Host plant direct defence against eggs of its specialist herbivore,Heliothis subflexa

1. The insect Heliothis subflexa Guenée is a specialist on plants in the genus Physalis. In the present study, the physical response of Physalis leaves to egg deposition by H. subflexa is described. 2. It was observed that the leaves of Physalis plants respond to the eggs of H. subflexa, while co‐occurring non‐host plants do not. Leaves of Physalis angulata L. and Physalis pubescens L. respond to H. subflexa eggs by the formation of (i) necrotic tissue, (ii) undifferentiated cells that form a bump (neoplasm) under the eggs of this herbivore, or (iii) both types of responses. 3. Greenhouse experiments showed that 64% of eggs laid on P. angulata elicited a response, and that a response to an egg decreased the probability of hatching. Further experiments in the field with P. angulata showed that the mean response to eggs by leaves was 31%, and that this response increased as temperature increased. Field experiments also confirmed that a plant response to an egg decreased the probability of hatching and increased the probability of removal from the plant by physical dislodgement or predation. 4. Eggs that elicited a response had a 25% lower probability of hatching and a 28% lower probability of remaining on the plant, resulting in an average fitness cost of 19.3% for H. subflexa. This is the first study to show an induced direct physical defence of a plant against eggs of a noctuid moth.     

THE GENETIC ARCHITECTURE OF A COMPLEX ECOLOGICAL TRAIT: HOST PLANT USE IN THE SPECIALIST MOTH,HELIOTHIS SUBFLEXA

We used genetic mapping to examine the genetic architecture of differences in host plant use between two species of noctuid moths, Heliothis subflexa, a specialist on Physalis spp., and its close relative, the broad generalist H. virescens. We introgressed H. subflexa chromosomes into the H. virescens background and analyzed 1462 backcross insects. The effects of H. subflexa‐origin chromosomes were small when measured as the percent variation explained in backcross populations (0.2–5%), but were larger when considered in relation to the interspecific difference explained (1.5–165%). Most significant chromosomes had effects on more than one trait, and their effects varied between years, sexes, and genetic backgrounds. Different chromosomes could produce similar phenotypes, suggesting that the same trait might be controlled by different chromosomes in different backcross populations. It appears that many loci of small effect contribute to the use of Physalis by H. subflexa. We hypothesize that behavioral changes may have paved the way for physiological adaptation to Physalis by the generalist ancestor of H. subflexa and H. virescens.     

The social wasp Polybia fastidiosuscula Saussure (Hymenoptera: Vespidae) uses herbivore‐induced maize plant volatiles to locate its prey

Social wasps in the Polybia genus are important for use as pest‐control agents in agricultural systems. The objective of this study was to investigate the behavioural responses of Polybia fastidiosuscula Saussure (Hymenoptera: Vespidae) to volatiles from maize, both constitutive volatiles and those induced by the herbivory of Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae). To assess the behavioural response of P. fastidiosuscula to S. frugiperda larvae, undamaged plants, S. frugiperda‐damaged plants, mechanically damaged plants, mechanically damaged plants plus regurgitant from larvae and extracts from various treatments, bioassays were conducted in a Y‐olfactometer. In addition, the volatiles from plants subjected to different treatments were collected via aeration, and they were quantified and identified. The wasps showed a greater preference for plants with damage induced either by larval feeding or by being mechanically damaged plus regurgitant than for undamaged plants or either larvae alone or mechanically damaged plants. Wasps were more attracted to extracts from plants + S. frugiperda larvae and to an extract from mechanically damaged plants + the regurgitant of larvae compared to hexane. The primary compounds induced by herbivory for 5–6 h after the beginning of the damage or regurgitant treatment were identified as α‐pinene, β‐myrcene, (Z)‐3‐hexenyl acetate, limonene, (E)‐ocimene, linalool, DMNT, (E)‐β‐farnesene, TMTT and indole. The results presented here show that the social wasp P. fastidiosuscula uses herbivore‐induced plant volatiles from maize to locate its prey.     

Herbivore-induced resistance in Betula pendula: the role of plant vascular architecture

We studied the role of plant vascular architecture in the determination of the spatial extent of herbivore induced responses within Betula pendula Roth saplings. The induced responses were measured in bioassays in terms of the relative growth rate of larvae of a geometrid moth, Epirrita autumnata. We hypothesised that the level of induced resistance of a certain leaf would be determined by the degree of vascular connectivity between the leaf in question and a damaged leaf, as suggested by recent theoretical and empirical studies. A comparison of the control plants with the damaged plants indicated that damaging one leaf of a sapling was sufficient to induce an increase in the resistance level. There were also differences among the leaves within a plant in the resistance level, but these differences could not be explained by the degree of vascular connectivity with the damaged leaf. These results suggest that the vascular connections have low power as explanations of the spread and spatial extent of the induced resistance in Betula pendula saplings Instead, the resistance level of all leaves within a sapling increased following the damage. We suggest that the pattern of increased resistance observed in this experiment may be beneficial for the young saplings studied. For young saplings at their early stages of development, it may be beneficial to be able to distribute the induction signal to all leaves as fast as possible and thus repel the herbivore totally. For a young sapling, the capability of repelling the herbivore totally might thus be a feasible strategy whereas an older sapling may tolerate localised damage better and compensate for the damage within the undamaged plant parts.     

Herbivore perception decreases photosynthetic carbon assimilation and reduces stomatal conductance by engaging 12‐oxo‐phytodienoic acid,mitogen‐activated protein kinase 4 and cytokinin perception

Herbivory‐induced changes in photosynthesis have been documented in many plant species; however, the complexity of photosynthetic regulation and analysis has thwarted progress in understanding the mechanism involved, particularly those elicited by herbivore‐specific elicitors. Here, we analysed the early photosynthetic gas exchange responses in Nicotiana attenuata plants after wounding and elicitation with Manduca sexta oral secretions and the pathways regulating these responses. Elicitation with M. sexta oral secretions rapidly decreased photosynthetic carbon assimilation (A_C) in treated and systemic (untreated, vascularly connected) leaves, which were associated with changes in stomatal conductance, rather than with changes in Rubisco activity and 1‐5 ribulose‐1,5‐bisphosphate turnover. Phytohormone profiling and gas exchange analysis of oral secretion‐elicited transgenic plants altered in phytohormone regulation, biosynthesis and perception, combined with micrografting techniques, revealed that the local photosynthetic responses were mediated by 12‐oxo‐phytodienoic acid, while the systemic responses involved interactions among jasmonates, cytokinins and abscisic acid signalling mediated by mitogen‐activated protein kinase 4. The analysis also revealed a role for cytokinins interacting with mitogen‐activated protein kinase 4 in CO₂‐mediated stomatal regulation. Hence, oral secretions, while eliciting jasmonic acid‐mediated defence responses, also elicit 12‐oxo‐phytodienoic acid‐mediated changes in stomatal conductance and A_C, an observation illustrating the complexity and economy of the signalling that regulates defence and carbon assimilation pathways in response to herbivore attack.     

Temporal dynamics of herbivore‐induced responses in Brassica juncea and their effect on generalist and specialist herbivores

Herbivore feeding may induce an array of responses in plants, and each response may have its own temporal dynamics. Precise timing of these plant responses is vital for them to have optimal effect on the herbivores feeding on the plant. This study measured the temporal dynamics of various systemically induced responses occurring in Brassica juncea (L.) Czern. (Brassicaceae) leaves after insect herbivory in India and The Netherlands. Morphological (trichomes, leaf size) and chemical (glucosinolates, amino acids, sugars) responses were analysed. The effects of systemic responses were assessed using a specialist [Plutella xylostella L. (Lepidoptera: Plutellidae)] and a generalist [Spodoptera litura Fabricius (Lepidoptera: Noctuidae)] herbivore. We tested the hypotheses that morphological responses were slower than chemical responses and that generalist herbivores would be more affected by induced responses than specialists. Glucosinolates and trichomes were found to increase systemically as quickly as 4 and 7 days after herbivore damage, respectively. Amino acids, sugars, and leaf size remained unaffected during this period. The generalist S. litura showed a significant feeding preference for undamaged leaves, whereas the specialist herbivore P. xylostella preferred leaves that were damaged 9 days before. Performance bioassays on generalist S. litura revealed that larvae gained half the weight on leaves from damaged plants as compared to larvae feeding on leaves from undamaged plants. These studies show that although morphological responses are somewhat slower than chemical responses, they also contribute to induced plant resistance in a relatively short time span. We argue that before considering induced responses as resistance factors, their effect should be assessed at various points in time with both generalist and specialist herbivores.     

Effect of the postfeeding interval on olfactory responses of thrips to herbivore‐induced cotton plants

We investigated the responses of 3 thrips species, Frankliniella schultzei Trybom, F. occidentalis Pergrande, and Thrips tabaci Lindeman (Thysanoptera: Thripidae) to herbivore‐damaged and undamaged cotton seedlings (Gossypium hirsutum L. [Malvales: Malvaceae]) at a range of time intervals following damage by adult Tetranychus urticae (Koch), adult T. ludeni (Zacher) (Acari: Tetranychidae) or Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) larvae in olfactometer assays. The intensity/frequency of the response of thrips to herbivore‐induced plants decreased with time and ultimately disappeared in all cases; however, the rate at which the response declined was related to the herbivore species that inflicted the damage. All 3 species of thrips were attracted to plants damaged by T. urticae for longer than they were to plants damaged by T. ludeni. The duration for which damaged plants remained attractive was also affected by the degree of damage inflicted on cotton seedlings. For example, F. schultzei was attracted to plants damaged by a higher density of two‐spotted spider mites (100/plant) for much longer than to plants damaged by a lower density of these mites (50/plant). The results reinforce previous studies that demonstrate that arrangement of variables influences the responses of thrips to their herbivore‐induced cotton host plants. Results also show that these responses are variable in time following herbivore damage to cotton plants, which further demonstrates how difficult it is to generalize about the functional significance of these interactions.     

Seasonality of herbivory and communication between individuals of sagebrush

Seasonal changes in herbivore numbers and in plant defenses are well known to influence plant–herbivore interactions. Some plant defenses are induced in response to herbivore attack or cues correlated with risk of attack although seasonal variation in these defenses is relatively poorly known. We previously reported that sagebrush becomes more resistant to its herbivores when neighboring plants have been experimentally clipped with scissors. In this study we asked whether herbivory to leaves of sagebrush varied seasonally and whether there was seasonal variation in natural levels of damage when neighbors were clipped. We found that sagebrush accumulated most chewing damage early in the season, soon after the spring flush of new leaves. This damage was caused by generalist grasshoppers, deer, specialist caterpillars, beetles, gall makers, and other less common herbivores. Sagebrush showed no evidence of preferentially abscising leaves that had been experimentally clipped. Experimental clipping by Trirhabda pilosa beetle larvae caused neighbors to accumulate less herbivore damage later that season, similar to results in which clipping was done with scissors. Induced resistance caused by experimentally clipping a neighbor was affected by season; plants with neighbors clipped in May accumulated less damage throughout the season relative to plants with unclipped neighbors or neighbors clipped later in the summer. We found a correlation between seasonal herbivore pressure, damage accumulated by plants, and induced responses to experimentally clipping neighbors. The causal mechanisms responsible for this correlation are unknown although a strong seasonal effect was clear.     

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.     

Elevated CO2 levels and herbivore damage alter host plant preferences

Interactions between the moth Spodoptera littoralis and two of its host plants, alfalfa (Medicago sativa) and cotton (Gossypium hirsutum) were examined, using plants grown under ambient (350 ppm) and elevated (700 ppm) CO₂ conditions. To determine strength and effects of herbivore‐induced responses assays were performed with both undamaged (control) and herbivore damaged plants. CO₂ and damage effects on larval host plant preferences were determined through dual‐choice bioassays. In addition, larvae were reared from hatching to pupation on experimental foliage to examine effects on larval growth and development. When undamaged plants were used S. littoralis larvae in consumed more cotton than alfalfa, and CO₂ enrichment caused a reduction in the preference for cotton. With damaged plants larvae consumed equal amounts of the two plant species (ambient CO₂ conditions), but CO₂ enrichment strongly shifted preferences towards cotton, which was then consumed three times more than alfalfa. Complementary assays showed that elevated CO₂ levels had no effect on the herbivore‐induced responses of cotton, whereas those of alfalfa were significantly increased. Larval growth was highest for larvae fed undamaged cotton irrespectively of CO₂ level, and lowest for larvae on damaged alfalfa from the high CO₂ treatment. Development time increased on damaged cotton irrespectively of CO₂ treatment, and on damaged alfalfa in the elevated CO₂ treatment. These results demonstrate that elevated CO₂ levels can cause insect herbivores to alter host plant preferences, and that effects on herbivore‐induced responses may be a key mechanism behind these processes. Furthermore, since the insects were shown to avoid foliage that reduced their physiological performance, our data suggest that behavioural host plant shifts result in partial escape from negative consequences of feeding on high CO₂ foliage. Thus, CO₂ enrichment can alter both physiology and behaviour of important insect herbivores, which in turn may to impact plant biodiversity.     

Effects of pepper (Capsicum chinense) genotypic diversity on insect herbivores

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Compound effects of induced plant responses on insect herbivores and parasitoids: implications for tritrophic interactions

1. Induced plant responses can affect herbivores either directly, by reducing herbivore development, or indirectly, by affecting the performance of natural enemies. Both the direct and indirect impacts of induction on herbivore and parasitoid success were evaluated in a common experimental system, using clonal poplar trees Populus nigra (Salicales: Salicaceae), the gypsy moth Lymantria dispar (L.) (Lepidoptera: Lymantriidae), and the gregarious parasitoid Glyptapanteles flavicoxis (Marsh) (Hymenoptera: Braconidae). 2. Female parasitoids were attracted to leaf odours from both damaged and undamaged trees, however herbivore‐damaged leaves were three times more attractive to wasps than undamaged leaves. Parasitoids were also attracted to herbivore larvae reared on foliage and to larval frass, but they were not attracted to larvae reared on artificial diet. 3. Prior gypsy moth feeding elicited a systemic plant response that retarded the growth rate, feeding, and survival of gypsy moth larvae, however induction also reduced the developmental success of the parasitoid. 4. The mean number of parasitoid progeny emerging from hosts fed foliage from induced trees was 40% less than from uninduced trees. In addition, the proportion of parasitised larvae that survived long enough to issue any parasitoids was lower on foliage from induced trees. 5. A conceptual and analytical model is provided to describe the net impacts of induced plant responses on parasitoids, and implications for tritrophic interactions and biological control of insect pests are discussed.     

Responses of Thrips tabaci to odours of herbivore‐induced cotton seedlings

Herbivore‐induced changes in plants have been widely viewed as defensive responses against further insect attack. However, changes in plants as a consequence of herbivore feeding can elicit various responses in herbivores; these are variable, context dependent, and often unpredictable. In this laboratory study, the responses of Thrips tabaci Lindeman (Thysanoptera: Thripidae) to volatiles emitted by intact and herbivore‐damaged or mechanically damaged cotton seedlings [Gossypium hirsutum L. (Malvaceae)] were investigated in dual‐choice olfactometer assays. Thrips tabaci showed increased attraction to seedlings subject to foliar mechanical damage and those with foliar damage inflicted by conspecifics or Tetranychus urticae Koch (Acari: Tetranychidae), upon which it preys. However, T. tabaci did not discriminate between intact seedlings and those with foliar damage inflicted by Helicoverpa armigera Hübner (Lepidoptera: Noctuidae), two other species of thrips, Frankliniella schultzei Trybom and Frankliniella occidentalis Pergrande (Thysanoptera: Thripidae), or those with root damage inflicted by Tenebrio molitor L. (Coleoptera: Tenebrionidae). Attraction of T. tabaci was also affected by herbivore density on damaged plants. That is, seedlings damaged by higher densities of T. urticae or T. tabaci were more attractive than seedlings damaged by lower densities of the corresponding arthropod. Although attracted to plants damaged by conspecifics or T. urticae, T. tabaci showed greater attraction to seedlings damaged by T. urticae than to seedlings damaged by conspecifics. Results are discussed in the context of the responses of F. schultzei and F. occidentalis to herbivore‐induced cotton seedlings, highlighting the complexity, variability, and unpredictability of the responses of even closely related species of insects to plants under herbivore attack.     

Fruit abscission by Physalis species as defense against frugivory

Fruit abscission as a response to herbivory is well-documented in many plant species, but its effect on further damage by mobile herbivores that survive fruit abscission is relatively unstudied. Physalis plants (Solanaceae) abscise fruit containing feeding larvae of their main frugivore, Heliothis subflexa Guenée (Lepidoptera: Noctuidae), a specialist moth. We tested the ability of H. subflexa larvae placed under the plant canopy to find and climb onto two architecturally different Physalis species. Physalis pubescens L., a low, shrub-like, spreading plant, abscises its fruit at a higher rate than Physalis angulata L., a tall arborescent plant. As a result, small larvae are more often dropped from P. pubescens . Third and fifth instars located P. pubescens faster and with a higher probability than P. angulata . Although fifth instars outperformed third-instar caterpillars at finding P. angulata , P. pubescens was located equally fast by the two instars. Heliothis subflexa located Physalis plants more successfully and more quickly than a close relative, the generalist Heliothis virescens Fabricius. The higher fruit abscission rates in P. pubescens may be an evolved response to its greater susceptibility to searching caterpillars.     

Within-plant distribution of induced resistance in apple seedlings: rapid acropetal and delayed basipetal responses

Induction of plant resistance by herbivory is a complex process, which follows a temporal dynamic and varies spatially at the within-plant scale. This study aimed at improving the understanding of the induction process in terms of time scale and within-plant allocation, using apple tree seedlings (Malus × domestica) as plant model. Feeding preferences of a leaf-chewing insect (Spodoptera littoralis) for previously damaged and undamaged plants were assessed for six different time intervals with respect to the herbivore damage treatment and for three leaf positions. In addition, main secondary defense compounds were quantified and linked to herbivore feeding preferences. Significant herbivore preference for undamaged plants (induced resistance) was first observed 3 days after herbivore damage in the most apical leaf. Responses were delayed in the other leaf positions, and induced resistance decreased within 10 days after herbivore damage simultaneously in all tested leaf positions. Chemical analysis revealed higher concentrations of the flavonoid phloridzin in damaged plants as compared to undamaged plants. This indicates that herbivore preference for undamaged apple plants may be linked to phloridzin, which is the main secondary metabolite of apple leaves. The observed time course and distribution of resistance responses within plants contribute to the understanding of induction processes and patterns, and support the optimal defense theory stating young tissue to be prioritized. Moreover, induced resistance responses occurred also basipetally in leaves below the damage site, which suggests that signaling pathways involved in resistance responses are not unidirectional.     

Induced resistance in mountain birch: defence against leaf-chewing insect guild and herbivore competition

Summary The effect of leaf damage simulating the feeding of early season insect herbivore species, e.g. Epirrita autumnata, to mountain birch, Betula pubescens ssp. tortuosa, on the performance of insect larvae was studied with eleven leaf-chewing sawfly species. I found variation in the results that was due to short- and long-term inducible responses and to the phenology of herbivore species. In general, early and mid-season species were more strongly affected by induced reactions than late-season species. This finding is in accordance with earlier results but I could show that the persistance of induced reactions rather than the influence of timing of damage is responsible for the result. The growth of the larvae of mid-season sawfly species was affected by both short- and long-term induced reactions. This result shows that early season species may escape short-term induced reactions of mountain birch in current year but may not avoid long-term effects. It is supposed that seasonal deterioration of leaf quality either masks the effects of induced defences or late-season species are better adapted to low-quality leaves. Some species show variation in their response to induced defence in different years. This may be due to yearly differences in induced reactions as well as to species-specific responses. Induced defence reactions may play a role in competitive interactions between herbivore species in leaf-chewing guild of mountain birch.