Generalist Herbivores Research Articles

Testing the shifting defense hypothesis for constitutive and induced resistance and tolerance

Biogeographical variation in herbivore communities may drive invasive plants to evolve lower defense against specialist herbivores but higher defense against generalist herbivores (shifting defense hypothesis, SDH). However, empirical tests on this topic have been strongly biased toward examining constitutive resistance and less is known about the evolution of induced resistance and tolerance. We examined constitutive and induced resistance and tolerance of the invasive plant, Alternanthera philoxeroides (alligator weed, Centrospermae: Amaranthaceae) against the specialist herbivore Agasicles hygrophila (Coleoptera, Chrysomelidae) and the generalist herbivore Spodoptera litura (Lepidoptera, Noctuidae), using genotypes from its introduced and native ranges. We found that introduced genotypes, compared to native genotypes, had higher constitutive resistance to the generalist herbivore, but similar constitutive resistance to the specialist herbivore. Furthermore, introduced genotypes, compared to native genotypes, had lower induced resistance to the generalist herbivore, but similar induced resistance to the specialist herbivore. Moreover, although introduced and native genotypes did not differ in their tolerance to the generalist, the introduced genotypes exhibited lower tolerance to the specialist than native ones. Therefore, while individual defense strategy does not indicate a shift from defense against the specialist to defense against the generalist, the results for the different defense strategies jointly support the SDH.

The role of volatile plant secondary metabolites as pre‐ingestive cues and potential toxins dictating diet selection by African elephants

Understanding the factors driving diet selection represents one of the main thrusts of contemporary foraging ecology. Many studies have focussed on nutritional factors and anti‐nutritional factors (such as tannins) that may describe diet selection of generalist mammalian herbivores, but these often do not explain the observed feeding patterns. Alternatively, generalist herbivores may be influenced by the presence, diversity and/or concentration of toxins. Plant volatiles have been understudied, yet may play an important role in this context. We aimed to determine whether diet selection by African elephants is better correlated with the presence and concentration of toxic plant secondary metabolites (PSMs) than with nutritional or anti‐nutritional factors. We also aimed to identify the specific aspects of the plant‐odour profiles that were correlated with dietary selection, which could be used as a pre‐ingestive cue for food selection. We found that elephant diet selection was not well described by crude protein, in vitro digestibility, tannin concentration and total polyphenol concentration. Instead, the best predictors of elephant diet choice were the number and absolute (total) emissions of potentially toxic volatile PSMs, specifically monoterpenes. Elephants avoided plant species that emitted a wider diversity of volatile PSMs, had higher total emissions, and higher numbers and emissions of monoterpenes. This suggests that PSMs with a high propensity to become toxins, such as monoterpenes, are likely a better indicator for elephant diet avoidance than nutritional or anti‐nutritional factors. Moreover, we demonstrated that elephants can differentiate between food items based on odour alone, specifically volatile monoterpenes, suggesting that these animals are relying on specific volatile cues emitted from plants to direct their foraging choices prior to ingesting selected plants.

Geography is more important than host plant use for the population genetic structure of a generalist insect herbivore.

Population divergence can occur due to mechanisms associated with geographic isolation and/or due to selection associated with different ecological niches. Much of the evidence for selection-driven speciation has come from studies of specialist insect herbivores that use different host plant species; however, the influence of host plant use on population divergence of generalist herbivores remains poorly understood. We tested how diet breadth, host plant species and geographic distance influence population divergence of the fall webworm (Hyphantria cunea; FW). FW is a broadly distributed, extreme generalist herbivore consisting of two morphotypes that have been argued to represent two different species: black-headed and red-headed. We characterized the differentiation of FW populations at two geographic scales. We first analysed the influence of host plant and geographic distance on genetic divergence across a broad continental scale for both colour types. We further analysed the influence of host plant, diet breadth and geographic distance on divergence at a finer geographic scale focusing on red-headed FW in Colorado. We found clear genetic and morphological distinction between red- and black-headed FW, and Colorado FW formed a genetic cluster distinct from other locations. Although both geographic distance and host plant use were correlated with genetic distance, geographic distance accounted for up to 3× more variation in genetic distance than did host plant use. As a rare study investigating the genetic structure of a widespread generalist herbivore over a broad geographic range (up to 3,000km), our study supports a strong role for geographic isolation in divergence in this system.

Rapid evolution of Medicago polymorpha during invasion shifts interactions with the soybean looper.

The Enemy Release Hypothesis posits that invasion of novel habitats can be facilitated by the absence of coevolved herbivores. However, a new environment and interactions with unfamiliar herbivores may impose selection on invading plants for traits that reduce their attractiveness to herbivores or for enhanced defenses compared to native host plants, leading to a pattern similar to enemy release but driven by evolutionary change rather than ecological differences. The Shifting Defense Hypothesis posits that plants in novel habitats will shift from specialized defense mechanisms to defense mechanisms effective against generalist herbivores in the new range. We tested these ideas by comparing herbivore preference and performance of native (Eurasia)‐ and invasive (New World)‐range Medicago polymorpha, using a generalist herbivore, the soybean looper, that co‐occurs with M. polymorpha in its New World invaded range. We found that soybean loopers varied in preference and performance depending on host genotype and that overall the herbivore preferred to consume plant genotypes from naïve populations from Eurasia. This potentially suggests that range expansion of M. polymorpha into the New World has led to rapid evolution of a variety of traits that have helped multiple populations become established, including those that may allow invasive populations to resist herbivory. Thus, enemy release in a novel range can occur through rapid evolution by the plant during invasion, as predicted by the Shifting Defense Hypothesis, rather than via historical divergence.

Circadian Network Interactions with Jasmonate Signaling and Defense.

Plants experience specific stresses at particular, but predictable, times of the day. The circadian clock is a molecular oscillator that increases plant survival by timing internal processes to optimally match these environmental challenges. Clock regulation of jasmonic acid (JA) action is important for effective defenses against fungal pathogens and generalist herbivores in multiple plant species. Endogenous JA levels are rhythmic and under clock control with peak JA abundance during the day, a time when plants are more likely to experience certain types of biotic stresses. The expression of many JA biosynthesis, signaling, and response genes is transcriptionally controlled by the clock and timed through direct connections with core clock proteins. For example, the promoter of Arabidopsis transcription factor MYC2, a master regulator for JA signaling, is directly bound by the clock evening complex (EC) to negatively affect JA processes, including leaf senescence, at the end of the day. Also, tobacco ZEITLUPE, a circadian photoreceptor, binds directly to JAZ proteins and stimulates their degradation with resulting effects on JA root-based defenses. Collectively, a model where JA processes are embedded within the circadian network at multiple levels is emerging, and these connections to the circadian network suggest multiple avenues for future research.

Divergent host plant utilization by adults and offspring is related to intra-plant variation in chemical defences.

Adult and juvenile herbivores of the same species can use divergent feeding strategies, and thus may inhabit and consume different parts of the plant. Because the expression of chemical defences often differs between host plant tissues, this variation may result in disparate performance outcomes for adult and juvenile conspecifics that feed on distinct dietary substrates. The goal of this study was to evaluate how host range may differ between adults and juveniles in a generalist herbivore. We addressed the impacts of among- and within-plant defence variation using the wood-feeding Asian longhorned beetle (Anoplophora glabripennis) and three host plants having a range of putative resistance. Impacts of host plants on adult and offspring performance were assessed using a series of controlled bioassays. We evaluated adult-feeding and egg-laying behaviours in choice and no-choice experiments using the different hosts, and subsequent offspring establishment. We then evaluated host plant chemical composition related to nutrition and defence. Different plants had strong impacts on adult performance, but these patterns did not extend to effects on offspring. Females were capable of developing eggs when provided Acer rubrum, but not Populus deltoides or Populus tomentosa. Females that produced eggs by feeding on A.rubrum, however, deposited eggs into all three plant species. Larvae hatched and consumed tissues in all three hosts. The differences between adult and juvenile utilization of Populus spp. were reflected in markedly higher salicinoid phenolic concentrations in bark (>2% dw), while wood had trace quantities. Our results demonstrate that plant resistance mechanisms can differentially act upon adult and juvenile life stages of a polyphagous herbivore when there is differential expression of chemical defences among plant tissue types. Anoplophora glabripennis has been a globally successful invader due in part to its broad host range, and our results suggest a mechanism that permits the beetle to exploit marginally resistant plants. This study has implications for how host range differs between insect feeding stages, which is particularly important for invasive, polyphagous species encountering novel food sources.

Chemical Responses of Nicotiana tabacum (Solanaceae) Induced by Vibrational Signals of a Generalist Herbivore.

Plants are able to sense their environment and respond appropriately to different stimuli. Vibrational signals (VS) are one of the most widespread yet understudied ways of communication between organisms. Recent research into the perception of VS by plants showed that they are ecologically meaningful signals involved in different interactions of plants with biotic and abiotic agents. We studied changes in the concentration of alkaloids in tobacco plants induced by VS produced by Phthorimaea operculella (Lepidoptera: Gelechiidae), a generalist caterpillar that naturally feeds on the plant. We measured the concentration of nicotine, nornicotine, anabasine and anatabine in four treatments applied to 11-weeks old tobacco plant: a) Co = undamaged plants, b) Eq = Playback equipment attached to the plant without VS, c) Ca = Plants attacked by P. operculella herbivory and d) Pl = playback of VS of P. operculella feeding on tobacco. We found that nicotine, the most abundant alkaloid, increased more than 2.6 times in the Ca and Pl treatments as compared with the Co and Eq treatments, which were similar between them. Nornicotine, anabasine and anatabine were mutually correlated and showed similar concentration patterns, being higher in the Eq treatment. Results are discussed in terms of the adaptive significance of plant responses to ecologically important VS stimuli.

Enemy release from the effects of generalist granivores can facilitate Bromus tectorum invasion in the Great Basin Desert.

The enemy release hypothesis (ERH) of plant invasion asserts that natural enemies limit populations of invasive plants more strongly in native ranges than in non‐native ranges. Despite considerable empirical attention, few studies have directly tested this idea, especially with respect to generalist herbivores. This knowledge gap is important because escaping the effects of generalists is a critical aspect of the ERH that may help explain successful plant invasions. Here, we used consumer exclosures and seed addition experiments to contrast the effects of granivorous rodents (an important guild of generalists) on the establishment of cheatgrass (Bromus tectorum) in western Asia, where cheatgrass is native, versus the Great Basin Desert, USA, where cheatgrass is exotic and highly invasive. Consistent with the ERH, rodent foraging reduced cheatgrass establishment by nearly 60% in western Asia but had no effect in the Great Basin. This main result corresponded with a region‐specific foraging pattern: rodents in the Great Basin but not western Asia generally avoided seeds from cheatgrass relative to seeds from native competitors. Our results suggest that enemy release from the effects of an important guild of generalists may contribute to the explosive success of cheatgrass in the Great Basin. These findings corroborate classic theory on enemy release and expand our understanding of how generalists can influence the trajectory of exotic plant invasions.

Negative frequency-dependent foraging behaviour in a generalist herbivore (Alces alces) and its stabilizing influence on food web dynamics.

Resource selection is widely appreciated to be context-dependent and shaped by both biological and abiotic factors. However, few studies have empirically assessed the extent to which selective foraging behaviour is dynamic and varies in response to environmental conditions for free-ranging animal populations. Here, we assessed the extent that forage selection fluctuated in response to different environmental conditions for a free-ranging herbivore, moose (Alces alces), in Isle Royale National Park, over a 10-year period. More precisely, we assessed how moose selection for coniferous versus deciduous forage in winter varied between geographic regions and in relation to (a) the relative frequency of forage types in the environment (e.g. frequency-dependent foraging behaviour), (b) moose abundance, (c) predation rate (by grey wolves) and (d) snow depth. These factors are potentially important for their influence on the energetics of foraging. We also built a series of food-chain models to assess the influence of dynamic foraging strategies on the stability of food webs. Our analysis indicates that moose exhibited negative frequency dependence, by selectively exploiting rare resources. Frequency-dependent foraging was further mediated by density-dependent processes, which are likely to be predation, moose abundance or some combination of both. In particular, frequency dependence was weaker in years when predation risk was high (i.e. when the ratio of moose to wolves was relatively low). Selection for conifers was also slightly weaker during deep snow years. The food-chain analysis indicates that the type of frequency-dependent foraging strategy exhibited by herbivores had important consequences for the stability of ecological communities. In particular, the dynamic foraging strategy that we observed in the empirical analysis (i.e. negative frequency dependence being mediated by density-dependent processes) was associated with more stable food web dynamics compared to fixed foraging strategies. The results of this study indicated that forage selection is a complex ecological process, varying in response to both biological (predation and moose density) and abiotic factors (snow depth) and over relatively small spatial scales (between regions). This study also provides a useful framework for assessing the influence of other aspects of foraging behaviour on the stability of food web dynamics.

Role of cyanogenic glycosides in the seeds of wild lima bean, Phaseolus lunatus: defense, plant nutrition or both?

Cyanogenic glycosides present in the seeds of wild lima bean plants are associated with seedling defense but do not affect seed germination and seedling growth. Wild lima bean plants contain cyanogenic glycosides (CNGs) that are known to defend the plant against leaf herbivores. However, seed feeders appear to be unaffected despite the high levels of CNGs in the seeds. We investigated a possible role of CNGs in seeds as nitrogen storage compounds that influence plant growth, as well as seedling resistance to herbivores. Using seeds from four different wild lima bean natural populations that are known to vary in CNG levels, we tested two non-mutually exclusive hypotheses: (1) seeds with higher levels of CNGs produce seedlings that are more resistant against generalist herbivores and, (2) seeds with higher levels of CNGs germinate faster and produce plants that exhibit better growth. Levels of CNGs in the seeds were negatively correlated with germination rates and not correlated with seedling growth. However, levels of CNGs increased significantly soon after germination and seeds with the highest CNG levels produced seedlings with higher CNG levels in cotyledons. Moreover, the growth rate of the generalist herbivore Spodoptera littoralis was lower in cotyledons with high-CNG levels. We conclude that CNGs in lima bean seeds do not play a role in seed germination and seedling growth, but are associated with seedling defense. Our results provide insight into the potential dual function of plant secondary metabolites as defense compounds and storage molecules for growth and development.

Evaluating mechanisms of plant‐mediated effects on herbivore persistence and occupancy across an ecoregion

AbstractContemporary climate change is rapidly creating one of the greatest challenges for management and conservation during the 21st century. Mountain ecosystems, which have a high degree of spatial heterogeneity and contain numerous habitat specialists, have been identified as particularly vulnerable. We used data from multiple years across sites spanning a >40 million ha ecoregion to test hypotheses regarding how community‐level characteristics of vegetation may affect a mammalian generalist herbivore, the American pika (Ochotona princeps). We examined patterns of pika persistence across sites in the hydrographic Great Basin, and occupancy within a subset of these sites. We used mixed‐effects logistic regression models to compare evidence in support of competing explanations for each pattern within an information–theoretic framework (using Akaike's information criterion). Models reflected four hypothesized classes of mechanisms related to nutritional ecology, ecosystem function, indirect indication of climatic effects, and (synergistic) combinations of these three classes. At the site level, models reflecting synergistic effects received the most support. At the within‐site level, support appeared to be split equally among hypotheses containing predictors related to either nutritional ecology or indirect climate effects. Well‐supported predictors included cover of invasive plant species, cover of more‐xeric plant species, species evenness, and proportion of graminoid species. Our results both (1) identify important aspects of vegetation communities that may influence herbivore distribution in mountainous areas across a large, diverse geographic region, and (2) contribute to an improved understanding of how mountain ecosystems may be affected by ongoing climate change, more broadly.

High-resolution QTL mapping in Tetranychus urticae reveals acaricide-specific responses and common target-site resistance after selection by different METI-I acaricides

Arthropod herbivores cause dramatic crop losses, and frequent pesticide use has led to widespread resistance in numerous species. One such species, the two-spotted spider mite, Tetranychus urticae, is an extreme generalist herbivore and a major worldwide crop pest with a history of rapidly developing resistance to acaricides. Mitochondrial Electron Transport Inhibitors of complex I (METI-Is) have been used extensively in the last 25 years to control T. urticae around the globe, and widespread resistance to each has been documented. METI-I resistance mechanisms in T. urticae are likely complex, as increased metabolism by cytochrome P450 monooxygenases as well as a target-site mutation have been linked with resistance.To identify loci underlying resistance to the METI-I acaricides fenpyroximate, pyridaben and tebufenpyrad without prior hypotheses, we crossed a highly METI-I-resistant strain of T. urticae to a susceptible one, propagated many replicated populations over multiple generations with and without selection by each compound, and performed bulked segregant analysis genetic mapping. Our results showed that while the known H92R target-site mutation was associated with resistance to each compound, a genomic region that included cytochrome P450-reductase (CPR) was associated with resistance to pyridaben and tebufenpyrad. Within CPR, a single nonsynonymous variant distinguished the resistant strain from the sensitive one. Furthermore, a genomic region linked with tebufenpyrad resistance harbored a non-canonical member of the nuclear hormone receptor 96 (NHR96) gene family. This NHR96 gene does not encode a DNA-binding domain (DBD), an uncommon feature in arthropods, and belongs to an expanded family of 47 NHR96 proteins lacking DBDs in T. urticae. Our findings suggest that although cross-resistance to METI-Is involves known detoxification pathways, structural differences in METI-I acaricides have also resulted in resistance mechanisms that are compound-specific.

Substrate specificity and promiscuity of horizontally transferred UDP-glycosyltransferases in the generalist herbivore Tetranychus urticae

Uridine diphosphate (UDP)-glycosyltransferases (UGTs) catalyze the addition of UDP-sugars to small hydrophobic molecules, turning them into more water-soluble metabolites. While their role in detoxification is well documented for vertebrates, arthropod UGTs have only recently been linked to the detoxification and sequestration of plant toxins and insecticides. The two-spotted spider mite Tetranychus urticae is a generalist herbivore notorious for rapidly developing resistance to insecticides and acaricides. We identified a set of eight UGT genes that were overexpressed in mites upon long-term acclimation or adaptation to a new host plant and/or in mite strains highly resistant to acaricides. Functional expression revealed that they were all catalytically active and that the majority preferred UDP-glucose as activated donor for glycosylation of model substrates. A high-throughput substrate screening of both plant secondary metabolites and pesticides revealed patterns of both substrate specificity and promiscuity. We further selected nine enzyme-substrate combinations for more comprehensive analysis and determined steady-state kinetic parameters. Among others, plant metabolites such as capsaicin and several flavonoids were shown to be glycosylated. The acaricide abamectin was also glycosylated by two UGTs and one of these was also overexpressed in an abamectin resistant strain. Our study corroborates the potential role of T. urticae UGTs in detoxification of both synthetic and natural xenobiotic compounds and paves the way for rapid substrate screening of arthropod UGTs.

Trichoplusia ni (Lepidoptera: Noctuidae) Qualitative and Quantitative Sequestration of Host Plant Carotenoids.

Carotenoids are fundamental precursors for hormones and antioxidants, and insects must acquire carotenoids from their diet. Previous research has shown that insects can selectively absorb dietary carotenoids, often modifying them qualitatively or quantitatively, and quantities may be proportional to those found in the diet. Trichoplusia ni Hübner is a generalist herbivore with host plants varying greatly in carotenoid profiles and concentrations. Larvae sequester carotenoids in their hemolymph, and carotenoid sequestration contributes to their cryptic green coloration. Our objectives were to compare the types of carotenoids found in T. ni and their host plants to determine whether qualitative changes occurred, and compare the amounts of sequestered carotenoids in T. ni reared upon different host plants to determine whether quantitative variation influences sequestration. To fulfill these objectives, larvae were fed romaine lettuce (Lactuca sativa L. [Asterales: Asteraceae] var. longifolia) or kale (Brassica oleracea L. [Brassicales: Brassicaceae] var. sabellica) for a period of 5 d, and sequestered carotenoids from the entire insect were resolved with thin-layer chromatography and measured with spectrophotometer. All carotenoids resolved from plants were also resolved from larvae, and although the carotenoids of plants differed quantitatively, the sequestered carotenoids did not differ between host plants. Regardless of host plant species, T. ni sequestered carotenoids at concentrations up to 20 times higher than the concentrations found in the plants. Future research may be able to explicitly identify enzyme systems involved in the transport and modification of carotenoids in T. ni and other animals.

The Role of Enemy-Mediated Competition in Determining Fitness of a Generalist Herbivore

The influence of indirect competition on insect communities is not well understood. We studied how the fitness of a gregarious lepidopteran species, fall webworm, Hyphantria cunea (Drury), is affected by another gregarious caterpillar species, western tent caterpillar, Malacosoma californicum (Packard). Fall webworm and tent caterpillars both feed on chokecherry, Prunus virginiana L., and create silken structures in which they reside until just prior to pupation. Preliminary observations suggested that predators and parasitoids reside in tent caterpillar tents after tents have been abandoned by the caterpillars, thus increasing the presence of enemies near tents. We hypothesized that when reared near tent caterpillar tents, fall webworms are less fit because of increased predation from nearby enemies (enemy-mediated competition). We tested our hypothesis at two field sites by placing fall webworm larvae in the field with or without a tent caterpillar tent nearby. We compared differences in survival rates, parasitism rates, and fitness between the larvae reared with and without tents nearby. At one field site, webworm larvae near tent caterpillar tents disappeared ∼30% faster than did larvae without tents nearby. Although survival or fitness did not differ significantly between larvae reared with or without tents nearby, distance separating fall webworm larvae from a tent caterpillar tent might be important. Our results suggested that enemy-mediated competition might affect the fitness of fall webworm larvae and should be investigated further.

Snail herbivory affects seedling establishment in a temperate forest in the Ozarks

Abstract Species‐specific herbivores are hypothesized to maintain plant diversity by preventing the dominance of any one plant species. However, a large proportion of herbivores have wide host ranges, and these generalists could have similar effects on plant community composition if they exhibit differences in their host preference. Here, we coupled laboratory and field experiments to test whether a common forest‐understorey snail (Neohelix alleni), a generalist herbivore, has the potential to influence forest composition through differential preference of their plant hosts. We first performed a cafeteria‐style experiment to test whether N. alleni showed feeding preferences among leaves of five tree species and one shrub common to temperate forests in Missouri, USA. We then conducted a factorial snail and deer exclusion experiment to decouple the effects of snail herbivory from those of white‐tailed deer on seedling establishment of 1‐month‐old newly germinated seedlings of these six woody species in the field. Finally, we examined whether variation in both snail feeding preference and experimentally measured effects of snails on seedling establishment across plant species were related to their relative abundance measured in a 12‐ha forest plot. In the laboratory, we found that snails preferred leaves of woody species that were less abundant in the forests relative to those species that were more common. In the forest, we found that experimental exclusion of snails had a stronger positive effect on seedling above‐ground biomass and survival over a 1‐year period than did exclusion of deer. Plant species found to be more preferred in the laboratory were also those that had lower seedling establishment in the forest due to the negative effects of snails. Synthesis. Collectively, our results suggest that greater susceptibility to snail herbivory limits seedling establishment, perhaps contributing to differences in tree species relative abundance. Although less appreciated than their insect and mammal counterparts, herbivory by snails may be significant drivers to the assembly of forest tree communities.

White-tailed deer browse preference for an invasive shrub, Amur honeysuckle (Lonicera maackii), depends on woody species composition

AbstractSelective browsing by abundant, generalist herbivores on preferred species could allow less-preferred invasive species to flourish. We tested such an effect by examining rates at which white-tailed deer (Odocoileus virginianus Zimmermann) consume Amur honeysuckle [Lonicera maackii (Rupr.) Herder], an invasive shrub, relative to native woody species across eight forested sites in southwestern Ohio. We tested three hypotheses: (1) deer prefer to browse on L. maackii versus other woody plants; (2) L. maackii is not a preferred source of browse, but is consumed where preferred foods are scarce; and (3) L. maackii provides an important food resource for deer in early spring when other foods are scarce. We used counts of browsed and unbrowsed twigs of each species to calculate, for each site, both the proportion of each species’ twigs browsed and the degree to which deer selectively favor each species (“electivity”) during early to mid-growing season. Across the eight sites, electivity of L. maackii correlated with the proportion of its twigs browsed, and both measures were negatively associated with the density of L. maackii twigs. Lonicera maackii electivity was negative at most sites, indicating it is generally not preferred, undermining hypothesis 1. The hypothesis that deer consume L. maackii when more-preferred foods are depleted was not supported, as there was no negative relationship between L. maackii browse and the density of twigs of more-preferred species. We found a negative relationship between the proportion of L. maackii twigs browsed and the density of L. maackii among sites, which supports the third hypothesis. This finding, combined with seasonal patterns of deer browse on L. maackii, indicates that this invasive shrub is an important source of browse for deer during early spring, regardless of its abundance.

The influence of warming on the biogeographic and phylogenetic dependence of herbivore-plant interactions.

Evolutionary experience and the phylogenetic relationships of plants have both been proposed to influence herbivore–plant interactions and plant invasion success. However, the direction and magnitude of these effects, and how such patterns are altered with increasing temperature, are rarely studied. Through laboratory functional response experiments, we tested whether the per capita feeding efficiency of an invasive generalist herbivore, the golden apple snail, Pomacea canaliculata, is dependent on the biogeographic origin and phylogenetic relatedness of host plants, and how increasing temperature alters these dependencies. The feeding efficiency of the herbivore was highest on plant species with which it had no shared evolutionary history, that is, novel plants. Further, among evolutionarily familiar plants, snail feeding efficiency was higher on those species more closely related to the novel plants. However, these biogeographic dependencies became less pronounced with increasing temperature, whereas the phylogenetic dependence was unaffected. Collectively, our findings indicate that the susceptibility of plants to this invasive herbivore is mediated by both biogeographic origin and phylogenetic relatedness. We hypothesize that warming erodes the influence of evolutionary exposure, thereby altering herbivore–plant interactions and perhaps the invasion success of plants.

Herbivore accumulation on invasive alien plants increases the distribution range of generalist herbivorous insects and supports proliferation of non-native insect pests

Invasive alien plant species have become dominant components of many landscapes, where they are indicators of ecological disequilibria. They coexist and compete with native plants, disrupting a wide range of trophic interactions, and must cope with a new array of herbivores. We examined factors influencing the occurrence of invertebrate herbivores on two invasive alien plant species, Acacia dealbata and Carpobrotus edulis, and surrounding native vegetation in the northwestern Iberian Peninsula. The aims were to assess the accumulation of herbivorous insects, including the distribution and frequency of herbivorous insects at different invasion levels (low, medium and high), and to evaluate whether introduced plants favour native or exotic, and generalist or specialist herbivorous insects in newly evolving plant–herbivore networks. To achieve these objectives, we surveyed A. dealbata along transects in five mixed pine forests and four shrublands, and C. edulis in nine coastal areas using quadrats. We identified nine herbivore species feeding on one or both of these species and determined that herbivore species composition in forest and coastal areas is directly tied to host preference, as well as related to invasion levels of A. dealbata in forest areas. On average, the introduced species in both the forest and coastal areas had approximately threefold more herbivore species (~ 50% of them exotics) than native plants. The introduced plants in forest areas had approximately eightfold more species interaction strength between herbivores and plants (a measure of the importance of each plant species to each herbivore). Overall, generalist herbivores were favoured by the invasion of the introduced plants. The results also demonstrate that exotic insects are supported by introduced plants, increasing the local occurrence and range of insect pests. Considering our observations, management strategies should be implemented to favour the restoration and protection of native areas for conserving biodiversity.

Plant defense against generalist herbivores in the forest understory: a phylogenetic comparison of native and invasive species

Invasive woody plants from Europe and Asia tend to be more productive than co-occurring native species in deciduous forests of Eastern North America, but community-scale drivers of invasion success remain unknown. If increased productivity in invaders relative to natives comes at the expense of reduced relative allocation to defenses, generalist herbivores may preferentially consume invasive species, potentially reducing the effectiveness of invader growth advantage. We compared leaf traits related to herbivory (nitrogen concentration, total phenolics, cellulose and lignin concentrations, and leaf dry matter content) in 20 phylogenetically paired native and non-native, invasive species and evaluated species palatability to two generalist herbivores: one non-native (European gypsy moth, Lymantria dispar) and one native (fall webworm, Hyphantria cunia). We also evaluated potential physiological tradeoffs between defense-related leaf traits, leaf production, and carbon storage in this group of species. Invasive plants had higher leaf N and lignin concentrations than related native species (0.5% and 5%, respectively), but gypsy moth and fall webworm growth were not associated with plant nativity. Plant defense traits did not predict differences in gypsy moth growth, but fall webworm growth was marginally negatively associated with leaf N. We found no evidence of tradeoffs between defense-related leaf traits, leaf production, and carbon storage, indicating a limited role for carbon-based tradeoffs relating to growth and herbivory in these species, although other unmeasured carbon pools including reproduction may impact these relationships. Woody invaders continue to spread in Eastern North American forest understories, but we found no evidence of inhibition or facilitation by generalist insect herbivores, despite differences among species in leaf traits associated with defense.