Tree resistance to Lymantria dispar caterpillars: importance and limitations of foliar tannin composition

The ability of foliar tannins to increase plant resistance to herbivores is potentially determined by the composition of the tannins; hydrolyzable tannins are much more active as prooxidants in the guts of caterpillars than are condensed tannins. By manipulating the tannin compositions of two contrasting tree species, this work examined: (1) whether increased levels of hydrolyzable tannins increase the resistance of red oak (Quercus rubra L.), a tree with low resistance that produces mainly condensed tannins, and (2) whether increased levels of condensed tannins decrease the resistance of sugar maple (Acer saccharum Marsh.), a tree with relatively high resistance that produces high levels of hydrolyzable tannins. As expected, when Lymantria dispar L. caterpillars ingested oak leaves coated with hydrolyzable tannins, levels of hydrolyzable tannin oxidation increased in their midgut contents. However, increased tannin oxidation had no significant impact on oxidative stress in the surrounding midgut tissues. Although growth efficiencies were decreased by hydrolyzable tannins, growth rates remained unchanged, suggesting that additional hydrolyzable tannins are not sufficient to increase the resistance of oak. In larvae on condensed tannin-coated maple, no antioxidant effects were observed in the midgut, and levels of tannin oxidation remained high. Consequently, neither oxidative stress in midgut tissues nor larval performance were significantly affected by high levels of condensed tannins. Post hoc comparisons of physiological mechanisms related to tree resistance revealed that maple produced not only higher levels of oxidative stress in the midgut lumen and midgut tissues of L. dispar, but also decreased protein utilization efficiency compared with oak. Our results suggest that high levels of hydrolyzable tannins are important for producing oxidative stress, but increased tree resistance to caterpillars may require additional factors, such as those that produce nutritional stress.     

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.     

Impact of plant architecture versus leaf quality on attack by leaf-tying caterpillars on five oak species

Because shelter-building herbivorous insect species often consider structural features of their host plants in selecting construction sites, their probability of attack is likely to be a function of some combination of plant architectural traits and leaf quality factors. We tested the hypothesis that plant architecture, in the form of the number of touching leaves, influences interspecific variation in attack by leaf-tying caterpillars in five species of sympatric Missouri oaks (Quercus). We compared colonization on control branches, in which both architecture and leaf quality were potentially important, with colonization on experimental branches for which we controlled for the effects of architecture by creating equal numbers of artificial ties. Colonization of artificial ties was highly correlated with natural colonization on neighboring control branches, suggesting that leaf quality factors and not architecture influenced interspecific variation in attack by leaf-tying caterpillars. Of the leaf quality factors measured (water, protein-binding capacity, nitrogen, specific leaf area, pubescence, and toughness), nitrogen was the most explanatory. With the exception of white oak, natural leaf tie colonization was positively correlated with nitrogen availability (ratio of nitrogen to protein-binding capacity), and negatively correlated with protein-binding capacity of leaf extracts. Both host plant species and subgenus oak influenced the community composition of leaf-tying caterpillars and the non-tying symbionts colonizing the ties. Host plant differences in leaf nitrogen content were positively correlated with pupal weight of one of two caterpillar species reared on all five host plant species. Thus, interspecific differences in nitrogen, nitrogen availability, and protein-binding capacity of leaf extracts are the best predictors at this time of interspecific differences in attack by leaf-tying caterpillars, in turn affecting their success on individual host plants in the laboratory.     

Physiological factors affecting the rapid decrease in protein assimilation efficiency by a caterpillar on newly‐mature tree leaves

Lymantria dispar L. caterpillars have a decreased ability to assimilate protein from mature leaves of red oak (Quercus rubra) compared with young, expanding leaves. The present study determines whether the drop in protein assimilation efficiency (PAE) occurs during the rapid phase of leaf maturation. Several mechanisms that might account for decreased PAE are also examined: mature leaf tissues could resist being chewed efficiently, protein in mature leaf tissues could become difficult to extract, and other nutrients in mature leaves might become growth limiting. The entire seasonal decrease in PAE occurs rapidly (in less than 2 weeks), when the leaves finished expanding. The maturation process is characterized by increased levels of fibre and decreased levels of water but no significant changes in the levels of protein or carbohydrates. Despite increased fibre in mature leaves, they are not chewed into larger food particles than are immature leaves. Carbohydrate assimilation efficiencies remain high on mature leaves, and signs of limiting water levels in larvae of L. dispar on mature leaves are not observed. The most important finding in the present study is the decreased extractability of protein in food particles from mature leaves, which plays a major role in explaining the rapid decrease in PAE. It is hypothesized that structural changes in cell walls during the rapid process of leaf maturation decrease protein extractability, which, in turn, greatly decreases the nutritional quality of mature oak leaves for caterpillars. The results of the present study therefore suggest a general mechanism to help explain the widely documented decrease in the nutritional quality of the mature leaves of many tree species for herbivorous insects.     

Nutrients are assimilated efficiently by Lymantria dispar caterpillars from the mature leaves of trees in the Salicaceae

The efficient aquisition of nutrients from leaves by insect herbivores increases their nutrient assimilation rates and overall fitness. Caterpillars of the gypsy moth (Lymantria dispar L.) have high protein assimilation efficiencies (PAE) from the immature leaves of trees such as red oak (Quercus rubra) and sugar maple (Acer saccharum) (71–81%) but significantly lower PAE from their mature leaves (45–52%). By contrast to this pattern, both PAE and carbohydrate assimilation efficiencies (CAE) remain high for L. dispar larvae on the mature leaves of poplar (Populus alba × Populus tremula) grown in greenhouse conditions. The present study tests two alternative hypotheses: (i) outdoor environmental stresses cause decreased nutrient assimilation efficiencies from mature poplar leaves and (ii) nutrients in the mature leaves of trees in the poplar family (Salicaceae) remain readily available for L. dispar larvae. When poplar trees are grown in ambient outdoor conditions, PAE and CAE remain high (approximately 75% and 78%, respectively) in L. dispar larvae, in contrast to the first hypothesis. When larvae feed on the mature leaves of species in the Salicaceae [aspen (Populus tremuloides), cottonwood (Populus deltoides), willow (Salix nigra) and poplar], PAE and CAE also remain high (68–76% and 72–92%, respectively), consistent with the second hypothesis. Larval growth rates are strongly associated with protein assimilation rates, and more strongly associated with protein assimilation rates than with carbohydrate assimilation rates. It is concluded that tree species in the Salicaceae are relatively high‐quality host plants for L. dispar larvae, in part, because nutrients in their mature leaves remain readily available.     

Matching the supply of bacterial nutrients to the nutritional demand of the animal host

Various animals derive nutrients from symbiotic microorganisms with much-reduced genomes, but it is unknown whether, and how, the supply of these nutrients is regulated. Here, we demonstrate that the production of essential amino acids (EAAs) by the bacterium Buchnera aphidicola in the pea aphid Acyrthosiphon pisum is elevated when aphids are reared on diets from which that EAA are omitted, demonstrating that Buchnera scale EAA production to host demand. Quantitative proteomics of bacteriocytes (host cells bearing Buchnera) revealed that these metabolic changes are not accompanied by significant change in Buchnera or host proteins, suggesting that EAA production is regulated post-translationally. Bacteriocytes in aphids reared on diet lacking the EAA methionine had elevated concentrations of both methionine and the precursor cystathionine, indicating that methionine production is promoted by precursor supply and is not subject to feedback inhibition by methionine. Furthermore, methionine production by isolated Buchnera increased with increasing cystathionine concentration. We propose that Buchnera metabolism is poised for EAA production at certain maximal rates, and the realized release rate is determined by precursor supply from the host. The incidence of host regulation of symbiont nutritional function via supply of key nutritional inputs in other symbioses remains to be investigated.     

Genetic and metabolic determinants of nutritional phenotype in an insect-bacterial symbiosis

The pervasive influence of resident microorganisms on the phenotype of their hosts is exemplified by the intracellular bacterium Buchnera aphidicola, which provides its aphid partner with essential amino acids (EAAs). We investigated variation in the dietary requirement for EAAs among four pea aphid (Acyrthosiphon pisum) clones. Buchnera-derived nitrogen contributed to the synthesis of all EAAs for which aphid clones required a dietary supply, and to none of the EAAs for which all four clones had no dietary requirement, suggesting that low total dietary nitrogen may select for reduced synthesis of certain EAAs in some aphid clones. The sequenced Buchnera genomes showed that the EAA nutritional phenotype (i.e. the profile of dietary EAAs required by the aphid) cannot be attributed to sequence variation of Buchnera genes coding EAA biosynthetic enzymes. Metabolic modelling by flux balance analysis demonstrated that EAA output from Buchnera can be determined precisely by the flux of host metabolic precursors to Buchnera. Specifically, the four EAA nutritional phenotypes could be reproduced by metabolic models with unique profiles of host inputs, dominated by variation in supply of aspartate, homocysteine and glutamate. This suggests that the nutritional phenotype of the symbiosis is determined principally by host metabolism and transporter genes that regulate nutrient supply to Buchnera. Intraspecific variation in the nutritional phenotype of symbioses is expected to mediate partitioning of plant resources among aphid genotypes, potentially promoting the genetic subdivision of aphid populations. In this way, microbial symbioses may play an important role in the evolutionary diversification of phytophagous insects.     

The Many Dimensions of Diet Breadth: Phytochemical,Genetic, Behavioral,and Physiological Perspectives on the Interaction between a Native Herbivore and an Exotic Host

From the perspective of an herbivorous insect, conspecific host plants are not identical, and intraspecific variation in host nutritional quality or defensive capacity might mediate spatially variable outcomes in plant-insect interactions. Here we explore this possibility in the context of an ongoing host breadth expansion of a native butterfly (the Melissa blue, Lycaeides melissa) onto an exotic host plant (alfalfa, Medicago sativa). We examine variation among seven alfalfa populations that differed in terms of colonization by L. melissa; specifically, we examined variation in phytochemistry, foliar protein, and plant population genetic structure, as well as responses of caterpillars and adult butterflies to foliage from the same populations. Regional patterns of alfalfa colonization by L. melissa were well predicted by phytochemical variation, and colonized patches of alfalfa showed a similar level of inter-individual phytochemical diversity. However, phytochemical variation was a poor predictor of larval performance, despite the fact that survival and weight gain differed dramatically among caterpillars reared on plants from different alfalfa populations. Moreover, we observed a mismatch between alfalfa supporting the best larval performance and alfalfa favored by ovipositing females. Thus, the axes of plant variation that mediate interactions with L. melissa depend upon herbivore life history stage, which raises important issues for our understanding of adaptation to novel resources by an organism with a complex life history.     

Plant quality and local adaptation undermine relocation in a bog specialist butterfly

The butterfly Boloria aquilonaris is a specialist of oligotrophic ecosystems. Population viability analysis predicted the species to be stable in Belgium and to collapse in the Netherlands with reduced host plant quality expected to drive species decline in the latter. We tested this hypothesis by rearing B. aquilonaris caterpillars from Belgian and Dutch sites on host plants (the cranberry, Vaccinium oxycoccos). Dutch plant quality was lower than Belgian one conferring lower caterpillar growth rate and survival. Reintroduction and/or supplementation may be necessary to ensure the viability of the species in the Netherlands, but some traits may have been selected solely in Dutch caterpillars to cope with gradual changes in host plant quality. To test this hypothesis, the performance of Belgian and Dutch caterpillars fed with plants from both countries were compared. Dutch caterpillars performed well on both plant qualities, whereas Belgian caterpillars could not switch to lower quality plants. This can be considered as an environmentally induced plastic response of caterpillars and/or a local adaptation to plant quality, which precludes the use of Belgian individuals as a unique solution for strengthening Dutch populations. More generally, these results stress that the relevance of local adaptation in selecting source populations for relocation may be as important as restoring habitat quality.     

Trophic influence on the development of the gypsy moth (Lepidoptera, Lymantriidae) and the forecast of its abundance

Results of a long-term investigation on the development of the Armenian population of Lymantria dispar L. are reported. A list of host plants with their comparative characteristics and data on their effect on different developmental phases of the pest (larvae, pupae, adults, and embryos) are given. Biochemical changes in the leaves of host plants are shown to play a significant role in regulation of the L. dispar population density. Population growth can be expected when population develops on primary hosts with sufficient content of nutrients in the leaves and a certain carbohydrate to protein ratio, as well as a certain ratio of water-and alkali-soluble protein fractions. Intense weakening of the plants caused by the larval feeding affects the food value of the leaves and, subsequently, the pest metabolism, negatively affecting its general physiological condition and fecundity, which eventually results in decline of the pest population.     

Host and symbiont genetic determinants of nutritional phenotype in a natural population of the pea aphid

A defining feature of the nutritional ecology of plant sap‐feeding insects is that the dietary deficit of essential amino acids (EAAs) in plant sap is supplemented by EAA‐provisioning microbial symbionts in the insect. Here, we demonstrated substantial variation in the nutritional phenotype of 208 genotypes of the pea aphid Acyrthosiphon pisum collected from a natural population. Specifically, the genotypes varied in performance (larval growth rates) on four test diets lacking the EAAs arginine, histidine and methionine or aromatic EAAs (phenylalanine and tryptophan), relative to the diet containing all EAAs. These data indicate that EAA supply from the symbiotic bacteria Buchnera can meet total aphid nutritional demand for only a subset of the EAA/aphid genotype combinations. We then correlated single nucleotide polymorphisms (SNPs) identified in the aphid and Buchnera genomes by reduced genome sequencing against aphid performance for each EAA deletion diet. This yielded significant associations between performance on the histidine‐free diet and Buchnera SNPs, including metabolism genes predicted to influence histidine biosynthesis. Aphid genetic correlates of performance were obtained for all four deletion diets, with associations on the arginine‐free diet and aromatic‐free diets dominated by genes functioning in the regulation of metabolic and cellular processes. The specific aphid genes associated with performance on different EAA deletion diets are largely nonoverlapping, indicating some independence in the regulatory circuits determining aphid phenotype for the different EAAs. This study demonstrates how variation in the phenotype of associations collected from natural populations can be applied to elucidate the genetic basis of ecologically important traits in systems intractable to traditional forward/reverse genetic techniques.     

Does insect herbivory on oak depend on the diversity of tree stands?

Studies on the effects of plant diversity on insect herbivory have produced conflicting results. Plant diversity has been reported to cause positive and negative responses of herbivores. Explanations for these conflicting responses include not only various population-level processes but also changes in plant quality that lead to changes in herbivore performance. In a tree diversity experiment, we investigated the effects of tree diversity on insect herbivory on oak in general and whether the effects of tree diversity on herbivore damage are reflected by the performance (leaf consumption, growth) of the generalist herbivore Lymantria dispar. Our study showed that the feeding damage caused by naturally occurring herbivores on oak trees decreased with increasing diversity of tree stands. The performance of L. dispar on oak leaves was not affected by tree diversity, neither in field nor laboratory experiments. Our results can be explained by the various processes behind the hypothesis of associational resistance.     

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.     

Variability in leaf traits,insect herbivory and herbivore performance within and among individuals of four broad-leaved tree species

Individual plants may vary in their suitability as hosts for insect herbivores. The adaptive deme formation hypothesis predicts that this variability will lead to the fine-scale adaptation of herbivorous insects to host individuals. We studied individual and temporal variation in the quality of leaves of the tree species ash, lime, common oak, and sycamore in the field as food for herbivores. We determined herbivore attack and leaf consumption and performance of the generalist caterpillars of Spodoptera littoralis in the laboratory. We further assessed the concentrations of carbon, nitrogen and water in the leaves.All measures of leaf tissue quality varied among and within individuals for all tree species. The level of herbivory differed among the tree individuals in lime, oak and sycamore, but not in ash. Within host individuals, differences in herbivory between the upper and lower crown layer varied in direction and magnitude depending on tree species. In feeding experiments, herbivore performance also varied among and within tree individuals. However, variation in palatability was not consistently related to the leaf traits measured or to herbivory levels in the field. The ranking of individuals with respect to the quality of leaf tissue for herbivorous insects varied between years in lime and oak. Thus, trees of both species might present moving targets for herbivores which prevents fine-scale adaptations. In contrast, among individuals of ash and sycamore the pattern of insect performance remained constant over 2 years. These species may be more suitable hosts for the formation of adapted demes in herbivores.     

Litter Breakdown and Microbial Succession on Two Submerged Leaf Species in a Small Forested Stream

Microbial succession during leaf breakdown was investigated in a small forested stream in west-central Georgia, USA, using multiple culture-independent techniques. Red maple (Acer rubrum) and water oak (Quercus nigra) leaf litter were incubated in situ for 128 days, and litter breakdown was quantified by ash-free dry mass (AFDM) method and microbial assemblage composition using phospholipid fatty acid analysis (PLFA), ribosomal intergenic spacer analysis (RISA), denaturing gradient gel electrophoresis (DGGE), and bar-coded next-generation sequencing of 16S rRNA gene amplicons. Leaf breakdown was faster for red maple than water oak. PLFA revealed a significant time effect on microbial lipid profiles for both leaf species. Microbial assemblages on maple contained a higher relative abundance of bacterial lipids than oak, and oak microbial assemblages contained higher relative abundance of fungal lipids than maple. RISA showed that incubation time was more important in structuring bacterial assemblages than leaf physicochemistry. DGGE profiles revealed high variability in bacterial assemblages over time, and sequencing of DGGE-resolved amplicons indicated several taxa present on degrading litter. Next-generation sequencing revealed temporal shifts in dominant taxa within the phylum Proteobacteria, whereas γ-Proteobacteria dominated pre-immersion and α- and β-Proteobacteria dominated after 1 month of instream incubation; the latter groups contain taxa that are predicted to be capable of using organic material to fuel further breakdown. Our results suggest that incubation time is more important than leaf species physicochemistry in influencing leaf litter microbial assemblage composition, and indicate the need for investigation into seasonal and temporal dynamics of leaf litter microbial assemblage succession.     

Hydrolyzable tannins as “quantitative defenses”: Limited impact against Lymantria dispar caterpillars on hybrid poplar

The high levels of tannins in many tree leaves are believed to cause decreased insect performance, but few controlled studies have been done. This study tested the hypothesis that higher foliar tannin levels produce higher concentrations of semiquinone radicals (from tannin oxidation) in caterpillar midguts, and that elevated levels of radicals are associated with increased oxidative stress in midgut tissues and decreased larval performance. The tannin-free leaves of hybrid poplar (Populus tremula × P. alba) were treated with hydrolyzable tannins, producing concentrations of 0%, 7.5% or 15% dry weight, and fed to Lymantria dispar caterpillars. As expected, larvae that ingested control leaves contained no measurable semiquinone radicals in the midgut, those that ingested 7.5% hydrolyzable tannin contained low levels of semiquinone radicals, and those that ingested 15% tannin contained greatly increased levels of semiquinone radicals. Ingested hydrolyzable tannins were also partially hydrolyzed in the midgut. However, increased levels of semiquinone radicals in the midgut were not associated with oxidative stress in midgut tissues. Instead, it appears that tannin consumption was associated with increased metabolic costs, as measured by the decreased efficiency of conversion of digested matter to body mass (ECD). Decreased ECD, in turn, decreased the overall efficiency of conversion of ingested matter to body mass (ECI). Contrary to our hypothesis, L. dispar larvae were able to maintain similar growth rates across all tannin treatment levels, in part, because of compensatory feeding. We conclude that hydrolyzable tannins act as “quantitative defenses” in the sense that high levels appear to be necessary to increase levels of semiquinone radicals in the midguts of caterpillars. However, these putative resistance factors are not sufficient to decrease the performance of tannin-tolerant caterpillars such as L. dispar.     

Genetic determination of tannins and herbivore resistance in <Emphasis Type="Italic">Quercus ilex</Emphasis>

Genetic variability of trees influences the chemical composition of tissues. This determines herbivore impact and, consequently, herbivore performance. We evaluated the independent effects of plant genotype and provenance on the tannin content of holm oak (Quercus ilex) and their consequences for herbivory and performance of gypsy moth (Lymantria dispar) larvae. Oak seedlings of 48 open-pollinated families from six populations were grown in a common garden in central Spain. Half the plants were subjected to defoliation by gypsy moth larvae and the other half were destructively sampled for chemical analysis. Tannin content of leaves did not differ significantly among populations but differed significantly among families. Estimates of heritability (h ²) and quantitative genetic differentiation among populations for tannin content (Q _ST) were 0.83 and 0.12, respectively. Defoliation was not related to the tannin content of plants nor to spine and trichome densities of leaves, although positive family–mean associations were observed between defoliation and both seed weight and plant height (P < 0.003). Among the oak populations, differential increase in larval weight gain with defoliation was observed. Leaf tannin content in Q. ilex is genetically controlled but does not influence defoliation or predict performance of the larvae. Different efficiencies of food utilisation depending on the oak genotypes indicate that other plant traits are influencing the feeding patterns and fitness of L. dispar and consequent population dynamics.     

Clumped distribution of oak leaf miners between and within plants

Leaf miners typically show non-random distributions both between and within plants. We tested the hypothesis that leaf miners on two oak species were clumped on individual host trees and individual branches and addressed whether clumping was influenced by aspects of plant quality and how clumping and/or interactions with other oak herbivores affected leaf-miner survivorship. Null models were used to test whether oak herbivores and different herbivore guilds co-occur at the plant scale. Twenty individual Quercus geminata plants and 20 Quercus laevis plants were followed over the season for the appearance of leaf miners and other herbivores, and foliar nitrogen, tannin concentration, leaf toughness and leaf water content were evaluated monthly for each individual tree. The survivorship of the most common leaf miners was evaluated by following the fate of marked mines in several combinations that involved intra- and inter-specific associations. We observed that all leaf miners studied were clumped at the plant and branch scale, and the abundance of most leaf-miner species was influenced by plant quality traits. Mines that occurred singly on leaves exhibited significantly higher survivorship than double and triple mines and leaves that contained a mine or a leaf gall and a mine and damage by chewers exhibited lowest survivorship. Although leaf miners were clumped at individual host trees, null model analyses indicated that oak herbivores do not co-occur significantly less than expected by chance and there was no evidence for biological mechanisms such as inter-specific competition determining community structure at the plant scale. Thus, despite co-occurrence resulting in reduced survivorship at the leaf scale, such competition was not strong enough to structure separation of these oak herbivore communities.     

Aboveground phytochemical responses to belowground herbivory in poplar trees and the consequence for leaf herbivore preference

Belowground (BG) herbivory can influence aboveground (AG) herbivore performance and food preference via changes in plant chemistry. Most evidence for this phenomenon derives from studies in herbaceous plants but studies in woody plants are scarce. Here we investigated whether and how BG herbivory on black poplar (Populus nigra) trees by Melolontha melolontha larvae influences the feeding preference of Lymantria dispar (gypsy moth) caterpillars. In a food choice assay, caterpillars preferred to feed on leaves from trees that had experienced attack by BG herbivores. Therefore, we investigated the effect of BG herbivory on the phytochemical composition of P. nigra trees alone and in combination with AG feeding by L. dispar caterpillars. BG herbivory did not increase systemic AG tree defences like volatile organic compounds, protease inhibitors and salicinoids. Jasmonates and salicylic acid were also not induced by BG herbivory in leaves but abscisic acid concentrations drastically increased together with proline and few other amino acids. Leaf coating experiments with amino acids suggest that proline might be responsible for the caterpillar feeding preference via presumptive phagostimulatory properties. This study shows that BG herbivory in poplar can modify the feeding preference of AG herbivores via phytochemical changes as a consequence of root‐to‐shoot signaling.     

The larval ecology of the butterfly Euphydryas desfontainii (Lepidoptera: Nymphalidae) in SW-Portugal: food plant quantity and quality as main predictors of habitat quality

Corresponding to theory, the persistence of metapopulations in fragmented landscapes depends on the area of suitable habitat patches and their degree of isolation, mediating the individual exchange between habitats. More recently, habitat quality has been highlighted as being equally important. We therefore assess the role of habitat area, isolation and quality for the occupancy of larval stages of the regionally threatened butterfly Euphydryas desfontainii occurring in grassland habitats comprising the host plant Dipsascus comosus. We put a special focus on habitat quality which was determined on two spatial scales: the landscape (among patches) and the within-patch level. On the landscape level, occupancy of caterpillars was determined by a presence-absence analysis at 28 host plant patches. On the within-patch level, oviposition site selection was studied by comparing 159 host plants with egg clutches to a random sample of 253 unoccupied host plants within six habitat patches. The occupancy of caterpillars and presence of egg clutches on host plants was then related to several predictors such as patch size and isolation on the landscape level and host plant characteristics and immediate surroundings on the within patch level. On the landscape level, only host plant abundance was related to the presence of caterpillars, while size and isolation did not differ between occupied and unoccupied patches. However, the weak discrimination of larval stages among patches changed on the within-patch level: here, several microclimatic predictors such as sunshine hours and topography, host plant morphology and phenology as well as further potential host plants in the immediate surroundings of the plant chosen for oviposition strongly determined the presence of egg clutches. We strongly suggest promoting the presence of the host plant in topographically and structurally rich habitat patches to offer potential for microclimatic compensation for a species considered threatened by climate change.