prunus serotina), and greatly reduced relative abundance of other tree species, notably pin cherry (Prunus pensylvanica) and birches (Betula spp.). To quantify effects of these changes on caterpillars, we sampled caterpillars from 960 branch tips of the 8 tree species that comprise 95% of trees in Allegheny hardwood forests: red maple (Acer rubrum), striped maple (Acer pensylvanicum), sugar maple (Acer saccharum), sweet birch (Betula lenta), yellow birch (Betula allegheniensis), American beech (Fagus grandifolia), black cherry, and pin cherry. We collected 547 caterpillar specimens that belonged to 66 Lepidoptera and 10 Hymenoptera species. Caterpillar density, species richness, and community composition differed significantly among tree species sampled. Pin cherry, nearly eliminated at high deer density, had the highest density and diversity of caterpillars. Pin cherry shared a common caterpillar community with black cherry, which was distinct from those of other tree hosts. As high deer density continues to replace diverse forests of cherries, maples, birches, and beech with monodominant stands of black cherry, up to 66% of caterpillar species may be eliminated. Hence, deer‐induced changes in forest vegetation are likely to ricochet back up forest food webs and therefore negatively affect species that depend on caterpillars and moths for food and pollination. Efectos Indirectos de la Sobreabundancia de Venados Pandémicos Inferida de Relaciones Orugas‐Huéspedes     

Geographic covariation of chemical quality of the host alga<Emphasis Type="Italic"> Fucus vesiculosus</Emphasis> with fitness of the herbivorous isopod<Emphasis Type="Italic"> Idotea baltica</Emphasis>

Environmental and/or genetic among-site variation in plant quality may influence growth and fecundity of specialized herbivores inhabiting a particular site. Such variation is important as it generates spatial variation in selection for traits related to plant–herbivore interaction. Littoral macroalgae are known to respond plastically to environmental variation by modifying their chemistry or morphology. We studied geographic variation in phlorotannin, nitrogen, protein, and sugar (fucose, mannitol, and melibiose) concentrations of the brown alga Fucus vesiculosus at 12 sites separated by 0.5 to 40 km in the naturally fragmented Archipelago Sea in the northern Baltic Sea. By this regional variation in algal chemistry we attempted to explain among-population variation in size and fecundity of the crustacean herbivore Idotea baltica. We observed high spatial variation in all the measured chemical characteristics of F. vesiculosus, as well as in female size and the number of eggs produced by the herbivores. Spatial variation in nitrogen or protein contents of the alga did not explain the variation of herbivore traits. However, egg size positively covaried with spatial variation in the concentration of mannitol, the major storage carbohydrate of the alga. Such a positive relationship may arise if I. baltica can utilize the nutritive value of a mannitol-rich diet thereby being better able to provision the developing eggs with energy-rich metabolites. Unexpectedly, the concentration of phlorotannins, secondary metabolites having a putative role in defense against herbivory, positively covaried with the size of the herbivore. Among-population variation in host plant chemistry and covariation of that with herbivore growth and reproduction imply that herbivores respond to the local quality of their host plants, and that geographical structuring of populations has to be taken into account in studies of plant–herbivore interactions.Communicated by M. Kühl, Helsingør     

Physiological benefits of feeding in the spring by Lymantria dispar caterpillars on red oak and sugar maple leaves: nutrition versus oxidative stress

The rapid growth of insects that feed on tree leaves in the spring is believed to be due to high nutritional quality. This study tested the hypothesis that both high nutritional quality and low levels of oxidative stress (i.e., toxicological effects) benefit caterpillars that feed in the spring. Fourth-instar larvae of Lymantria dispar (Lepidoptera: Lymantriidae) were used to bioassay the leaves of two contrasting host plants in the spring and summer: red oak (Quercus rubra), a high-quality host, and sugar maple (Acer saccharum), a low-quality host. On spring foliage, the combined effects of rapid consumption rate, efficient nutrient assimilation, and high nutritional quality allowed larvae to grow rapidly and attain larger body mass. Ellagitannins, a major source of oxidative stress in the midgut, were at higher concentrations in the spring than in the summer in maple leaves, but were at negligible levels throughout the growing season in oak. Thus, the impact of phenolic defenses (measured as semiquinone free radicals and oxidized glutathione in the midgut) was not decreased in spring-feeding larvae. Instead, oxidative stress in larvae on maple remained at elevated levels in the spring and summer. By contrast, larvae that fed on oak had consistently low levels of oxidative stress. We conclude that oak and maple were better host plants in the spring because of their higher nutritional quality, and not because of a lower effectiveness of their chemical defenses. This work emphasizes the need to measure not only foliar nutritional and phenolic chemistry but also specific physiological responses in the herbivore, such as oxidative stress. These physiological mechanisms add to our understanding of why spring-feeding life-history strategies have evolved in some insect herbivores.