Associational effects against a leaf beetle mediate a minority advantage in defense and growth between hairy and glabrous plants

Based on the accumulation of evidence, the risk of herbivory depends not only on the traits of a plant but also on those of neighboring plants. Despite the potential importance of frequency-dependent interactions in the evolutionary stability of anti-herbivore defense, we know little about such associational effects between defended and undefended plants within a species. In this study, we determined whether the intraspecific associational effects against the oligophagous leaf beetle, Phaedon brassicae, caused a minority advantage in defense and growth between trichome-producing (hairy) and trichomeless (glabrous) plants of Arabidopsis halleri subsp. gemmifera. We experimentally demonstrated that the magnitude of herbivory and the number of adult beetles on hairy plants decreased when hairy plants were a minority, whereas the leaf damage and the beetle abundance did not differ between hairy and glabrous plants when glabrous plants were a minority. By contrast, the larvae of P. brassicae occurred less when hairy plants were a majority. We also found a reciprocal minority advantage in the biomass production for both hairy and glabrous plants. Additionally, the adults tended to attack glabrous leaves more rapidly than hairy ones, particularly when the beetles were starved or experienced glabrous diets. Furthermore, in the absence of herbivory, the growth of hairy plants tended to be slower than glabrous plants, which indicated a cost for the production of trichomes. Our study suggests that associational effects are a mechanism for the maintenance of trichome dimorphism by contributing to negative frequency-dependent growth.     

Coexistence of trichome variation in a natural plant population: a combined study using ecological and candidate gene approaches

The coexistence of distinct phenotypes within populations has long been investigated in evolutionary ecology. Recent studies have identified the genetic basis of distinct phenotypes, but it is poorly understood how the variation in candidate loci is maintained in natural environments. In this study, we examined fitness consequences and genetic basis of variation in trichome production in a natural population of Arabidopsis halleri subsp. gemmifera. Half of the individuals in the study population produced trichomes while the other half were glabrous, and the leaf beetle Phaedon brassicae imposed intensive damage to both phenotypes. The fitness of hairy and glabrous plants showed no significant differences in the field during two years. A similar result was obtained when sibling hairy and glabrous plants were transplanted at the same field site, whereas a fitness cost of trichome production was detected under a weak herbivory condition. Thus, equivalent fitness of hairy and glabrous plants under natural herbivory allows their coexistence in the contemporary population. The pattern of polymorphism of the candidate trichome gene GLABROUS1 (GL1) showed no evidence of long-term maintenance of trichome variation within the population. Although balancing selection under fluctuating biotic environments is often proposed to explain the maintenance of defense variation, the lack of clear evidence of balancing selection in the study population suggests that other factors such as gene flow and neutral process may have played relatively large roles in shaping trichome variation at least for the single population level.     

Tests of associational defence provided by hairy plants for glabrous plants of Arabidopsis halleri subsp. gemmifera against insect herbivores

1. Trichome‐producing (hairy) and trichomeless (glabrous) plants of Arabidopsis halleri subsp. gemmifera were investigated to test whether plant resistance to herbivory depends on the plants' phenotypes and/or the phenotypes of neighbouring plants (associational effects). 2. A common garden experiment was conducted in which the relative frequency of hairy and glabrous plants was manipulated. Two species of leaf‐chewing insects (larvae of a white butterfly and a cabbage sawfly) were found less often on hairy plants than on glabrous plants. By contrast, the numbers of aphids and flea beetles did not differ significantly between hairy and glabrous plants. For none of these insects did abundance depend on the frequency of the two plant morphs. 3. A field survey was conducted in two natural populations of A. halleri. In the first population, a species of white butterfly was the dominant herbivore, and hairy plants incurred less leaf damage than glabrous plants across 2 years. By contrast, in the other population, where flea beetles were dominant, there were no consistent differences in leaf damage between the two types of plants. In neither of the two populations was any evidence found of associational effects. 4. This study did not provide any conclusive evidence of associational effects of anti‐herbivore resistance, but it was discovered that trichomes can confer resistance to certain herbivores. Given the results of previous work by the authors on associational effects against a flightless leaf beetle, such associational effects of the trichome dimorphism of A. halleri were herbivore‐specific.     

Trichome production and variation in young plant resistance to the specialist insect herbivore Plutella xylostella among natural populations of Arabidopsis lyrata

The strength of plant‐herbivore interactions varies spatially and through plant ontogeny, which may result in variable selection on plant defense, both among populations and life‐history stages. To test whether populations have diverged in herbivore resistance at an early plant stage, we quantified oviposition preference and larval feeding by Plutella xylostella (L.) (Lepidoptera: Plutellidae) on young (5–6 weeks old) Arabidopsis lyrata (L.) O'Kane & Al‐Shehbaz (Brassicaceae) plants, originating from 12 natural populations, six from Sweden and six from Norway. Arabidopsis lyrata can be trichome‐producing or glabrous, with glabrous plants usually receiving more damage from insect herbivores in natural populations. We used the six populations polymorphic for trichome production to test whether resistance against P. xylostella differs between the glabrous and the trichome‐producing morph among young plants. There was considerable variation among populations in the number of eggs received and the proportion of leaf area consumed by P. xylostella, but not between regions (Sweden vs. Norway) or trichome morphs. Rosette size explained a significant portion of the variation in oviposition and larval feeding. The results demonstrate that among‐population variation in resistance to insect herbivory can be detected among very young individuals of the perennial herb A. lyrata. They further suggest that trichome densities are too low at this plant developmental stage to contribute to resistance, and that the observed among‐population variation in resistance is related to differences in other plant traits.     

Fine‐scale frequency differentiation along a herbivory gradient in the trichome dimorphism of a wild Arabidopsis

Geographic variation is commonly observed in plant resistance traits, where plant species might experience different selection pressure across a heterogeneous landscape. Arabidopsis halleri subsp. gemmifera is dimorphic for trichome production, generating two morphs, trichome‐producing (hairy) and trichomeless (glabrous) plants. Trichomes of A. halleri are known to confer resistance against the white butterfly, cabbage sawfly, and brassica leaf beetle, but not against flea beetles. We combined leaf damage, microclimate, and microsatellite loci data of 26 A. halleri populations in central Japan, to explore factors responsible for fine‐scale geographic variation in the morph frequency. We found that hairy plants were less damaged than glabrous plants within populations, but the among‐site variation was the most significant source of variation in the individual‐level damage. Fixation index () of a putative trichome locus exhibited a significant divergence along population‐level damage with an exception of an outlier population, inferring the local adaptation to herbivory. Notably, this outlier was a population wherein our previous study reported a balancing role of the brassica leaf beetle Phaedon brassicae on the morph frequency. This differentiation of the trichome locus was unrelated to neutral genetic differentiation (evaluated by of microsatellite loci) and meteorological factors (including temperature and solar radiation). The present findings, combined with those of our previous work, provide suggestive evidence that herbivore‐driven divergence and occasional outbreak of a specific herbivore have jointly contributed to the ecogeographic pattern in the frequency of two morphs.     

Antiherbivore defenses alter natural selection on plant reproductive traits

While many studies demonstrate that herbivores alter selection on plant reproductive traits, little is known about whether antiherbivore defenses affect selection on these traits. We hypothesized that antiherbivore defenses could alter selection on reproductive traits by altering trait expression through allocation trade‐offs, or by altering interactions with mutualists and/or antagonists. To test our hypothesis, we used white clover, Trifolium repens, which has a Mendelian polymorphism for the production of hydrogen cyanide—a potent antiherbivore defense. We conducted a common garden experiment with 185 clonal families of T. repens that included cyanogenic and acyanogenic genotypes. We quantified resistance to herbivores, and selection on six floral traits and phenology via male and female fitness. Cyanogenesis reduced herbivory but did not alter the expression of reproductive traits through allocation trade‐offs. However, the presence of cyanogenic defenses altered natural selection on petal morphology and the number of flowers within inflorescences via female fitness. Herbivory influenced selection on flowers and phenology via female fitness independently of cyanogenesis. Our results demonstrate that both herbivory and antiherbivore defenses alter natural selection on plant reproductive traits. We discuss the significance of these results for understanding how antiherbivore defenses interact with herbivores and pollinators to shape floral evolution.     

Frequency-dependent herbivory by a leaf beetle,Phaedon brassicae,on hairy and glabrous plants of Arabidopsis halleri subsp. gemmifera

Frequency-dependent prey choice by natural enemies may influence the coexistence of multiple prey types, but little is known about whether frequency-dependent foraging choice occurs in herbivory on plants showing resistance polymorphism within a single population. Here we examined frequency-dependent foraging by a crucifer-feeding leaf beetle, Phaedon brassicae, on trichome-producing (hairy) and trichomeless (glabrous) plants coexisting within a natural population of the perennial herb Arabidopsis halleri subsp. gemmifera. Larvae of P. brassicae fed on hairy leaves showed slower growth than those fed on glabrous leaves. Although adult beetles consumed similar amounts of leaves when they were fed either hairy or glabrous leaves in no-choice conditions, our choice experiment showed that adult beetles fed at less than the proportionally expected level on hairy leaves compared to glabrous leaves when the hairy leaves were less or equally abundant. Both types of leaves were consumed at the proportionally expected levels when the hairy leaves were more abundant than the glabrous leaves. In a natural population, the leaf damage on the hairy plants was negatively correlated with the local proportion of the glabrous plants in a 1-m diameter patch across 2 years, while correlations between the leaf damage on the glabrous plants and their proportion differed between the 2 years. Additionally, we found five glucosinolates in leaves of A. halleri, but their accumulation did not differ between hairy and glabrous plants. Our experimental results indicate that hairy plants incur less herbivory by P. brassicae when glabrous plants are abundant. The field pattern provides evidence suggestive of frequency-dependent herbivory acting on hairy plants. The present study highlights one of the putative mechanisms of maintaining plant resistance polymorphism.     

Cost of trichome production and resistance to a specialist insect herbivore in <Emphasis Type="Italic">Arabidopsis lyrata</Emphasis>

Theory predicts that trade-offs between resistance to herbivory and other traits positively affecting fitness can maintain genetic variation in resistance within plant populations. In the perennial herb Arabidopsis lyrata, trichome production is a resistance trait that exhibits both qualitative and quantitative variation. Using a paternal half-sib design, we conducted two greenhouse experiments to ask whether trichomes confer resistance to oviposition and leaf herbivory by the specialist moth Plutella xylostella, and to examine potential genetic constraints on evolution of increased resistance and trichome density. In addition, we examined whether trichome production is induced by insect herbivory. We found strong positive genetic and phenotypic correlations between leaf trichome density and resistance to leaf herbivory, demonstrating that the production of leaf trichomes increases resistance to leaf damage by P. xylostella. Also resistance to oviposition tended to increase with increasing leaf trichome density, but genetic and phenotypic correlations were not statistically significant. Trichome density and resistance to leaf herbivory were negatively correlated genetically with plant size in the absence of herbivores, but not in the presence of herbivores. There was no evidence of increased trichome production after leaf damage by P. xylostella. The results suggest that trichome production and resistance to leaf herbivory are associated with a cost and that the direction of selection on resistance and trichome density depends on the intensity of herbivory.     

Trichomes of Lycopersicon species and their hybrids: effects on pests and natural enemies

1 The cultivated tomato, Lycopersicon esculentum, is an economically important worldwide crop. Current pest management techniques rely heavily on pesticides but trichome‐based host‐plant resistance may reduce pesticide use. 2 A review of the literature is provided on trichomes of wild Lycopersicon species and the effects of trichome‐based host‐plant resistance on arthropods. Solvents have been used to remove glandular trichome exudates and the resulting dimminution of their effects quantified. Correlational approaches to assess the relationship between the different trichome types and effects on pests have also been used. 3 Most studies have focused on Lepidoptera and Hemiptera, although some work has included Coleoptera, Diptera and Acarina, and both antibiotic and antixenotic effects have been demonstrated. 4 Natural enemies are a cornerstone of international pest management and this review discusses how the compatibility of this approach with trichome‐based host‐plant resistance is uncertain because of the reported negative effects of trichomes on one dipteran, one hemipteran and several Hymenoptera. 5 For trichome‐based host‐plant resistance to be utilized as a pest management tool, trichomes of wild species need to be introgressed into the cultivated tomato. Hybrids between the cultivated tomato and the wild species Lycopersicon hirsutum f. glabratum, Lycopersicon pennellii and Lycopersicon cheesmanii f. minor have been produced and useful levels of resistance to Acarina, Diptera and Hemiptera pests have been exhibited, although these effects may be tempered by effects on natural enemies. 6 This review proposes that studies on genetic links between fruit quality and resistance, field studies to determine the compatibility of natural enemies and trichome‐based host‐plant resistance, and a strong focus on L. cheesmanii f. minor, are all priorities for further research that will help realize the potential of this natural defence mechanism in pest management.     

Trichomes of Lycopersicon species and their hybrids: effects on pests and natural enemies

1 The cultivated tomato, Lycopersicon esculentum, is an economically important worldwide crop. Current pest management techniques rely heavily on pesticides but trichome‐based host‐plant resistance may reduce pesticide use. 2 A review of the literature is provided on trichomes of wild Lycopersicon species and the effects of trichome‐based host‐plant resistance on arthropods. Solvents have been used to remove glandular trichome exudates and the resulting dimminution of their effects quantified. Correlational approaches to assess the relationship between the different trichome types and effects on pests have also been used. 3 Most studies have focused on Lepidoptera and Hemiptera, although some work has included Coleoptera, Diptera and Acarina, and both antibiotic and antixenotic effects have been demonstrated. 4 Natural enemies are a cornerstone of international pest management and this review discusses how the compatibility of this approach with trichome‐based host‐plant resistance is uncertain because of the reported negative effects of trichomes on one dipteran, one hemipteran and several Hymenoptera. 5 For trichome‐based host‐plant resistance to be utilized as a pest management tool, trichomes of wild species need to be introgressed into the cultivated tomato. Hybrids between the cultivated tomato and the wild species Lycopersicon hirsutum f. glabratum, Lycopersicon pennellii and Lycopersicon cheesmanii f. minor have been produced and useful levels of resistance to Acarina, Diptera and Hemiptera pests have been exhibited, although these effects may be tempered by effects on natural enemies. 6 This review proposes that studies on genetic links between fruit quality and resistance, field studies to determine the compatibility of natural enemies and trichome‐based host‐plant resistance, and a strong focus on L. cheesmanii f. minor, are all priorities for further research that will help realize the potential of this natural defence mechanism in pest management.     

Relaxation of herbivore‐mediated selection drives the evolution of genetic covariances between plant competitive and defense traits

Insect herbivores are important mediators of selection on traits that impact plant defense against herbivory and competitive ability. Although recent experiments demonstrate a central role for herbivory in driving rapid evolution of defense and competition‐mediating traits, whether and how herbivory shapes heritable variation in these traits remains poorly understood. Here, we evaluate the structure and evolutionary stability of the G matrix for plant metabolites that are involved in defense and allelopathy in the tall goldenrod, Solidago altissima. We show that G has evolutionarily diverged between experimentally replicated populations that evolved in the presence versus the absence of ambient herbivory, providing direct evidence for the evolution of G by natural selection. Specifically, evolution in an herbivore‐free habitat altered the orientation of G , revealing a negative genetic covariation between defense‐ and competition‐related metabolites that is typically masked in herbivore‐exposed populations. Our results may be explained by predictions of classical quantitative genetic theory, as well as the theory of acquisition‐allocation trade‐offs. The study provides compelling evidence that herbivory drives the evolution of plant genetic architecture.     

Trichome production and spatiotemporal variation in herbivory in the perennial herb Arabidopsis lyrata

Allocation theory suggests that the optimal level of resistance against herbivores should vary with the risk of herbivory if allocation to resistance is costly. The perennial herb Arabidopsis lyrata has a genetically based polymorphism for trichome production and occurs in a glabrous and a trichome-producing form. Leaf trichomes (hairs) can protect plants against insect herbivores, and may increase tolerance to drought and UV-radiation. To examine the functional significance of trichome production, we documented the frequency of glabrous plants and damage by insect herbivores in 30 A. lyrata populations in Sweden and Norway. The proportion of glabrous plants ranged from 0.10 to 0.71 (median=0.44) in polymorphic populations; 7 of 12 populations in Norway and 14 of 18 populations in Sweden were monomorphic glabrous, i.e. with fewer than 5% trichome-producing plants. The mean proportion of the leaf area removed by herbivores varied substantially among populations and years. With few exceptions, glabrous plants were more damaged than trichome-producing plants in polymorphic populations. The intensity of herbivory quantified as the mean damage to glabrous plants tended to be higher in polymorphic populations than in populations monomorphic for the glabrous morph and was higher in Sweden than in Norway. In Norway, both the magnitude of herbivore damage and the frequency of trichome-producing plants tended to decrease with increasing altitude. The results indicate that leaf trichomes contribute to resistance against herbivorous insects in A. lyrata , and suggest that herbivore-mediated selection contributes to the maintenance of the polymorphism in trichome production.     

Reducing the impact of insect herbivory in eucalypt plantations through management of extrinsic influences on tree vigour

In this paper it is argued that concepts developed in ecologically derived insect–plant interaction models can contribute directly to the management of insect herbivory in eucalypt plantations. Common to most species of commercially planted eucalypt is their genetic potential for early rapid growth. Several plant defence theories predict that intrinsically fast growing plants are able to tolerate relatively high levels of herbivory. The risk of this strategy failing increases when plants are exposed to external stressful factors that reduce canopy growth and vigour. Results from a young Eucalyptus camaldulensis plantation stressed by moisture deficit and two young Eucalyptus dunnii plantations, stressed by flooding and weed competition, respectively, are summarized. In all three cases, the stress‐inducing agents reduced canopy growth rates and architecture so that the proportion of leaf tissue damaged by insects increased and the tree’s ability to tolerate this damage decreased. Therefore, alleviating tree stress through improved silvicultural practices or improved site selection techniques may indirectly reduce the impact of insect herbivory. In resource‐limiting environments, an alternative approach may be to plant eucalypt species that although slower growing, are predicted to have better defended foliage. Manipulation of these natural antiherbivore plant strategies are not exclusive of other management approaches, such as the need for routine surveillance of key pest insects or the genetic selection of natural insect resistance and selective chemical control techniques, but should be viewed as an overarching concept for plantation health.     

Molecular and ecological plant defense responses along an elevational gradient in a boreal ecosystem

Plants have the capacity to alter their phenotype in response to environmental factors, such as herbivory, a phenomenon called phenotypic plasticity. However, little is known on how plant responses to herbivory are modulated by environmental variation along ecological gradients. To investigate this question, we used bilberry (Vaccinium myrtillus L.) plants and an experimental treatment to induce plant defenses (i.e., application of methyl jasmonate; MeJA), to observe ecological responses and gene expression changes along an elevational gradient in a boreal system in western Norway. The gradient included optimal growing conditions for bilberry in this region (ca. 500 m a.s.l.), and the plant's range limits at high (ca. 900 m a.s.l.) and low (100 m a.s.l.) elevations. Across all altitudinal sites, MeJA‐treated plants allocated more resources to herbivory resistance while reducing growth and reproduction than control plants, but this response was more pronounced at the lowest elevation. High‐elevation plants growing under less herbivory pressure but more resource‐limiting conditions exhibited consistently high expression levels of defense genes in both MeJA‐treated and untreated plants at all times, suggesting a constant state of “alert.” These results suggest that plant defense responses at both the molecular and ecological levels are modulated by the combination of climate and herbivory pressure, such that plants under different environmental conditions differentially direct the resources available to specific antiherbivore strategies. Our findings are important for understanding the complex impact of future climate changes on plant–herbivore interactions, as this is a major driver of ecosystem functioning and biodiversity.     

Induced responses in Ipomoea hederacea: simulated mammalian herbivory induces resistance and susceptibility to insect herbivores

Multispecies interactions between plants and natural enemies are ubiquitous, and often lead to diffuse interactions between plants and their herbivores. Non-specific induced responses, where responses induced by one species affect other species, are one potential mechanism generating diffuse interactions. Using 57 inbred lines of the Ivyleaf morning glory, Ipomoea hederacea, in a greenhouse experiment, we examined whether simulated mammalian herbivory induced responses that could affect plant resistance to the generalist insect herbivore, Spodoptera exigua. Inbred lines were highly variable for induced responses, ranging from induced resistance to induced susceptibility, with the rank-order for resistance in inbred lines changing between clipping and control treatments. We failed to detect significant genetic correlations between induced responses and trichome density, or that clipping modified the negative relationship between trichome density and Spodoptera exigua consumption and biomass. Our results suggest that non-specific induced responses can mediate the diffuse evolutionary relationship between I. hederacea and its herbivores, and that genetic variation in induced responses are an important component of this interaction. Handling Error: Heikki Hokkanen     

Herbivore‐induced changes in flower scent and morphology affect the structure of flower–visitor networks but not plant reproduction

Herbivory induces various responses in plants, thus altering the plants’ phenotype in chemical and morphological traits. Herbivore‐induced changes in vegetative plant parts, plant‐physiological mechanisms, and effects on plant‐animal interactions have been intensively studied from species to community level. In contrast, we are just beginning to examine herbivore‐induced effects on reproductive plant parts and flower–visitor interactions, especially in a community context. We investigated the effect of herbivory at different plant developmental stages on plant growth, floral and vegetative phenotype and reproduction in Sinapis arvensis (Brassicaceae). Additionally, we tested how herbivore‐induced plant responses affect flower–visitor interactions and plant reproduction in species‐rich communities. Our results indicate that the timing of herbivory affects the magnitude of changes in plant traits. Herbivory in early but not in late development accelerated the plant's flowering phenology, reduced vegetative growth, increased stem trichome density and altered floral morphology and scent. These findings suggest age‐dependent tradeoffs between growth, defense and reproduction. Herbivore‐induced changes in flower traits also affected flower–visitor interactions in a community context with effects on the structure of flower–visitor networks. However, changes in the network structure had neglectable effects on plant reproduction, i.e. plants were able to compensate altered flower visitor behavior. Thus, herbivory is a source of intraspecific variation in reproductive traits, which can be behaviorally relevant for potential pollinators. However, plants were capable to maintain reproductive success suggesting a tolerance against herbivory. We conclude that in our study system induced direct or indirect defenses that have often been shown to decrease negative effects of herbivores on vegetative plant parts come at no costs for plant reproduction.     

Evidence for rapid evolutionary change in an invasive plant in response to biological control

We present evidence that populations of an invasive plant species that have become re‐associated with a specialist herbivore in the exotic range through biological control have rapidly evolved increased antiherbivore defences compared to populations not exposed to biocontrol. We grew half‐sib families of the invasive plant Lythrum salicaria sourced from 17 populations near Ottawa, Canada, that differed in their history of exposure to a biocontrol agent, the specialist beetle Neogalerucella calmariensis. In a glasshouse experiment, we manipulated larval and adult herbivory to examine whether a population's history of biocontrol influenced plant defence and growth. Plants sourced from populations with a history of biocontrol suffered lower defoliation than naïve, previously unexposed populations, strongly suggesting they had evolved higher resistance. Plants from biocontrol‐exposed populations were also larger and produced more branches in response to herbivory, regrew faster even in the absence of herbivory and were better at compensating for the impacts of herbivory on growth (i.e. they exhibited increased tolerance). Furthermore, resistance and tolerance were positively correlated among genotypes with a history of biocontrol but not among naïve genotypes. Our findings suggest that biocontrol can rapidly select for increased defences in an invasive plant and may favour a mixed defence strategy of resistance and tolerance without an obvious cost to plant vigour. Although rarely studied, such evolutionary responses in the target species have important implications for the long‐term efficacy of biocontrol programmes.     

Ceropegia thaithongiae sp. nov. (Apocynaceae,Asclepiadoideae) from northern Thailand and preliminary observations on impacts of climate change on its only known population

A new pendulous herb, Ceropegia thaithongiae Kidyoo (Apocynaceae, Asclepiadoideae), is described from northern Thailand. This plant is strictly endemic to Chiang Dao Wildlife Sanctuary and critically endangered owing to it living in an ecosystem affected by frequent natural disturbances. It is here described, illustrated and compared to the similar species, C. mairei. Both species have fusiform rootstocks, 1–2‐flowered cymes, deeply bifid outer corona lobes and linear, hairy inner corona lobes. However, the new species can be distinguished by its glabrous stem, leaf covered with hairs on both surfaces and glabrous corolla tube and lobes.     

Plant genotype and induced responses affect resistance to herbivores on evening primrose (Oenothera biennis)

Abstract. 1. Although both genotype and induced responses affect a plant's resistance to herbivores, little is known about their relative and interactive effects. This study examined how plant genotype of a native plant (Oenothera biennis) and induced plant responses to herbivory affect resistance to, and interactions among, several herbivores. 2. In a field experiment, genetic and environmental variation among habitats led to variation in the amount of early season damage and plant quality. The pattern of variation in early season infestation by spittlebugs (Philaenus spumarius, a piercing–sucking herbivore) negatively correlated with oviposition preference by a later feeding specialist weevil (Tyloderma foveolatum, a leaf‐chewer). 3. To determine if plant genotype and induced responses to herbivory might be responsible for these field patterns, we performed no‐choice and choice bioassays using four genotypes of O. biennis that varied in resistance. Plants were induced by either spittlebugs or weevils and assays measured the responses of the same specialist weevil as well as a generalist caterpillar (Spodoptera exigua). 4. Resistance to adult weevils was largely unaffected by plant genotype, while they experienced induced resistance following damage by conspecific weevils in no‐choice assays. Caterpillars were more strongly affected by plant genotype than induced responses in both no‐choice and choice assays, but they also fed less and experienced higher mortality on plants previously damaged by weevils. In contrast to the pattern suggested by the field experiment, spittlebugs did consistently induce resistance against either weevils or caterpillars in the bioassay experiment. 5. These results support recent findings that show herbivore species can compete via induced plant responses. Additionally, a quantitative review of the literature demonstrates that plant genotype tends to be more important than interspecific competition among herbivores (plant‐mediated or otherwise) in affecting herbivore preference and performance.