Consequences of sequential attack for resistance to herbivores when plants have specific induced responses

Plants in nature are attacked sequentially by herbivores, and theory predicts that herbivore-specific responses allow plants to tailor their defenses. We present a novel field test of this hypothesis, and find that specific responses of Solanum dulcamara lead to season-long consequences for two naturally colonizing herbivores, irrespective of the second herbivore to attack plants. This result indicates that responses induced by the initial herbivore made plants less responsive to subsequent attack. We show that initial herbivory by flea beetles and tortoise beetles induce distinct plant chemical responses. Initial herbivory by flea beetles lowered the occurrence of conspecifics and tortoise beetles relative to controls. Conversely, initial herbivory by tortoise beetles did not influence future herbivory. Remarkably, the experimentally imposed second herbivore to feed on plants did not modify consequences (induced resistance or lack thereof) of the first attacker. Induction of plant chemical responses was consistent with these ecological effects; i.e. the second herbivore did not modify the plant's initial induced response. Thus, canalization of the plant resistance phenotype may constrain defensive responses in a rapidly changing environment.     

Herbivore-induced responses and patch heterogeneity affect abundance of arthropods on plants

Abstract.  1. Plants respond to herbivore damage by inducing defences that can affect the abundance of herbivores and predators. These tritrophic interactions may be influenced by heterogeneity in plant neighbourhood.
2. In the present study, the effects of induced responses on the abundance of herbivores (flea beetles and aphids), omnivores (pirate bugs and thrips), and predators (lady beetles and spiders) on individual plants and their neighbours between and within patches composed of three tomato plants was investigated.
3. Herbivore damage was manipulated to create homogeneous patches where either all or none of the plants had defences induced by herbivore damage, and heterogeneous patches where only one of the plants was induced.
4. Arthropod abundance on plants at different scales was compared by testing between patch effects (patch level), for neighbourhood effects at the plant phenotype level (neighbourhood level), and between near and far plants (within patch position).
5. At the patch level , plants in homogeneously induced patches contained fewer flea beetles and pirate bugs, but more lady beetles, compared with homogeneously non-induced patches. There was no effect of patch type on the abundance of aphids, thrips, and spiders on plants.
6. At the neighbourhood level , induced plants in heterogeneous patches contained more flea beetles and pirate bugs compared with induced plants in homogeneous patches, indicating that the abundance of some herbivores and omnivores on induced plants varied depending on the phenotype of the other plants within the patch. Within patch position, there was no evidence that the abundance of herbivores or predators on non-induced plants was affected by proximity to an induced plant.
7. Therefore, variation in plant neighbourhood generated by induced plant responses affected the abundance of three arthropods from three feeding guilds.     

Density dependence in insect performance within individual plants: induced resistance to Spodoptera exigua in tomato

Net intraspecific density dependence experienced by insect herbivores at the scale of single plants can be a function both of induced resistance in the plant and other interactions among individual herbivores. Theory suggests that non‐linearity in the form of this density dependence can influence the effects of plants on herbivore population dynamics. This study examined both net density dependence at the scale of single plants, and changes in plant quality with herbivore density for Spodoptera exigua caterpillars on tomato plants. One experiment measured the growth of caterpillars moving freely about the plant at different densities, the distribution of damage by these caterpillars, and the quality of the plant as food for caterpillars (growth of caterpillars on undamaged leaf tissue excised from the plant). A second experiment measured plant quality for plants with different amounts of damage by caterpillars confined to particular leaves in mesh bags. Growth of S. exigua caterpillars was found to be negatively density dependent, and this was in part due to decreases in plant quality both as herbivore density increased and as the amount of damage increased. The response of plant quality to herbivores was found to have non‐linear features; there was both a threshold below which no significant decreases in quality (as measured by herbivore growth) occurred, and the decrease in herbivore performance saturated at the highest damage levels. In addition, it was found that caterpillar damage was significantly more aggregated than expected when multiple caterpillars occupy a single plant. This study confirms that host plants have the potential to be a source of density dependence that affects herbivore performance.     

Response of Brassica oleracea to temporal variation in attack by two herbivores affects preference and performance of a third herbivore

1. Plants are frequently under attack by multiple insect herbivores, which may interact indirectly through herbivore‐induced changes in the plant's phenotype. The identity, order, and timing of herbivore arrivals may influence the outcome of interactions between two herbivores. How these aspects affect, in turn, subsequently arriving herbivores that feed on double herbivore‐induced plants has not been widely investigated. 2. This study tested whether the order and timing of arrival of two inducing herbivores from different feeding guilds affected the preference and performance of a subsequently arriving third herbivore, caterpillars of Mamestra brassicae L. (Lepidoptera: Noctuidae). Aphids [Brevicoryne brassicae L. (Hemiptera: Aphididae)] and caterpillars [Plutella xylostella L. (Lepidoptera: Yponomeutidae)] were introduced onto wild Brassica oleracea L. (Brassicaceae) plants in different sequences and with different arrival times. The effects of these plant treatments on M. brassicae caterpillars were assessed in pairwise preference tests and no‐choice performance tests. 3. The caterpillars of M. brassicae preferred to feed from undamaged plants rather than double herbivore‐induced plants. Compared with undamaged plants, they preferred plant material on which aphids had arrived first followed by caterpillars, whereas they avoided plant material with the reverse order of herbivore arrival. Performance of the caterpillars increased with increasing arrival time between herbivore infestations in double herbivore‐induced plants. Although M. brassicae grew faster on plants induced by aphids than on those induced by caterpillars alone, its performance was not affected by the order of previous herbivore arrival. 4. These results imply that the timing of colonisation by multiple herbivores determines the outcome of plant‐mediated herbivore–herbivore interactions.     

Heterogeneity of plant phenotypes caused by herbivore-specific induced responses influences the spatial distribution of herbivores

Abstract.  1. Herbivory can induce resistance in a plant and the induced phenotype may be disfavoured by subsequent herbivores. Yet, as the distance between plants in a population increases, limited mobility may make a herbivore more likely to feed and oviposit on host plants in its immediate surroundings.
2. The present study tested whether a herbivore's preference and distribution across plants with different induced phenotypes was influenced by the spatial distribution of plants. A fragmented population of Solanum dulcamara plants was created. This consisted of discrete, spatially separated patches with different histories of damage, either herbivory from adult flea beetles ( Psylliodes affinis ), tortoise beetles ( Plagiometriona clavata ), or mechanical damage. Each patch was separated by 7 m and consisted of 12 plants that were spaced 30 cm apart. Then a fixed number of adult tortoise beetles were introduced to each patch, and movement and oviposition within and between spatially separate homogeneous patches (receiving one type of damage) were compared with movement and oviposition within heterogeneous patches (containing all three types of damage) over the growing season.
3. Flea beetle and tortoise beetle herbivory consistently induced different phytochemical responses in S. dulcamara (polyphenol oxidase and peroxidase), and adult tortoise beetles avoided oviposition on the flea beetle induced plants within heterogeneous patches. However, between homogeneous patches, plant phenotype did not influence oviposition. Colonisation by naturally occurring flea beetle adults followed a similar pattern.
4. These results suggest that the heterogeneity of plant phenotypes can influence herbivore choice and distribution at small but not large spatial scales.     

Shared signals -'alarm calls' from plants increase apparency to herbivores and their enemies in nature

The attraction of natural enemies of herbivores by volatile organic compounds as an induced indirect defence has been studied in several plant systems. The evidence for their defensive function originates mainly from laboratory studies with trained parasitoids and predators; the defensive function of these emissions for plants in natural settings has been rarely demonstrated. In native populations and laboratory Y-tube choice experiments with transgenic Nicotiana attenuata plants unable to release particular volatiles, we demonstrate that predatory bugs use terpenoids and green leaf volatiles (GLVs) to locate their prey on herbivore-attacked plants. By attracting predators with volatile signals, this native plant reduces its herbivore load – demonstrating the defensive function of herbivore-induced volatile emissions. However, plants producing GLVs are also damaged more by flea beetles. The implications of these conflicting ecological effects for the evolution of induced volatile emissions and for the development of sustainable agricultural practices are discussed.     

Herbivore‐induced plant responses in Brassica oleracea prevail over effects of constitutive resistance and result in enhanced herbivore attack

1. Plant responses to herbivore attack may have community‐wide effects on the composition of the plant‐associated insect community. Thereby, plant responses to an early‐season herbivore may have profound consequences for the amount and type of future attack. 2. Here we studied the effect of early‐season herbivory by caterpillars of Pieris rapae on the composition of the insect herbivore community on domesticated Brassica oleracea plants. We compared the effect of herbivory on two cultivars that differ in the degree of susceptibility to herbivores to analyse whether induced plant responses supersede differences caused by constitutive resistance. 3. Early‐season herbivory affected the herbivore community, having contrasting effects on different herbivore species, while these effects were similar on the two cultivars. Generalist insect herbivores avoided plants that had been induced, whereas these plants were colonised preferentially by specialist herbivores belonging to both leaf‐chewing and sap‐sucking guilds. 4. Our results show that community‐wide effects of early‐season herbivory may prevail over effects of constitutive plant resistance. Induced responses triggered by prior herbivory may lead to an increase in susceptibility to the dominant specialists in the herbivorous insect community. The outcome of the balance between contrasting responses of herbivorous community members to induced plants therefore determines whether induced plant responses result in enhanced plant resistance.     

Order of herbivore arrival on wild cabbage populations influences subsequent arthropod community development

In plant–arthropod associations, the first herbivores to colonise a plant may directly or indirectly affect community assembly on that particular plant. Whether the order of arrival of different arthropod species further modulates community assembly and affects plant fitness remains unclear. Using wild Brassica oleracea plants in the field, we manipulated the order of arrival of early‐season herbivores that belong to different feeding guilds, namely the aphid Brevicoryne brassicae and caterpillars of Plutella xylostella. We investigated the effect of herbivore identity and order of arrival on community assembly on two B. oleracea plant populations during two growth seasons. For this perennial plant, we evaluated whether foliar herbivory also affected herbivore communities on the flowers and if these interactions affected plant seed production. Aphid infestation caused an increase in parasitoid abundance, but caterpillars modulated these effects, depending on the order of herbivore infestation and plant population. In the second growth season, when plants flowered, the order of infestation of leaves with aphids and caterpillars more strongly affected abundance of herbivores feeding on the flowers than those feeding on leaves. Infestation with caterpillars followed by aphids caused an increase in flower‐feeding herbivores compared to the reversed order of infestation in one plant population, whereas the opposite effects were observed for the other plant population. The impact on plant seed set in the first reproductive year was limited. Our work shows that the identity and arrival order of early season herbivores may have long‐term consequences for community composition on individual plants and that these patterns may vary among plant populations. We discuss how these community processes may affect plant fitness and speculate on the implications for evolution of plant defences.     

The effect of rhizosphere microbes outweighs host plant genetics in reducing insect herbivory

Rhizosphere microbes affect plant performance, including plant resistance against insect herbivores; yet, a direct comparison of the relative influence of rhizosphere microbes versus plant genetics on herbivory levels and on metabolites related to defence is lacking. In the crucifer Boechera stricta, we tested the effects of rhizosphere microbes and plant population on herbivore resistance, the primary metabolome, and select secondary metabolites. Plant populations differed significantly in the concentrations of six glucosinolates (GLS), secondary metabolites known to provide herbivore resistance in the Brassicaceae. The population with lower GLS levels experienced ~60% higher levels of aphid (Myzus persicae) attack; no association was observed between GLS and damage by a second herbivore, flea beetles (Phyllotreta cruciferae). Rhizosphere microbiome (disrupted vs. intact native microbiome) had no effect on plant GLS concentrations. However, aphid number and flea beetle damage were respectively about three‐ and seven‐fold higher among plants grown in the disrupted versus intact native microbiome treatment. These differences may be attributable to shifts in primary metabolic pathways previously implicated in host defence against herbivores, including increases in pentose and glucoronate interconversion among plants grown with an intact microbiome. Furthermore, native microbiomes with distinct community composition (as estimated from 16s rRNA amplicon sequencing) differed two‐fold in their effect on host plant susceptibility to aphids. The findings suggest that rhizosphere microbes, including distinct native microbiomes, can play a greater role than population in defence against insect herbivores, and act through metabolic mechanisms independent of population.     

Plant sex and induced responses independently influence herbivore performance, natural enemies and aphid-tending ants

Sex is an ecologically important form of genetic variation in dioecious plants, with males and females generally differing in constitutive resistance to herbivores. Yet little is known about sexual dimorphism with respect to induced or indirect defense, or whether sex-based differences are underlain by trade-offs among modes of defense. We compared male and female Valeriana edulis plants for constitutive and induced direct resistance to two herbivores, an early-season caterpillar and a late-season aphid, and for constitutive and induced indirect resistance in terms of abundance of natural enemies and aphid-tending ants. No sexual dimorphism was found in constitutive direct plant resistance, yet the sexes differed for constitutive indirect resistance, with 78?% more natural enemies and 117?% more ants present on females than males. Past feeding damage by caterpillars induced direct and indirect resistance in both males and females, increasing caterpillar development time by 26?% and the abundance of natural enemies by 147?%. Caterpillar feeding did not induce direct resistance with respect to caterpillar final mass or aphid performance. In all cases, there were no interactions between the effects of caterpillar damage and plant sex. In summary, plant sexual dimorphism and induced responses to herbivore damage independently influenced herbivore performance and the composition of arthropod communities at higher trophic levels.     

Co(i)-ordinating defenses: NaCOI1 mediates herbivore- induced resistance in Nicotiana attenuata and reveals the role of herbivore movement in avoiding defenses

Arabidopsis and tomato plants mutated in the F-box protein COI1 mediating jasmonate (JA) responses are more susceptible to herbivores in laboratory trials, but the exact mechanisms of COI1-mediated resistance are not known. We silenced COI1 by transformation with an inverted repeat construct (ir-coi1) in Nicotiana attenuata, a plant the direct and indirect defenses of which against various herbivores have been well studied. ir-coi1 plants are male sterile and impaired in JA-elicited direct [nicotine, caffeoylputrescine and trypsin proteinase inhibitor (TPI) activity] and indirect (cis-alpha-bergamotene emission) defense responses; responses not elicited by JA treatment (ethylene production and flower TPI activity) were unaffected. Larvae of Manduca sexta, a common herbivore of N. attenuata, gained three times more mass feeding on ir-coi1 than on wild-type (WT) plants in glasshouse experiments. By regularly moving caterpillars to unattacked leaves of the same plant, we demonstrate that larvae on WT plants can grow and consume leaves as fast as those on ir-coi1 plants, a result that underscores the role of COI1 in mediating locally induced resistance in attacked leaves, and the importance of herbivore movement in avoiding the induced defenses of a plant. When transplanted into native habitats in the Great Basin Desert, ir-coi1 plants suffer greatly from damage by the local herbivore community, which includes herbivores not commonly found on N. attenuata WT plants. Choice assays with field-grown plants confirmed the increased attractiveness of ir-coi1 plants for both common and unusual herbivores. We conclude that NaCOI1 is essential for induced resistance in N. attenuata, and that ir-coi1 plants highlight the benefits of herbivore movement for avoiding induced defenses.     

Leafhopper-induced plant resistance enhances predation risk in a phytophagous beetle

Many herbivores elicit biochemical, physiological, or morphological changes in their host plants that render them more resistant to co-occurring herbivores. Yet, despite the large number of studies that investigate how induced resistance affects herbivore preference and performance, very few have simultaneously explored the cascading effects of induction on higher trophic levels and consequences for prey suppression. In our study system, early-season herbivory by leafhoppers elevated plant resistance to subsequent attack by chrysomelid beetles sharing the same host plant. Notably, beetles feeding on leafhopper-damaged plants incurred developmental penalties (e.g., prolonged time in early larval instars) that rendered them more susceptible to predation by natural enemies. As a result, the combined bottom-up effect of leafhopper-induced resistance and the top-down effect of enhanced predation resulted in the synergistic suppression of beetle populations. These results emphasize that higher trophic level dynamics should be considered in conjunction with induced resistance to better understand how plants mediate interspecific interactions in phytophagous insect communities.     

The jasmonate pathway alters herbivore feeding behaviour: consequences for plant defences

The jasmonate pathway is a highly conserved defensive cascade in plants that regulates the induction of resistance against herbivores; however, its role in herbivore feeding behaviour remains unknown. We used a mutant tomato plant (def‐1) deficient in the production of jasmonate‐related defensive proteins to test the hypothesis that genotypes with a reduced ability to induce resistance have a higher and more concentrated pattern of herbivore damage. Wild‐type and def‐1 plants received either damage by Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) caterpillars or no damage. After treatment, we tested for systemic responses by allowing a free roaming S. exigua caterpillar to feed on the undamaged portions of plants. Weight‐gain and leaf consumption of S. exigua were highest on def‐1 plants, regardless of prior herbivore damage. Def‐1 plants also had fewer numbers of leaves and leaflets eaten, and fewer feeding holes, which was indicative of a more concentrated distribution of damage on mutant compared to wild‐type plants. Following these results, we mimicked the amount and distribution of feeding damage that wild‐type or jasmonate‐deficient plants would receive on wild‐type plants to test whether changes in feeding behaviour may feedback to influence the expression of induced resistance. We mimicked the distribution of damage in wild‐type and jasmonate‐deficient plants by allowing caterpillars to feed on either one (leaf 1 or 2) or two leaves (leaf 1 and 2). Increased herbivore damage resulted in higher proteinase inhibitor (PI) activity, a jasmonate‐regulated defensive protein, and lower S. exigua performance on wild‐type but not jasmonate‐deficient plants. Compared to undamaged plants, a concentrated pattern of herbivore damage increased systemic resistance; these induced responses were greater on leaflets with stronger vascular connections to the damaged leaf. A more dispersed pattern of caterpillar damage altered the expression of induced responses, but the outcome depended on the specific pattern of damage. When leaf 1 was damaged and then leaf 2, the undamaged (third) leaf (which is more strongly connected to leaf 1 than 2) expressed reduced the PI activity compared to plants receiving concentrated damage to leaf 1; whereas in plants where leaf 2 was first damaged and then leaf 1, there were no differences in PI activity in leaf 3 compared to plants receiving concentrated damage to leaf 2. Thus, induction of the jasmonate pathway may not only determine the amount and distribution of feeding damage by herbivores, but this may feedback to affect the subsequent expression of plant defence.     

Specificity of induced resistance in wild radish: causes and consequences for two specialist and two generalist caterpillars

Inducible plant resistance against herbivores is becoming a paradigm of plant–herbivore ecology. Fundamental to understanding induced resistance and its evolutionary ecology is specificity of "induction" and "effects". Specificity in the induction of resistance refers to whether plant damage by various herbivores causes the same response in plants. Specificity in the effects of induced resistance refers to whether induction has the same consequences (i.e., reduced preference or performance) for various herbivores. I examined both specificity of induction and effect employing four lepidopteran herbivores and wild radish plants, a system for which fitness benefits and costs of induction have been documented for the plant. Variation in the specificity of induction and effects of induced plant resistance was found; however, this variation was not associated with diet specialization in the herbivores (i.e., specialists vs generalists). Induction caused by Plutella (specialist) and Spodoptera (generalist) resulted in general resistance to all of the herbivores, induction caused by Pieris (specialist) induced resistance only to Spodoptera (generalist) and Pieris , and plant damage by Trichoplusia (generalist) failed to induce resistance and reduce the performance of any of the herbivores. To the contrary, plants damaged by Trichoplusia supported enhanced growth of subsequently feeding Trichoplusia compared to uninduced controls. These results add a novel level of complexity to interactions between plants and leaf chewing caterpillars. Within the same guild of feeders, some herbivores cause strong induced resistance, no induced resistance, or induced susceptibility. Similarly, caterpillar species were variable in the level to which induced resistance affected their performance. Such interactions limit the possibility of pairwise coevolution between plants and herbivores, and suggest that coevolution can only be diffuse.     

Preference and performance of Lepidoptera varies with tree age in juniper woodlands

1. As trees age, they undergo significant physiological and morphological changes. Nevertheless, tree ontogeny and its impacts on herbivores are often overlooked as determinants of plant–herbivore population dynamics and the strength of plant–herbivore interactions. 2. Juniperus (Cupressaceae) is a dominant, long‐lived conifer that serves as the sole host to a specialised assemblage of caterpillars. Over the past 150 years, several juniper species in western North America have expanded their geographic occupancy at local and regional scales, which has resulted in an increase in the number of immature trees on the landscape. Using assays in the laboratory, the effects of tree ontogeny on caterpillar performance and oviposition preference for two juniper specialist caterpillars, Callophrys gryneus (Lycaenidae) and Glena quinquelinearia (Geometridae), were examined. The study considered whether responses to tree ontogeny were consistent across caterpillar species and juniper host species. 3. Tree age was found to be a reliable predictor of caterpillar performance, with caterpillars developing more quickly and growing larger when fed foliage from young trees. Differences in the phytochemical diversity between foliage from trees of different ages might help to explain observed differences in caterpillar performance. Interestingly, the specialist butterfly, C. gryneus, displayed an oviposition preference for foliage from old‐growth Juniperus osteosperma trees, despite the fact that larvae of this species performed poorly on older trees. 4. It is concluded that young juniper trees are an important resource for the specialised Lepidopteran community and that tree ontogeny is an important component of intraspecific variation, which contributes to the structure of plant–herbivore communities.     

Interactions between plants and herbivores: A review of plant defense

Ecologists have long ignored or underestimated the importance of plant–herbivore interactions owing to the diversities of herbivores, plant defensive strategies and ecological systems. In this review, we briefly discussed the categories of herbivores. Then we reviewed the major types of plant defenses against herbivores. Selective forces of herbivore pressures have led to the evolution of various defensive mechanisms in plants, which can be classified into (i) resistance traits that reduce the amount of damage received, including physical, chemical, and biotic traits; (ii) tolerance mechanisms that decrease the impact of herbivore damage, and (iii) escape strategies that reduce the probability of plants to be found by herbivores. These strategies have been studied at different levels from molecular genetics and genomics, to chemistry and physiology, to community and ecosystem ecology. We summarized the development of the methodology for studying plant defenses against herbivores. Particularly, 24 of those hypotheses and models, which are influential in the international community concerning the relationship between plants and herbivores, including the defensive mimicry hypothesis, the compensatory continuum hypothesis, the slow-growth-high-mortality hypothesis, etc, were introduced and grouped into four categories according to plant defense strategies in the present review. Finally, we also reviewed the research progress of plant–herbivore interactions in China, and discussed the perspectives of studies on plant–herbivore interactions.     

Contrasting cascades: insectivorous birds increase pine but not parasitic mistletoe growth

1. Intraguild predation occurs when top predators feed upon both intermediate predators and herbivores. Intraguild predators may thus have little net impact on herbivore abundance. Variation among communities in the strength of trophic cascades (the indirect effects of predators on plants) may be due to differing frequencies of intraguild predation. Less is known about the influence of variation within communities in predator-predator interactions upon trophic cascade strength. 2. We compared the effects of a single predator community between two sympatric plants and two herbivore guilds. We excluded insectivorous birds with cages from ponderosa pine Pinus ponderosa trees parasitized by dwarf mistletoe Arceuthobium vaginatum. For 3 years we monitored caged and control trees for predatory arthropods that moved between the two plants, foliage-feeding caterpillars and sap-feeding hemipterans that were host-specific, and plant damage and growth. 3. Excluding birds increased the abundance of ant-tended aphids on pine and resulted in an 11% reduction in pine woody growth. Mutualist ants protected pine-feeding aphids from predatory arthropods, allowing aphid populations to burgeon in cages even though predatory arthropods also increased in cages. By protecting pine-feeding aphids from predatory arthropods but not birds, mutualist ants created a three-tiered linear food chain where bird effects cascaded to pine growth via aphids. 4. In contrast to the results for tended aphids on pine, bird exclusion had no net effects on untended pine herbivores, the proportion of pine foliage damaged by pine-feeding caterpillars, or the proportion of mistletoe plants damaged by mistletoe-feeding caterpillars. These results suggest that arthropod predators, which were more abundant in cages as compared with control trees, compensated for bird predation of untended pine and mistletoe herbivores. 5. These contrasting effects of bird exclusion support food web theory: where birds were connected to pine by a linear food chain, a trophic cascade occurred. Where birds fed as intraguild predators, the reticulate food webs linking birds to pine and mistletoe resulted in no net effects on herbivores or plant biomass. Our study shows that this variation in food web structure occurred between sympatric plants and within plants between differing herbivore guilds.     

Inbreeding in horsenettle influences herbivore resistance

Abstract 1. Plant traits (e.g. nutrition, allelochemistry) are an important determinant of the feeding preferences and performance of insect herbivores. Recent evidence suggests that plant inbreeding can affect plant–insect interactions by impacting host‐plant quality and resistance to herbivory. 2. The effect of inbreeding on host‐plant quality for, and resistance against, the tobacco hornworm, Manduca sexta L., was assessed in the wild solanaceous weed horsenettle, Solanum carolinense L. Caterpillar preference, relative growth rate (RGR), total leaf consumption (TC), and per cent total nitrogen in leaves were examined using selfed and outcrossed progeny of eight maternal plants. 3. Inbreeding significantly influenced insect preference, with caterpillars preferring leaf discs from selfed versus outcrossed plants. There was also a breeding effect for RGR and TC, with both higher on selfed plants. No breeding effect for per cent total nitrogen was observed. 4. The results of this study indicate that inbreeding decreased resistance against the tobacco hornworm, but did not affect plant quality. Decreased plant resistance will likely alter interactions with the herbivore community and could also have important consequences for plant–herbivore–natural enemy interactions.     

Specificity of induced plant responses to specialist herbivores of the common milkweed Asclepias syriaca

Induced plant responses to herbivory appear to be universal, yet the degree to which they are specific to sets of herbivores is poorly understood. The generalist/specialist hypothesis predicts that generalist herbivores are more often negatively affected by host plant defenses, wheras specialists may be either unaffected by or attracted to these same "plant defenses". Therefore, specialists should be less predictable than generalists in their responses to induced plant resistance traits. To better understand the variation in plant responses to herbivore attack, and the impacts these responses have on specialist herbivores, we conducted a series of experiments examining pairwise interactinos between two specialaist herbivores of the common milkweed ( Asclepias syriaca ). We damaged plants mechnically, with swamp milkweed beetles ( Labidomera clivicollis ), or with monarchs ( Danaus plexippus ), and then asessed specificity of elicitation, both by measuring a putative defensive trait (latex volume) and by challenging plants with insects of both species in bioasays. Latex production increased by 34% and 13% following beetle and monarch herbivory, respectively, but only beetles significantly elevated latex production compared to undamaged controls. While beetle growth was negatively affected by latex across all experiments, beetles were not affected by previous damage caused by conspecifies or by monarchs. In contrast, monarchs feeding on previously damaged plants were 20% smaller, and their response was the same on plants damaged mechnically or by either herbivore. Therefore, these specialist herbivores exhibit both specificity of elicitation in plant responses and specificity of effects in response to prior damage.     

The Myriad Plant Responses to Herbivores

Abstract Plant responses to herbivores are complex. Genes activated on herbivore attack are strongly correlated with the mode of herbivore feeding and the degree of tissue damage at the feeding site. Phloem-feeding whiteflies and aphids that produce little injury to plant foliage are perceived as pathogens and activate the salicylic acid (SA)-dependent and jasmonic acid (JA)/ethylene-dependent signaling pathways. Differential expression of plant genes in response to closely related insect species suggest that some elicitors generated by phloem-feeding insects are species-specific and are dependent on the herbivore's developmental stage. Other elicitors for defense-gene activation are likely to be more ubiquitous. Analogies to the pathogen-incompatible reactions are found. Chewing insects such as caterpillars and beetles and cell-content feeders such as mites and thrips cause more extensive tissue damage and activate wound-signaling pathways. Herbivore feeding is not equivalent to mechanical wounding. Wound responses are a part of the induced responses that accompany herbivore feeding. Herbivores induce direct defenses that interfere with herbivore feeding, growth and development, fecundity, and fertility. In addition, herbivores induce an array of volatiles that creates an indirect mechanism of defense. Volatile blends provide specific cues to attract herbivore parasites and predators to infested plants. The nature of the elicitors for volatile production is discussed.