Plant-herbivore Coevolution Research Articles

Spatial and temporal variability in the structure of the multiple-herbivore community of horsenettle, and evidence for evolutionary responses in host-plant resistance

The geographic mosaic model of plant–herbivore coevolution asserts that interactions between a plant species and an herbivore species vary in intensity among populations across the plant’s geographic range. Despite this model’s intuitive appeal, data to investigate its implications for the type of complex, multiple-herbivore communities that occur in nature are scant. This paper reports on the results of 2 years of field surveys of damage by five leaf herbivores and one stem herbivore in four Solanum carolinense (horsenettle) populations, combined with results of a common-garden study quantifying the mean resistance levels of the plants from each field against each of the six herbivores. The relative amounts of damage caused by each species (representing the “herbivore-community structure”) differed significantly among the four fields. The plants were much more heavily damaged in the 2nd year than in the first, but the herbivore-community structure remained stable within each field between years. Overall, the amount of damage by species of herbivores in a field tended to be positively correlated with the plants’ levels of resistance that were measured in the common garden (r = 0.40, P = 0.05). Specifically, for five of the six herbivores, greater damage in the field was associated with greater plant resistance. This result suggests that horsenettle’s evolution of resistance against specific herbivores can occur rather quickly within fields, creating a local-scale mosaic of populations specifically adapted to the particular structure of the herbivore community that they are facing, but that herbivore-community structure is not strongly determined by plant resistance.

Searching for genes that make plants susceptible to spider mites as a target for resistance breeding

Herbivorous mites must have to overcome several barriers that are produced by plants to prevent herbivores from obtaining food and successfully colonize them. How this interaction plays out is determined by a set of complex molecular mechanisms that trigger physiological changes in plants to deter mites, and in mites to withstand deterrence and together these determine their degree of compatibility. We demonstrated that spider mites secrete specialized salivary proteins into plants that suppress plant defenses and therefore play a decisive role in the plant-mite interaction. To these proteins we refer as effectors (Villarroel et al. 2016; Jonckheere et al. 2016). Elucidating how effectors suppress plant defenses can aid plant resistance breeding and help to better understand plant-herbivore co-evolution.

The Effects of Various Carbohydrates and Tannic Acid on the Food Consumption and Growth Performance of Uresiphita gilvata (Lepidoptera: Crambidae)

The effects of carbohydrates and tannen on the development and food preferences of Uresiphita gilvata larvae were investigated in this study. In addition, with the addition of tannin, the importance of plant-herbivore co-evolution in the food choice of the larvae was mentioned. For this reason, thirteen different artificial diets were prepared. Each diet contains sucrose, glucose, galactose, maltose, fructose, arabinose, mannose, or starch at the same concentration. To investigate the effect of tannic acid, 5 % tannic acid was added to the diets each containing sucrose, starch, glucose or maltose at the same concentration. According to the results of the study, sucrose was consumed by larvae more than glucose and fructose. However, the maximum food consumption of U. gilvata larvae was on the diet containing arabinose and their minimum consumption was on the diet containing mannose. Galactose is more consumed by larvae. However, intake galactose can not converted to pupal lipid by larvae. The addition of tannin to the diet reduced the amount of consumption of diet the dry pupal weight and lipid amount of pupae. However, the addition of tannin to the diet increased the amount of protein and extended the development time of pupae.

Plant-herbivore coevolution and plant speciation.

More than five decades ago, Ehrlich and Raven proposed a revolutionary idea-that the evolution of novel plant defense could spur adaptive radiation in plants. Despite motivating much work on plant-herbivore coevolution and defense theory, Ehrlich and Raven never proposed a mechanism for their "escape and radiate" model. Recent intriguing mechanisms proposed by Marquis etal. include sympatric divergence, pleiotropic effects of plant defense traits on reproductive isolation, and strong postzygotic isolation, but these may not be general features of herbivore-mediated speciation. An alternate view is that herbivores may impose strong divergent selection on defenses in allopatric plant populations, with plant-herbivore coevolution driving local adaptation resulting in plant speciation. Building on these ideas, we propose three scenarios that consider the role of herbivores in plant speciation. These include (1) vicariance, subsequent coevolution within populations and adaptive divergence between geographically isolated populations, (2) colonization of a new habitat lacking effective herbivores followed by loss of defense and then re-evolution and coevolution of defense in response to novel herbivores, and (3) evolution of a new defense followed by range expansion, vicariance, and coevolution. We discuss the general role of coevolution in plant speciation and consider outstanding issues related to understanding: (1) the mechanisms behind cospeciation of plants and insects, (2) geographic variation in defense phenotypes, (3) how defensive traits and geography map on plant phylogenies, and (4) the role of herbivores in driving character displacement in defense phenotypes of related species in sympatry.

Toxicity of Milkweed Leaves and Latex: Chromatographic Quantification Versus Biological Activity of Cardenolides in 16 Asclepias Species.

Cardenolides are classically studied steroidal defenses in chemical ecology and plant-herbivore coevolution. Although milkweed plants (Asclepias spp.) produce up to 200 structurally different cardenolides, all compounds seemingly share the same well-characterized mode of action, inhibition of the ubiquitous Na+/K+ ATPase in animal cells. Over their evolutionary radiation, milkweeds show a quantitative decline of cardenolide production and diversity. This reduction is contrary to coevolutionary predictions and could represent a cost-saving strategy, i.e. production of fewer but more toxic cardenolides. Here we test this hypothesis by tandem cardenolide quantification using HPLC (UV absorption of the unsaturated lactone) and a pharmacological assay (in vitro inhibition of a sensitive Na+/K+ ATPase) in a comparative study of 16 species of Asclepias. We contrast cardenolide concentrations in leaf tissue to the subset of cardenolides present in exuding latex. Results from the two quantification methods were strongly correlated, but the enzymatic assay revealed that milkweed cardenolide mixtures often cause stronger inhibition than equal amounts of a non-milkweed reference cardenolide, ouabain. Cardenolide concentrations in latex and leaves were positively correlated across species, yet latex caused 27% stronger enzyme inhibition than equimolar amounts of leaf cardenolides. Using a novel multiple regression approach, we found three highly potent cardenolides (identified as calactin, calotropin, and voruscharin) to be primarily responsible for the increased pharmacological activity of milkweed cardenolide mixtures. However, contrary to an expected trade-off between concentration and toxicity, later-diverging milkweeds had the lowest amounts of these potent cardenolides, perhaps indicating an evolutionary response to milkweed's diverse community of specialist cardenolide-sequestering insect herbivores.

Coevolution can explain defensive secondary metabolite diversity in plants.

Many plant species produce defensive compounds that are often highly diverse within and between populations. The genetic and cellular mechanisms by which metabolite diversity is produced are increasingly understood, but the evolutionary explanations for persistent diversification in plant secondary metabolites have received less attention. Here we consider the role of plant-herbivore coevolution in the maintenance and characteristics of diversity in plant secondary metabolites. We present a simple model in which plants can evolve to invest in a range of defensive toxins, and herbivores can evolve resistance to these toxins. We allow either single-species evolution or reciprocal coevolution. Our model shows that coevolution maintains toxin diversity within populations. Furthermore, there is a fundamental coevolutionary asymmetry between plants and their herbivores, because herbivores must resist all plant toxins, whereas plants need to challenge and nullify only one resistance trait. As a consequence, average plant fitness increases and insect fitness decreases as number of toxins increases. When costs apply, the model showed both arms race escalation and strong coevolutionary fluctuation in toxin concentrations across time. We discuss the results in the context of other evolutionary explanations for secondary metabolite diversification.

Insect herbivore‐associated organisms affect plant responses to herbivory

SummaryIn nature, plants interact with many organisms and need to integrate their responses to these diverse community members. Knowledge on plant–insect relationships has accumulated rapidly during the last decades. Yet most studies on direct or indirect defences of plants against herbivory have treated herbivores as individual stressors. However, herbivores often consist of communities themselves, comprising organisms such as parasites and symbionts, which may have important effects on the herbivore phenotype, and consequently on interactions of the herbivore with its food plant. Here, we review how herbivore‐associated organisms affect plant–herbivore interactions. Organisms associated with herbivores can directly affect how a plant interacts with their herbivorous hosts, by interfering with plant signal‐transduction pathways, repressing the expression of plant defence‐related genes, or altering plant secondary metabolism. In addition, herbivore‐associated organisms can also affect plant responses indirectly by their effect on the behaviour and physiology of their herbivore host. The changes in plant phenotype that arise from herbivore‐associated organisms may subsequently affect interactions with other community members, thereby impacting community dynamics. Furthermore, herbivore‐associated organisms may act as a hidden driving force of plant–herbivore coevolution. Therefore, to understand plant–herbivore interactions it is important to realize that every single herbivorous insect constitutes a community in itself.

Herbivory at marginal populations: Consequences for maternal fitness and vegetative differentiation

Margins of distribution of plant species constitute natural areas where the impact of the antagonistic interactions is expected to be higher and where changes in the dynamics of plant-herbivore coevolution could promote intraspecific differentiation in (co)evolving plant traits. In the present study, we investigated how differences in the average herbivory level affect maternal fitness in core continuous and marginal disjunct populations of Daphne laureola in an effort to assess the role of herbivores limiting plant distribution. Furthermore, we investigated intraspecific differentiation in vegetative traits and their potential connection to divergent selection by herbivores in both groups of populations. Our results did not support increased herbivory at the species margin but did support a difference in the effect of herbivory on maternal fitness between core continuous and marginal disjunct populations of D. laureola. In addition, herbivores did not exert phenotypic selection consistent with the geographic variation in studied plant traits. Therefore, the geographic variation of vegetative traits of D. laureola seems to be consequence of environmental heterogeneity more than result of geographically divergent selection by herbivores.

Plant‐herbivore coevolution in a changing world

AbstractCurrent anthropogenic environmental change causes rapid loss of biodiversity. Although the effects of the main causes of this loss (habitat fragmentation, climate change, and invasive species) on single species have been widely studied, the effects on species interactions are poorly understood. In particular, we do not yet understand how these phenomena affect the evolutionary processes that impact species interactions. Coevolution is a dominant process that organizes the web of life: most species are involved in at least one coevolved interaction. Due to rapid human modification of landscapes it is important to understand how subsequent changes in biotic and abiotic environment and in the level and distribution of genetic variation, as well as changes in population structures, influence the elements of the coevolutionary process. In this review, we synthesize recent development of theoretical work on the coevolution of interacting species with conservation genetics and the impact of anthropogenic environmental changes on single species to address the potential effects of habitat fragmentation, climate change, and invasive species on plant‐herbivore coevolution.

Leaf shape variation and herbivore consumption and performance: a case study with Ipomoea hederacea and three generalists

The effect of leaf shape variation on plant-herbivore interactions has primarily been studied from the perspective of host seeking behavior. Yet for leaf shape to affect plant-herbivore coevolution, there must be reciprocal effects of leaf shape variation on herbivore consumption and performance. We investigated whether alternative leaf morphs affected the performance of three generalist insect herbivores by taking advantage of a genetic polymorphism and developmental plasticity in leaf shape in the Ivyleaf morning glory, Ipomoea hederacea. Across four experiments, we found variable support for an effect of leaf shape genotype on insects. For cabbage loopers (Trichoplusia ni) and corn earworms (Helicoverpa zea) we found opposing, non-significant trends: T. ni gained more biomass on lobed genotypes, while H. zea gained more biomass on heart-shaped genotypes. For army beetworms (Spodoptera exigua), the effects of leaf shape genotype differed depending on the age of the plants and photoperiod of growing conditions. Caterpillars feeding on tissue from older plants (95 days) grown under long day photoperiods had significantly greater consumption, dry biomass, and digestive efficiency on lobed genotypes. In contrast, there were no significant differences between heart-shaped and lobed genotypes for caterpillars feeding on tissue from younger plants (50 days) grown under short day photoperiods. For plants grown under short days, we found that S. exigua consumed significantly less leaf area when feeding on mature leaves than juvenile leaves, regardless of leaf shape genotype. Taken together, our results suggest that the effects of leaf shape variation on insect performance are likely to vary between insect species, growth conditions of the plant, and the developmental stage and age of leaves sampled.

Differential attack on diploid, tetraploid, and hexaploid Solidago altissima L. by five insect gallmakers

Genetic variation among plants can influence host choice and larval performance in insect herbivores. Ploidy (cytotype) variation is a particularly dramatic form of plant genetic variation, and where diploid and polyploid cytotypes of a species occur in sympatry, they may provide herbivores with choices that are distinguished by profound and genome-wide genetic differences. We tested for non-random attack by five gallmaking insect herbivores on diploid, tetraploid, and hexaploid cytotypes of the goldenrod Solidago altissima L., working in seven midwestern US populations where the ploidies co-occur on spatial scales relevant to insect host choice. For four of the five herbivores, attack was non-random with respect to ploidy at one or more sites. Ploidy effects on attack were complex: the ploidy subjected to highest attack varied both across herbivores within sites and (for most herbivores) across sites within herbivores. Ploidy effects on attack will alter rates of encounter between insect herbivores-either increasing or decreasing the likelihood of two herbivores sharing a host plant ramet, compared with the case with no effects of ploidy. Plant ploidy variation appears likely to have a major impact on insect community organization, and perhaps on plant-herbivore coevolution, but that impact is likely to be spatially heterogeneous.

Can tolerance traits impose selection on herbivores?

Plant tolerance reduces the fitness consequences of herbivore and natural enemy damage, while resistance reduces the amount of damage suffered. In contrast to resistance, tolerance is often assumed to not affect herbivore performance and evolution. Evidence from the literature, however, suggests that it is possible for plant tolerance to affect herbivore performance and evolution, and potentially plant–herbivore coevolution. First, for cases when genetic correlations between resistance and tolerance are due to pleiotropy, the genes and loci for tolerance and resistance are the same, and as such both traits will affect herbivore performance directly. Second, it is possible that the physiological basis and mechanisms of plant tolerance – for example, changes in plant physiology and resource allocation – directly alter herbivore fitness characters. In this paper, I review the evidence for these potential effects of plant tolerance on herbivore performance, and suggest straightforward experiments to evaluate these possibilities. More generally, I propose that this untested assumption is constraining our view of plant–herbivore coevolution.

Consequences of Plant-Herbivore Coevolution on the Dynamics and Functioning of Ecosystems

The potential consequences of plant–herbivore coevolution for ecosystem functioning are investigated using a simple nutrient-limited ecosystem model in which plant and herbivore traits are subject to adaptive dynamics. Although the ecological model is very simple and always reaches a stable equilibrium in the absence of evolution, coevolution can generate a great diversity of dynamical behaviors. The evolutionary dynamics can lead to a stable equilibrium. If the evolution of plants is fast enough, certain values of the trade-off parameters lead to complex evolutionary cycles bounded by physiological constraints. The dynamical behavior of the model is very different when the dynamics of inorganic nutrient is ignored and plant competition is modeled by a logistic growth function. This emphasizes the importance of including explicit nutrient dynamics in studies of plant–herbivore coevolution.

PAIRWISE VERSUS DIFFUSE NATURAL SELECTION AND THE MULTIPLE HERBIVORES OF SCARLET GILIA, IPOMOPSIS AGGREGATA.

Recent theoretical studies have argued that plant-herbivore coevolution proceeds in a diffuse rather than a pairwise manner in multispecies interactions when at least one of two conditions are met: (1) genetic correlations exist between plant resistances to different herbivore species; and (2) ecological interactions between herbivores sharing a host plant cause nonadditive impacts of herbivory on plant fitness. We present results from manipulative field experiments investigating the single and interactive fitness effects of three types of herbivory on scarlet gilia (Ipomopsis aggregata) over two years of study. We utilize these data to test whether selection imposed by herbivore attack on date of first flowering is pairwise (independent) or diffuse (dependent) in nature. Our results reveal complex patterns of the fitness effect of herbivores. Simulated early season browsing had a strong negative fitness effect on plants and also reduced subsequent insect attack. Surprisingly, this ecological interaction did not translate into significant interactions between clipping and insect manipulations on plant fitness. However, we detected a significant interaction between seed fly and caterpillar herbivory on plant fitness, with the negative effect of either insect being greatest when occurring alone. These results suggest that herbivore-imposed selection may have pairwise and diffuse components. In our selection analysis of flowering phenology, we discovered significant pairwise linear selection imposed by clipping, diffuse linear selection imposed by insects, and diffuse nonlinear selection imposed by clipping and insect attack acting simultaneously. Our results reveal that the evolution of flowering phenology in scarlet gilia may be in response to diffuse and pairwise natural selection imposed by multiple herbivores. We discuss the evolution of resistance characters in light of diffuse versus pairwise forms of linear and nonlinear selection and stress the complexity of selection imposed by suites of interacting species.

Plant-Herbivore Coevolution in a Spatially and Genetically Explicit Model

A coevolutionary model was developed to test interactions between diploid plants and herbivores using genetic algorithms on a spatial lattice. Simulated plants carried defensive genes and herbivores carried genes coding for resistance (metabolism of herbivore defense) in gene-for-gene synchrony. Collectively these genes are referred to as defensive/resistance genes (DR-genes). Genes were linked on chromosomes. Regulatory genes modified both dominance at these DR loci and the tradeoff cost involved in producing either defense or resistance. We tested the effects of varying a) the number of DR-loci, b) the ratio of the number of herbivore:plant generations, c) the shape (square vs. long and thin) and function (torus vs. island) of the lattice, and d) herbivore encounter rate on plant progeny dispersal distance. Increasing both the number of DR-genes and the ratio of herbivore:plant generations caused a tighter coevolutionary response between plants and herbivores. Plant defense was highly sensitive to herbivory but not to increasing encounter rates. Plant DR-genes were selectively disadvantageous with only one lucus but selectively favored with two or more loci. Increasing the number of herbivore:plant generations caused increased fluctuations in herbivore resistance gene frequencies and a decrease in the lag time in herbivore response to changes in plant defensive gene frequencies. The relationship between heterotroph and autotroph DR-genes increased exponentially with increasing numbers of DR-loci. This relationship suggests that autotrophs benefit from increased diversity of defense that causes a relative increase in cost for the heterotrophs. The shape of the lattice interacted with lattice function, resulting in high species persistence on wraparound habitats and the greatest extinction likelihood on rectangular islands. Low to moderate herbivore encounter rates increased plant progeny dispersal distance while high herbivore encounter rates tended to reduce dispersal distance. The frequencies of genes coding for plant defense and herbivore resistance were dynamic for thousands of generations, despite the homogeneous lattice. This interaction may increase extinction probabilities in fragmented habitats.

Plant-Herbivore Coevolution in a Spatially and Genetically Explicit Model

A coevolutionary model was developed to test interactions between diploid plants and herbivores using genetic algorithms on a spatial lattice. Simulated plants carried defensive genes and herbivores carried genes coding for resistance (metabolism of herbivore defense) in gene-for-gene synchrony. Collectively these genes are referred to as defensive/resistance genes (DR-genes). Genes were linked on chromosomes. Regulatory genes modified both dominance at these DR loci and the tradeoff cost involved in producing either defense or resistance. We tested the effects of varying a) the number of DR-loci, b) the ratio of the number of herbivore:plant generations, c) the shape (square vs. long and thin) and function (torus vs. island) of the lattice, and d) herbivore encounter rate on plant progeny dispersal distance.Increasing both the number of DR-genes and the ratio of herbivore:plant generations caused a tighter coevolutionary response between plants and herbivores. Plant defense was highly sensitive to herbivory but not to increasing encounter rates. Plant DR-genes were selectively disadvantageous with only one lucus but selectively favored with two or more loci. Increasing the number of herbivore:plant generations caused increased fluctuations in herbivore resistance gene frequencies and a decrease in the lag time in herbivore response to changes in plant defensive gene frequencies. The relationship between heterotroph and autotroph DR-genes increased exponentially with increasing numbers of DR-loci. This relationship suggests that autotrophs benefit from increased diversity of defense that causes a relative increase in cost for the heterotrophs. The shape of the lattice interacted with lattice function, resulting in high species persistence on wraparound habitats and the greatest extinction likelihood on rectangular islands. Low to moderate herbivore encounter rates increased plant progeny dispersal distance while high herbivore encounter rates tended to reduce dispersal distance. The frequencies of genes coding for plant defense and herbivore resistance were dynamic for thousands of generations, despite the homogeneous lattice. This interaction may increase extinction probabilities in fragmented habitats.

Adaptive responses of generalist herbivores to competition: Convergence or divergence

Generalist herbivores are often faced with a choice of eating abundant, low quality foods or rare high quality foods. An adaptive forger in this situation should have mixed responses (Abrams, 1990) to the densities of the two food types. Competition from another generalist herbivore species will generally cause convergence in foraging time allocations when both species are strictly food limited. Competition will usually cause a convergent response in traits determining the relative consumption rates of the two food categories. Divergent responses are possible when consumers are not strictly food limited. Divergent responses are also expected in some other traits related to resource use, such as gut size. Evidence for the predicted responses and implications for plant-herbivore coevolution are discussed.

Plant Chemistry and Host Range in Insect Herbivores

Bernays and Graham (1988) argue at the outset of their interesting paper that the role of plant chemistry in plant-herbivore coevolution has been overemphasized, and that natural enemies, especially predators, may be the dominant factor in the evolution of host range. There has, of course, never been any doubt that factors other than the biochemical characteristics of plants may help determine host range of herbivorous insects. Moreover, close relationships between insects and a narrow range of plants may be promoted by the evolution of concealment from predators in relation to a single background. The plant provides the substrate for the larvae, not just their food (Ehrlich and Raven 1964). Also well known is that even in cases of extreme specialist herbivores, nonchemical factors may finetune host ranges. For example, Holdren and Ehrlich (1982) concluded that the larvae of the butterfly Euphydryas editha in Colorado were restricted by ecological factors to Castilleja linariibfolia, even though two other nutritionally satisfactory, closely related host plants were available. All three plants species are members of the Scrophulariaceae, and presumably contain the iridoid glycosides prerequisite to any plant being a suitable Euphydryas host (Bowers 1983). We believe that the examples presented by Bernays and Graham do not yet allow us to reject the traditional understanding of the dominance of host biochemistry in the coevolution of plants and insect herbivores, and thus in determination of diet breadth. That arthropod herbivores can rapidly evolve new oviposition choices and host utilization abilities certainly does not contradict the idea that host plant chemistry plays a key role. After all, the ability of insect pests to evolve resistance to synthetic pesticides in 10 generations or fewer is legendary, and equivalent performances in response to plant chemicals are to be expected. Certainly, populations of some plants seem to be under selective pressure to produce an array of defenses that make it difficult for herbivores to evolve resistance (e.g., Dolinger etal. 1973). The observation that, in some cases, deterrent chemical constituents are comparatively harmless suggests that some herbivores are avoiding certain plants for reasons other than their toxicity. That, however, is not strong evidence that characteristics other than the edibility of plants (which has other components besides noxiousness) are mediating those plant-herbivore relationships. The preferential choice by grasshoppers of C3 plants over C4 plants, apparently because large fractions of the edible sheath cells of the latter cannot be digested (Caswell and Reed 1975, 1976), is but one example. Furthermore, evidence suggesting that grasshopper survival, size, and growth rate are greater on native vs. nonnative grasses implies a substantial degree of evolutionary response to the native grass defenses even among comparatively generalist insects (Joern 1988). As Bernays and Graham note, putative cases must be examined with great care. Nevertheless, suppose that a well-stratified sampling reveals that the deterrents that keep herbivores from eating harmless, edible host plants usually are nontoxic chemicals. Then we indeed will have to find new explanations (such as constraints applied by predators) for the restricted diets commonly observed (at least in butterflies, the most thoroughly studied major group of insect herbivores). We find that result unlikely, however. Moreover, examples accumulate of chemicals that make plants inedible to insect herbivores. For example, specialist insects that attack milkweeds (Asclepiadaceae) commonly cut the veins of the leaves they are eating, blocking the flow of latex to their feeding sites distad of the cuts (Dussourd and Eisner 1987). Experimentally cutting the veins makes milkweeds accessible to generalists that do not normally feed on them, while insects that eat a latex-rich leaf find their mandibles badly gummed. One must also be cautious in evaluating the selective impact of insects that are extremely in relation to their host plants. As Ehrlich and Birch (1967) pointed out, that impact is easily understated. For instance, the cactus Opuntia is no longer a plague in Queensland and the moth Cactoblactis, which was introduced to keep Opuntia under control, is extremely rare in relation to the cactus. The tiny lycaenid butterfly Glaucopsyche lygdamuus, for another example, might be thought scarce and inconspicuous in relation to its lupine host plants (overwhelmingly so if com-

Species Richness and Faunal Similarity of Arboreal Insect Herbivores

Factors affecting the species richness and faunal similarities of leaf eating Lepidoptera and sawflies on woody plants were analysed using British and Finnish data. Host plant frequency, the number of plant species per taxon and the number of relatives of host plants explained a total of 62-85% of the variation in macrolepidopteran species richness on Finnish trees and shrubs. On British trees the range and number of species per plant taxon explained together 47-52% of the variation in microand macrolepidopteran species richness. The number of plant species per host plant taxon explained 87% of the sawfly species richness on Finnish tree and shrub genera. The average frequency of host plant genera explained a significant portion of the residual variation, increasing the coefficient of determination to 97%. Macrolepidopteran faunal similarities were based on taxonomic and habitat similarities of host plant genera. In sawflies the faunal similarity between plant genera was low and was based on taxonomic relatedness. Connections between theories of plant-herbivore coevolution and species richness of herbivores are described and discussed.

Response of Slugs to Unacceptable Food Items

(1) Recent studies in plant-herbivore coevolution have concentrated on adaptations that have allowed herbivores to overcome plant defences and suggest that generalist herbivores evolve an ability to tolerate unacceptable plants in their habitat. (2) This study was designed to test the hypothesis that relatively unacceptable plants will be more acceptable to generalist herbivores (slugs) from the same habitat than to slugs from other populations that are not normally exposed to these plants. (3) Two slug species that occur in both waste-sites and woodland were selected for feeding trials with one relatively unacceptable plant species from each habitat. (4) The hypothesis was not supported. In fact, the unacceptable plants were initially fed on more by the slugs not usually exposed to them. (5) However, the 'naive' slugs showed a reduced amount of feeding on the unacceptable plants in a second feeding trial, suggesting that they learned to avoid these plants. (6) These two aspects of feeding behaviour, namely extensive feeding on a novel food and learning to avoid unacceptable food items, probably allow the tracking of new potential foods in a changing habitat while reducing the time spent in unnecessary feeding on unacceptable food items. The importance of these factors is discussed in relation to the climatic and energetic constraints on slug activity.