Generalist Herbivores Research Articles

Interactions count: plant origin, herbivory and disturbance jointly explain seedling recruitment and community structure

Herbivory and disturbance are major drivers of biological invasions, but it is unclear how they interact to determine exotic vs. native seedling recruitment and what consequences arise for biodiversity and ecosystem functioning. Previous studies neglected the roles of different, potentially interacting, guilds of generalist herbivores such as rodents and gastropods. We therefore set up a full-factorial rodent exclusion x gastropod exclusion x disturbance x seed-addition experiment in a grassland community in Central Germany and measured early seedling recruitment, as well as species richness, species composition and aboveground biomass. Gastropod herbivory reduced the positive effect of disturbance on seedling recruitment, particularly for exotic species. Rodent herbivory had weak positive effects on seedling recruitment at undisturbed sites, irrespective of species origin. This effect was likely driven by their strong negative effect on productivity. Interactive effects between both herbivore guilds became only evident for species richness and composition. How many species established themselves depended on disturbance, but was independent of species origin. The fewer exotic species that established themselves increased productivity to a stronger extent compared to native species. Our study highlights that joint effects of disturbance, herbivory and species origin shape early recruitment, while they only weakly affect biodiversity and ecosystem functioning.

Effects of an invasive shrub, Lonicera maackii, and a generalist herbivore, white-tailed deer, on forest floor plant community composition

Both overabundant ungulates and invasive shrubs negatively impact forest floor plants, but few studies have investigated their interaction. We tested for direct effects and interactions of white-tailed deer, Odocoileus virginianus, and the invasive shrub, Lonicera maackii, on forest floor plant richness and composition in five sites in southwest Ohio. In each site a deer exclosure and an unfenced plot (each 20×20m) were established in 2010; in half of each plot all L. maackii was removed. Each spring and summer, 2011–2014, percent cover of each plant species in 50×20cm subplots was recorded.Herb layer species richness was greater where L. maackii was removed, and this effect tended to be stronger where deer were excluded. Species composition in the spring was not affected by treatments, whereas summer species composition (in one year) was affected by L. maackii treatment. Cover of graminoids, spring herbs, and vines were lower where L. maackii was intact, whereas cover of shrubs and tree seedlings were lower, and cover of bare ground higher, where deer had access.Our findings reveal that deer and L. maackii negatively impact different components of the forest herb layer. However, the combined effects on species richness tended to be antagonistic (less than additive), possibly due to deer browse reducing the cover of this invasive shrub. This suggests that the diversity of the herb layer will be enhanced more by both removal of invasive shrubs and reduction of the deer population than by either management action alone.

Continent-Wide Climatic Variation Drives Local Adaptation in North American White Clover.

Climate-associated clines in adaptive polymorphisms are commonly cited as evidence of local adaptation within species. However, the contribution of the clinally varying trait to overall fitness is often unknown. To address this question, we examined survival, vegetative growth, and reproductive output in a central US common garden experiment using 161 genotypes of white clover (Trifolium repens L.) originating from 15 locations across North America. White clover is polymorphic for cyanogenesis (hydrogen cyanide release upon tissue damage), a chemical defense against generalist herbivores, and climate-associated cyanogenesis clines have repeatedly evolved across the species range. Over a 12-month experiment, we observed striking correlations between the population of origin and plant performance in the common garden, with climatic distance from the common garden site predicting fitness more accurately than geographic distance. Assessments of herbivore leaf damage over the 2015 growing season indicated marginally lower herbivory on cyanogenic plants; however, this effect did not result in increased fitness in the common garden location. Linear mixed modeling suggested that while cyanogenesis variation had little predictive value for vegetative growth, it is as important as climatic variation for predicting reproductive output in the central United States. Together, our findings suggest that knowledge of climate similarity, as well as knowledge of locally favored adaptive traits, will help to inform transplantation strategies for restoration ecology and other conservation efforts in the face of climate change.

Similarity in volatile communities leads to increased herbivory and greater tropical forest diversity.

A longstanding paradigm in ecology is that there are positive associations between herbivore diversity, specialization, and plant species diversity, with a focus on taxonomic diversity. However, phytochemical diversity is also an informative metric, as insect herbivores interact with host plants not as taxonomic entities, but as sources of nutrients, primary metabolites, and mixtures of attractant and repellant chemicals. The present research examines herbivore responses to phytochemical diversity measured as volatile similarity in the tropical genus Piper. We quantified associations between naturally occurring volatile variation and herbivory by specialist and generalist insects. Intraspecific similarity of volatile compounds across individuals was associated with greater overall herbivory. A structural equationmodel supported the hypothesis that plot level volatile similarity caused greater herbivory by generalists, but not specialists, which led to increased understory plant richness. These results demonstrate that using volatiles as a functional diversity metric is informative for understanding tropical forest diversity and indicate that generalist herbivores contribute to the maintenance of diversity.

Relationship of different feeding groups of true bugs (Hemiptera: Heteroptera) with habitat and landscape features in Pannonic salt grasslands

Eastern European grasslands are still inhabited by a rich arthropod fauna, but the drivers and mechanisms influencing their communities have to be understood to ensure their future survival. Heteroptera communities were studied in 20 plot-pairs in Pannonic salt steppe–salt marsh mosaics in Hungary. The effects of vegetation characteristics, landscape diversity and the proportion of surrounding grasslands on the composition, species richness and abundance of different feeding groups of true bugs (carnivores, specialist and generalist herbivores) were examined using ordinations and mixed-effect models. We found distinct herbivorous assemblages corresponding to microtopography-driven differences in water regime and vegetation between steppe and marsh plots, but this pattern was less pronounced in carnivorous assemblages. A higher species richness of true bugs was found in the more diverse steppe vegetation than in the salt marsh vegetation, while the abundance pattern of true bugs was opposite. Landscape diversity had a positive effect on the species richness and abundance of generalist herbivores and carnivores. Our results suggested that generalist herbivores and carnivores appear to drive diversity patterns in the local landscape due to their high dispersal abilities and the broader range of resources they can utilize. Specialist herbivores strongly influence the local insect biomass in relation to the distribution and density of their host plants. The present study highlights the importance of both habitat and landscape diversity for local insect diversity in Pannonic salt grasslands and suggests that the main threats for arthropod diversity are those processes and activities that homogenize these areas.

The Perennial Penstemon: Variation in Defensive Chemistry Across Years, Populations, and Tissues.

Plants produce a variety of secondary metabolites that function as a defense against their natural enemies. Production of these secondary metabolites is genetically controlled, but is also phenotypically plastic and varies in response to both biotic and abiotic factors. Therefore, plant species may vary widely in their chemical defenses and such variation can be evident at temporal, spatial and tissue levels. Focusing on the chemical defenses of a native Colorado wildflower, Penstemon virgatus, we assessed the variation in iridoid glycoside (IG) content across two non-consecutive growing seasons, six natural populations and three tissue types: leaves, stems and flowers. Our results indicate that P. virgatus plants contain high concentrations of IGs (mean=23.36% dry weight of leaves) and that IGs were differentially allocated among tissue types. Leaves contained the highest concentration of IGs, which varied quantitatively between sampling years, among plant populations, and plant parts. We also quantified leaf herbivore damage at all six populations but we found very little herbivore damage. Our study indicates that the IG concentrations of P. virgatus plants are both spatially and temporally variable. Furthermore, the high concentrations of secondary metabolites combined with the low levels of damage suggest that these plants are well defended against generalist herbivores.

A Generalist Herbivore Copes with Specialized Plant Defence: the Effects of Induction and Feeding by Helicoverpa armigera (Lepidoptera: Noctuidae) Larvae on Intact Arabidopsis thaliana (Brassicales) Plants.

Plants of the Brassicaceae are defended from feeding by generalist insects by constitutively-expressed and herbivory-induced glucosinolates (GS). We induced Arabidopsis plants 1, 16 and 24h prior to allowing neonate larvae of the generalist Helicoverpa armigera to feed on whole plants for 72h. These plants were subsequently retested with another group of neonates for a further 72h. We used wild-type A. thaliana Col-0, and mutant lines lacking indolic GS, aliphatic GS or all GS. We hypothesized that larvae would not grow well on defended plants (WT) compared to those lacking GS, and would not grow well if plants had been primed or fed on for longer, due to the expected induced GS. There was survivorship on all lines suggesting H. armigera is a suitable generalist for these experiments. Larvae performed less well on wild-type and no indolic lines than on no aliphatic and no GS lines. Larvae distributed feeding damage extensively in all lines, more so on wild type and no-indolic lines. Contrary to expectations, larvae grew better on plants that had been induced for 1 to 16h than on un-induced plants suggesting they moved to and selected less toxic plant parts within a heterogeneously defended plant. Performance declined on all lines if plants had been induced for 24h, or had been fed upon for a further 72h. However, contrary to expectation, individual and total GS did not increase after these two treatments. This suggests that Arabidopsis plants induce additional (not GS) defenses after longer induction periods.

Interactions between Plant Metabolites Affect Herbivores: A Study with Pyrrolizidine Alkaloids and Chlorogenic Acid.

The high structural diversity of plant metabolites suggests that interactions among them should be common. We investigated the effects of single metabolites and combinations of plant metabolites on insect herbivores. In particular we studied the interacting effects of pyrrolizidine alkaloid (PAs), and chlorogenic acid (CGA), on a generalist herbivore, Frankliniella occidentalis. We studied both the predominantly occurring PA N-oxides and the less frequent PA free bases. We found antagonistic effects between CGA and PA free bases on thrips mortality. In contrast PA N-oxides showed synergistic interactions with CGA. PA free bases caused a higher thrips mortality than PA N-oxides while the reverse was through for PAs in combination with CGA. Our results provide an explanation for the predominate storage of PA N-oxides in plants. We propose that antagonistic interactions represent a constraint on the accumulation of plant metabolites, as we found here for Jacobaea vulgaris. The results show that the bioactivity of a given metabolite is not merely dependent upon the amount and chemical structure of that metabolite, but also on the co-occurrence metabolites in, e.g., plant cells, tissues and organs. The significance of this study is beyond the concerns of the two specific groups tested here. The current study is one of the few studies so far that experimentally support the general conception that the interactions among plant metabolites are of great importance to plant-environment interactions.

Expressional divergence of insect GOX genes: From specialist to generalist glucose oxidase

Insect herbivores often secrete glucose oxidase (GOX) onto plants to counteract plant defenses and potential pathogens. Whether generalist herbivores always have significantly higher GOX activities than their specialist counterparts at any comparable stage or conditions and how this is realized remain unknown. To address these two general questions, we subjected larvae of a pair of sister species differed mainly in host range, the generalist Helicoverpa armigera and its specialist counterpart Helicoverpa assulta, to the same sets of stage, protein to digestible carbohydrate (P:C) ratio, allelochemical or host plant treatments for simultaneous analyses of GOX transcripts and activities in their labial glands. GOX activity and transcripts are upregulated concurrently with food ingestion and body growth, downregulated with stopping ingestion and wandering for pupation in both species. The three tested host plants upregulated GOX transcripts, and to a lesser extent, GOX activity in both species. There were significant differences in both GOX transcripts and activity elicited by allelochemicals, but only in GOX transcripts by P:C ratios in both species. GOX activities were higher in H. armigera than H. assulta in all the comparable treatments, but GOX transcripts were significantly higher either in generalists or in specialists, depending on the developmental stages, host plants, P:C ratio and allelochemicals they encounter. These data indicate that the greater GOX activity in generalist herbivores is not achieved by greater transcription rate, but by greater transcript stability, greater translation rate, better enzyme stability and/or their combination.

Leaf rust infection reduces herbivore-induced volatile emission in black poplar and attracts a generalist herbivore.

Plants release complex volatile blends after separate attack by herbivores and pathogens, which play many roles in interactions with other organisms. Large perennials are often attacked by multiple enemies, but the effect of combined attacks on volatile emission is rarely studied, particularly in trees. We infested Populus nigra trees with a pathogen, the rust fungus Melampsora larici-populina, and Lymantria dispar caterpillars alone and in combination. We investigated poplar volatile emission and its regulation, as well as the behavior of the caterpillars towards volatiles from rust-infected and uninfected trees. Both the rust fungus and the caterpillars alone induced volatile emission from poplar trees. However, the herbivore-induced volatile emission was significantly reduced when trees were under combined attack by the herbivore and the fungus. Herbivory induced terpene synthase transcripts as well as jasmonate concentrations, but these increases were suppressed when the tree was additionally infected with rust. Caterpillars preferred volatiles from rust-infected over uninfected trees. Our results suggest a defense hormone crosstalk upon combined herbivore-pathogen attack in poplar trees which results in lowered emission of herbivore-induced volatiles. This influences the preference of herbivores, and might have other far-reaching consequences for the insect and pathogen communities in natural poplar forests.

Dietary self‐selection and rules of compromise by fifth‐instar Vanessa cardui

AbstractWe examined dietary self‐selection and rules of compromise for protein (P) and digestible carbohydrate (C) intake by fifth‐instar Vanessa cardui L. (Lepidoptera: Nymphalidae: Nymphalini). We presented six fat‐free diet pairs to larvae in a choice trial to determine the ‘intake target’. In addition, we fed larvae seven fat‐free single diets differing in dietary nutrient ratio in no‐choice trials to determine the rules of compromise they exhibit when constrained to a singular, sub‐optimal dietary source. In choice trials, caterpillars regulated nutrient intake to a ratio of 1 protein to 1.09 carbohydrate (1P:1.09C), exhibiting tighter regulation of protein than of carbohydrate. Furthermore, larvae from different diet pair treatments did not differ in pupal mass or stadium duration. In no‐choice experiments, larvae reduced consumption on increasingly protein‐biased diets and increased consumption on increasingly carbohydrate‐biased diets, relative to a 1P:1C ratio diet. Differences in carbohydrate consumption were much greater between no‐choice treatments than differences in protein consumption. Dietary nutrient ratio affected pupal mass when accounting for initial larval mass. Pupal mass decreased as nutrient ratio was shifted off of 1P:1C, but to a greater extent when the ratio was skewed toward carbohydrate. Stadium duration increased as nutrient ratio diverged from 1P:1C, being more pronounced when shifted toward carbohydrate than toward protein. Regulation to near 1P:1C is consistent with results found for other Lepidoptera, and the rule of compromise exhibited by V. cardui is consistent with that expected for a generalist herbivore.

Epiphytic bacteria on lettuce affect the feeding behavior of an invasive pest slug

Plant–animal interactions are not isolated pairwise relationships but are always accompanied by diverse assemblages of microbes. Additional to direct effects of microorganisms on their hosts, recent investigations demonstrated that bacteria associated with plants can modify the behavior of organisms of higher trophic levels. However, in the context of herbivory, functions of non-phytopathogenic bacteria colonizing leaf surfaces remain understudied. This study showed that naturally occurring epiphytic bacteria affect the feeding behavior of a generalist herbivore. Epiphytic bacteria isolated from leaves of <em>Lactuca sativa</em> var. <em>capitata</em> were screened for their potential to influence feeding choices of the slug <em>Arion vulgaris</em>. Cultivated bacteria were inoculated in artificial food substrates or on sterile leaves of gnotobiotic lettuce plants and were offered to slugs in different behavioral bioassays. A large proportion of bacterial strains tested induced behavioral alterations in the feeding choices of slugs. Behavioral responses of slugs were further modified by antibiotic treatment of slugs prior to choice tests indicating that both bacteria associated with plants and animals affect plant–animal interactions. Our results emphasize the important role of bacteria in plant–animal interactions and suggest a prominent role of bacteria in herbivory in natural, horticultural, and agricultural systems.

Antagonism between two root-associated beneficial Pseudomonas strains does not affect plant growth promotion and induced resistance against a leaf-chewing herbivore.

Plant growth-promoting microbes residing on the roots may cooperate or compete, thereby affecting their collective benefit to the host plant. Pseudomonas simiae WCS417r (formerly known as P. fluorescens WCS417r) and Pseudomonas fluorescens SS101 are well known for their ability to induce systemic resistance in Arabidopsis. Here, we evaluate how these species interact on the roots of Arabidopsis thaliana Col-0 and how their co-inoculation affects plant defense to the leaf-chewing herbivore Mamestra brassicae and plant growth promotion. WCS417r and SS101, applied individually to root tips or at two different positions along the roots, established similar population densities on Arabidopsis roots. When co-inoculated at the same position on the roots, however, WCS417r established significantly higher population densities than SS101. Both upon single inoculation and co-inoculation, the two pseudomonads induced the same level of induced systemic resistance against the caterpillar M. brassicae and the same increase in plant biomass. These results suggest that combined inoculation of both Pseudomonas strains does not significantly modify the plant's defensive capacity compared to individual inoculation, resulting in a similar effect on performance of the generalist herbivore M. brassicae.

The correlation between leaf-surface and leaf-tissue secondary metabolites: a case study with pyrrolizidine alkaloids in Jacobaea hybrid plants

Usually whole plant or whole leaf extracts are analyzed to study the chemical ecology of insect-plant interactions. For herbivore species the contact with the leaf surface enables them to estimate the quality of the plant. The relationship between the leaf-surface and leaf-tissue secondary metabolites (SMs) could offer important new insights in insect-plant interactions mediated by SMs. Pyrrolizidine alkaloids (PAs), typical defense chemicals in Jacobaea species, are repellent for generalist herbivores but are attractive to specialists. Explore whether the PAs on the leaf surface are a reliable representation of the PAs in the leaf tissue in PA-containing plants. The concentration of individual PAs present on the leaf surface and in the corresponding leaf tissue from 37 genotypes (one plant from each genotype) of an F2 generation of a cross between Jacobaea vulgaris and Jacobaea aquatica was measured by high performance liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). PAs were removed from the leaf surface by extraction with a slightly acidic aqueous solution. The total amount of PAs present on the surface of the leaves was only 0.015% (range 0.001–0.163%) of the total amount present in the leaf tissue. Most PAs present in the leaf tissue were also found on the surface, except for jaconine, dehydrojaconine, dehydrojacoline and usaramine N-oxide. Positive correlations between leaf-surface and leaf-tissue concentrations were found for most of the jacobine-like and otosenine-like PAs, but correlations for total PA, senecionine- and erucifoline-like PAs were not significant. These results indicate that PA variation on the leaf surface only partially reflects the PA variation in the leaf tissue. Because most herbivores are affected in a different manner by individual PAs, this result means that the leaf surface does not give a reliable estimate of plant quality to herbivores.

Herbivore-induced chemical and molecular responses of the kelps Laminaria digitata and Lessonia spicata.

Kelps are founding species of temperate marine ecosystems, living in intertidal coastal areas where they are often challenged by generalist and specialist herbivores. As most sessile organisms, kelps develop defensive strategies to restrain grazing damage and preserve their own fitness during interactions with herbivores. To decipher some inducible defense and signaling mechanisms, we carried out metabolome and transcriptome analyses in two emblematic kelp species, Lessonia spicata from South Pacific coasts and Laminaria digitata from North Atlantic, when challenged with their main specialist herbivores. Mass spectrometry based metabolomics revealed large metabolic changes induced in these two brown algae following challenges with their own specialist herbivores. Targeted metabolic profiling of L. spicata further showed that free fatty acid (FFA) and amino acid (AA) metabolisms were particularly regulated under grazing. An early stress response was illustrated by the accumulation of Sulphur containing amino acids in the first twelve hours of herbivory pressure. At latter time periods (after 24 hours), we observed FFA liberation and eicosanoid oxylipins synthesis likely representing metabolites related to stress. Global transcriptomic analysis identified sets of candidate genes specifically induced by grazing in both kelps. qPCR analysis of the top candidate genes during a 48-hours time course validated the results. Most of these genes were particularly activated by herbivore challenge after 24 hours, suggesting that transcriptional reprogramming could be operated at this time period. We demonstrated the potential utility of these genes as molecular markers for herbivory by measuring their inductions in grazed individuals of field harvested L. digitata and L. spicata. By unravelling the regulation of some metabolites and genes following grazing pressure in two kelps representative of the two hemispheres, this work contributes to provide a set of herbivore-induced chemical and molecular responses in kelp species, showing similar inducible responses upon specialist herbivores in their respective ecosystems.

Generalist social bees maximize diversity intake in plant species‐rich and resource‐abundant environments

AbstractNumerous studies revealed a positive relationship between biodiversity and ecosystem functioning, suggesting that biodiverse environments may not only enhance ecosystem processes, but also benefit individual ecosystem members by, for example, providing a higher diversity of resources. Whether and how the number of available resources affects resource collection and subsequently consumers (e.g., through impacting functions associated with resources) have, however, been little investigated, although a better understanding of this relationship may help explain why the abundance and richness of many animal species typically decline with decreasing plant (resource) diversity. Using a social bee species as model (Tetragonula carbonaria), we investigated how plant species richness—recorded for study sites located in different habitats—and associated resource abundance affected the diversity and functionality (here defined as nutritional content and antimicrobial activity) of resources (i.e., pollen, nectar, and resin) collected by a generalist herbivorous consumer. The diversity of both pollen and resin collected strongly increased with increasing plant/tree species richness, while resource abundance was only positively correlated with resin diversity. These findings suggest that bees maximize resource diversity intake in (resource) diverse habitats. Collecting more diverse resources did, however, not increase their functionality, which appeared to be primarily driven by the surrounding (plant) source community in our study. In generalist herbivores, maximizing resource diversity intake may therefore primarily secure collection of sufficient amounts of resources across the entire foraging season, but it also ensures that the allocated resources meet all functional needs. Decreasing available resource diversity may thus impact consumers primarily by reduced resource abundance, but also by reduced resource functionality, particularly when resources of high functionality (e.g., from specific plant species) become scarce.

Effects of deer on the photosynthetic performance of invasive and native forest herbs.

Overabundant generalist herbivores can facilitate non-native plant invasions, presumably through direct and indirect modifications to the environment that affect plant performance. However, ecophysiological mechanisms behind ungulate-mediated plant invasions have not been well-studied. At a long-term Odocoileus virginianus (white-tailed deer) exclusion site in a temperate deciduous forest, we quantified deer-mediated ecophysiological impacts on an invasive biennial Alliaria petiolata (garlic mustard) and two palatable native herbaceous perennials, Maianthemum racemosum and Trillium grandiflorum. In mid-summer, we found that leaf-level light availability was higher in unfenced areas compared with areas fenced to exclude deer. Alliaria in unfenced areas exhibited 50 % higher mean maximum photosynthetic rates compared with fenced areas. Further, specific leaf area decreased by 48 % on average in unfenced areas, suggesting leaf structural responses to higher light levels. Similarly, Maianthemum had 42 % higher mean photosynthetic rates and 33 % decreased mean specific leaf area in unfenced areas, but these functional advantages were likely countered by high rates of deer herbivory. By contrast, Trillium exhibited significantly lower (26 %) maximum photosynthetic rates in unfenced areas, but SLA did not differ. Deer-mediated differences in light saturated photosynthetic rates for all three species were only significant during months with overstory tree canopy cover, when light availability in the herb layer was significantly lower in fenced areas. Alliaria's enhanced photosynthetic rates implicate overabundant deer, a situation that is nearly ubiquitous across its invaded range. Collectively, our results provide empirical evidence that generalist herbivores can alter non-native plant physiology to facilitate invasion.

Past freeze–thaw events on Pinus seeds increase seedling herbivory

AbstractSeed and seedling survival are key components of plant population persistence. Although freeze–thaw events are experienced by many dispersed seeds in temperate ecosystems, it is unclear whether freeze–thaw stresses experienced by seeds can alter seedling susceptibility to herbivores during the growing season. We evaluated how freezing stress (temperature at −6°C for 6 h) experienced by seeds of two conifer species (Pinus resinosa and Pinus strobus) affects seedling herbivory by a generalist herbivore (Spodoptera exigua). For both Pinus species, herbivores consumed twofold more seedling biomass from freeze–thaw‐treated plants than seedlings from the constant temperature treatment. Herbivores grew ~66% faster when feeding on freeze–thaw P. resinosa seedlings relative to controls, but herbivore growth rate did not differ between treatments for P. strobus. Our results show that the thermal environment experienced by a seed can have subsequent effects on plant–herbivore interactions, suggesting that (1) early ontogenetic stress could be a cryptic, yet unappreciated, determinant of future herbivory and (2) increasingly frequent cold events, such as those projected under winter climate change, may amplify seedling herbivory and reduce recruitment in managed and natural conifer forests.

Plant traits and plant biogeography control the biotic resistance provided by generalist herbivores

Summary Globalization and climate change trigger species invasions and range shifts, which reshuffle communities at an exceptional rate and expose plant migrants to unfamiliar herbivores. Dominant hypotheses to predict plant success are based on evolutionary novelty: either herbivores are maladapted to consume novel plants (enemy release hypothesis), or novel plants are maladapted to deter herbivores (biotic resistance hypothesis). Since novelty can work both ways, it fails to consistently predict when herbivores will consume novel over non‐novel plants. Surprisingly, the value of using plant traits to predict herbivore consumption of novel plants remains largely unexplored. We hypothesized that (i) plant traits explain generalist herbivore consumption rates of novel and non‐novel plants, and (ii) any effect of novelty will be grounded in consistent trait differences between native and novel plants. Lastly, we expected to find (iii) differences in plant traits and plant consumption rates across latitude. To test these hypotheses, we measured the consumption rate of plant species for a tropical and a temperate generalist herbivore in controlled feeding trials by offering them a large variety of 40 plant species from different geographical origins. Therefore, whether a plant was novel depended on the herbivore used, allowing us to disentangle plant identity from plant novelty. We also measured plant chemical traits and determined whether traits, geographical origin or novelty best explained herbivore consumption rates. Both generalist herbivores consumed more of plants with a high nitrogen‐to‐phenolic compounds ratio, irrespective of the plant's novelty to the herbivore. A pattern of increasing plant's nitrogen‐to‐phenolics ratio with latitude could explain why both the tropical and temperate herbivore consumed more of plants from temperate regions. Plant novelty and its geographical origin no longer explained consumption rates once differences in nitrogen‐to‐phenolic compounds ratio were taken into account. We show that differences in plant traits along a latitudinal cline determine herbivore consumption rates, irrespective of whether plants are novel or familiar. Therefore, we propose that integrating evolutionary novelty theory with plant traits and biogeography will increase our understanding of the consequences of plant species migration beyond biogeographical barriers. A lay summary is available for this article.

White-tailed deer browse on an invasive shrub with extended leaf phenology meets assumptions of an apparent competition hypothesis.

It has been hypothesized that invasive plant species with extended leaf phenology (ELP) elevate generalist herbivore populations, increasing herbivory on native plants (apparent competition). This hypothesis assumes that consumption of the invasive is associated with periods of ELP, the invasive is a major component of the herbivore's diet, and that it is more nutritious than native plants during periods of ELP. We tested these assumptions by estimating the proportion of the white-tailed deer diet comprised of Lonicera maackii, an invasive shrub with ELP, quantifying the seasonal pattern of deer browse on this invasive shrub, and comparing its nutritional quality to leafless woody stems. In the Miami University Natural Areas in southwest Ohio we quantified the frequency of leafy twigs of woody species 0.3-2.1 m high in three habitats (deciduous forest, Juniperus virginiana forest, and forest-field edge). Monthly we quantified deer browse on marked L. maackii twigs, and estimated the mass of leaf and stem tissue consumed with allometric relationships using diameter and length of unbrowsed twig portions. We estimated the percent of the annual deer diet comprised of L. maackii by dividing the sum of these estimates by the product of deer abundance (estimated by pellet-based distance sampling) and consumption estimates from the literature. Crude protein of L. maackii stems and leaves was determined by C:N analyser. In each habitat the frequency of L. maackii was greater than all other woody species combined. We estimated L. maackii comprised 14-47 % of the annual deer diet. Deer browsed L. maackii each month, but consumption was high in early spring and late summer. Crude protein of leafy twigs of L. maackii in early spring was 12.9 %, much higher than leafless twigs of native species on-site. These findings support the assumptions of the hypothesis that invasive plants with ELP impact native plants via deer-mediated apparent competition.