Comparing responses of generalist and specialist herbivores to various cyanogenic plant features

Plants are obliged to defend themselves against multiple generalist and specialist herbivores. Whereas plant cyanogenesis is considered an efficient defence against generalists, it is thought to affect specialists less. In the present study, we analysed the function of various cyanogenic features of lima bean [Phaseolus lunatus L. (Fabaceae)] during interaction with different herbivores. Three cyanogenic features were analysed, i.e., cyanogenic potential (HCNp; concentration of cyanogenic precursors), β‐glucosidase activity, and cyanogenic capacity (HCNc; release of cyanide per unit time). In no‐choice and free‐choice feeding trials, five lima bean accessions were offered to generalist desert locust [Schistocerca gregaria Forskål (Orthoptera: Acrididae)] and specialist Mexican bean beetle [Epilachna varivestis Mulsant (Coleoptera: Coccinellidae)]. The HCNc was the most important parameter determining host plant selection by generalists, whereas choice behaviour of specialists was strongly affected by HCNp. Although locusts were effectively repelled by high HCNc, this cue was misleading for the detection of suitable host plants, as extensive consumption of low HCNc plant material resulted in strong intoxication of locusts. Balancing cyanide in consumed leaf area, the quantitative release of gaseous cyanide during feeding, and cyanide in faeces suggested that specialists metabolized significantly lower rates of cyanide per consumed leaf material than generalists. We hypothesize that specialists are able to avoid toxic concentrations of cyanide by using HCNp rather than HCNc as a cue for host plant quality, and that they exhibit mechanisms that reduce incorporation of host plant cyanide.     

Secretory avocado idioblast oil cells: evidence of their defensive role against a non-adapted insect herbivore

We tested the hypothesis that avocado idioblast oil cells play a defensive role against herbivorous insects. Toxicities of the intact avocado idioblast oil cells and the extracted idioblast oil were compared for three insect herbivores. Spodoptera exigua (Hübner) larvae are generalists that do not feed on avocados. By contrast, Sabulodes aegrotata (Guenée) and Pseudoplusia includens (Walker) larvae are generalist herbivores that readily feed on avocados. All bioassays were performed at a naturally occurring concentration of idioblast oil cells (2% w/w). Choice experiments showed that S. exigua larvae avoided diet treated with avocado idioblast oil cells and consume more control than treated diet. In contrast, idioblast oil cells had no significant antifeedant effects on the adapted S. aegrotata and P. includens larvae. Subsequent experiments designed to assess resistance mechanisms separated pre-ingestive (behavioral) and post-ingestive (physiological) effects of the avocado idioblast oil cells, and the extracted idioblast oil, on the two adapted herbivores. Post-ingestive adaptation was the mechanism that allows feeding. Because the impact of the avocado idioblast oil cells was greatest on the performance of non-adapted S. exigua, additional experiments determined that larvae fed diet containing the oil cells had higher mortality and reduced larval growth compared to controls. Developmental times were significantly prolonged for the survivors. Thus, increased mortality, reduced developmental rates, and antifeedant activity in the non-adapted insect indicate that defense against non-adapted herbivores may be an important function of idioblast cells in avocados.     

Effects of cyanogenesis in bracken fern (Pteridium aquilinum) on associated insects

Abstract. 1. The relationship between cyanogenesis in bracken fern and the insect fauna feeding on the plant was investigated over a 3 year period. The most common insects between May and July, while cyanide levels were high, were the sawflies Strongylogaster impressata Provancher, S.multicincta Norton, Aneug-menus fzavipes (Norton), the aphid Macrosiphum euphorbiae (Thomas) and a microlepidopteran species of Monochroa .
2. Collections of insects from cyanogenic and acyanogenic fronds showed significantly fewer sawflies on the cyanogenic fronds. The aphid and the microlepidopteran were randomly distributed with respect to cyanogenicity.
3. Feeding tests for two of the sawfly species ( Simpressata and Smulticincta ) showed that larvae grew more slowly and had a higher mortality when raised on cyanogenic fronds than on acyanogenic ones.
4. Field collected cyanogenic bracken fronds were found to have sustained less damage from chewing herbivores compared with acyanogenic fronds.     

Inhibition of lipoxygenase affects induction of both direct and indirect plant defences against herbivorous insects

Herbivore-induced plant defences influence the behaviour of insects associated with the plant. For biting–chewing herbivores the octadecanoid signal-transduction pathway has been suggested to play a key role in induced plant defence. To test this hypothesis in our plant—herbivore—parasitoid tritrophic system, we used phenidone, an inhibitor of the enzyme lipoxygenase (LOX), that catalyses the initial step in the octadecanoid pathway. Phenidone treatment of Brussels sprouts plants reduced the accumulation of internal signalling compounds in the octadecanoid pathway downstream of the step catalysed by LOX, i.e. 12-oxo-phytodienoic acid (OPDA) and jasmonic acid. The attraction of Cotesia glomerata parasitoids to host-infested plants was significantly reduced by phenidone treatment. The three herbivores investigated, i.e. the specialists Plutella xylostella, Pieris brassicae and Pieris rapae, showed different oviposition preferences for intact and infested plants, and for two species their preference for either intact or infested plants was shown to be LOX dependent. Our results show that phenidone inhibits the LOX-dependent defence response of the plant and that this inhibition can influence the behaviour of members of the associated insect community.     

Symbiotic soil microorganisms as players in aboveground plant–herbivore interactions – the role of rhizobia

Rhizobia play a key role for performance of leguminous plants and ecosystem productivity. However, no studies to date have investigated the importance of the rhizobial symbiosis for legume–herbivore interactions. The additional nitrogen provided by the rhizobia improves the nutritional quality of plants, but may also be used for the synthesis of defence compounds. We performed greenhouse experiments with nodulating and non-nodulating, as well as cyanogenic and acyanogenic strains of Trifolium repen s to study the effects of rhizobia Rhizobium leguminosarum on plant growth and the performance of the chewing herbivore Spodoptera littoralis and the phloem-sucking aphid Myzus persicae . We demonstrate that for nodulating strains of T. repens rhizobia increased plant growth and the performance of Spodoptera littoralis . However, this positive effect of rhizobia on the caterpillars did not occur in a cyanogenic clover strain. Reproduction of the phloem-sucking aphid Myzus persicae was inconsistently affected by rhizobia. Our study provides evidence that the additional nitrogen provided by the rhizobia may be used for the production of nitrogen-based defence compounds, thereby counteracting positive effects on the performance of chewing herbivores. The symbiosis with rhizobia is therefore an important driver of legume–herbivore interactions.     

Can Epichloë endophytes enhance direct and indirect plant defence?

Induced or constitutive production of secondary metabolites is a successful plant defence strategy against herbivores which can be mediated by plant associated micro-organisms. Several grass species can be associated with an endophytic fungus of the genus Epichloë which produces herbivore toxic or deterring alkaloids. Besides these direct defences, herbivorous insects are controlled via indirect plant defence mechanisms by attracting predators. Recent studies indicate that Epichloë endophytes can improve the grass emitted volatile organic compounds towards herbivore deterrence. Due to their defensive mutualistic function, we hypothesize that Epichloë altered plant volatiles can attract aphid predators and contribute to an increased indirect plant defence. With a common garden study, we show that hoverfly (Syrphidae) larvae and pupae were more abundant on endophyte-infected plants compared to uninfected plants. Our results indicate that the Epichloë endophyte provides, besides direct defence (alkaloid), indirect plant defence by improving the plant odor attracting more olfactory foraging aphid predators. Future research is needed in order to understand: (I) whether endophyte-mediated changes in plant volatiles are induced herbivore specific, (II) whether there is a trade-off between endophyte-mediated direct and indirect plant defence, (III) whether the endophyte produces volatiles or induces a change in plant-derived volatiles, (IV) the role of plant signals in endophyte-mediated 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.     

Impact of the dual defence system of Plantago lanceolata (Plantaginaceae) on performance,nutrient utilisation and feeding choice behaviour of Amata mogadorensis larvae (Lepidoptera,Erebidae)

Iridoid glycosides are plant defence compounds with potentially detrimental effects on non-adapted herbivores. Some plant species possess β-glucosidases that hydrolyse iridoid glycosides and thereby release protein-denaturing aglycones. To test the hypothesis that iridoid glycosides and plant β-glucosidases form a dual defence system, we used Plantago lanceolata and a polyphagous caterpillar species. To analyse the impact of leaf-age dependent differences in iridoid glycoside concentrations and β-glucosidase activities on insect performance, old or young leaves were freeze-dried and incorporated into artificial diets or were provided freshly to the larvae. We determined larval consumption rates and the amounts of assimilated nitrogen. Furthermore, we quantified β-glucosidase activities in artificial diets and fresh leaves and the amount of iridoid glycosides that larvae feeding on fresh leaves ingested and excreted. Compared to fresh leaves, caterpillars grew faster on artificial diets, on which larval weight gain correlated positively to the absorbed amount of nitrogen. When feeding fresh young leaves, larvae even lost weight and excreted only minute proportions of the ingested iridoid glycosides intact with the faeces, indicating that the hydrolysis of these compounds might have interfered with nitrogen assimilation and impaired larval growth. To disentangle physiological effects from deterrent effects of iridoid glycosides, we performed dual choice feeding assays. Young leaves, their methanolic extracts and pure catalpol reduced larval feeding in comparison to the respective controls, while aucubin had no effect on larval consumption. We conclude that the dual defence system of P. lanceolata consisting of iridoid glycosides and β-glucosidases interferes with the nutrient utilisation via the hydrolysis of iridoid glycosides and also mediates larval feeding behaviour in a concentration- and substance-specific manner.     

An ecological genomic approach challenging the paradigm of differential plant responses to specialist versus generalist insect herbivores

A general prediction of the specialist/generalist paradigm indicates that plant responses to insect herbivores may depend on the degree of ecological specialization of the insect attacker. However, results from a single greenhouse experiment evaluating the responses of the model plant Arabidopsis thaliana to three specialist (Plutella xylostella, Pieris rapae, and Brevicoryne brassicae) and three generalist (Trichoplusia ni, Spodoptera exigua, and Myzus persicae) insect species did not support the previous prediction. Using an ecological genomic approach, we assessed plant responses in terms of herbivore-induced changes in genome-wide gene expression, defense-related pathways, and concentrations of glucosinolates (i.e., secondary metabolites that are ubiquitously present in cruciferous plants). Our results showed that plant responses were not influenced by the degree of specialization of insect herbivores. In contrast, responses were more strongly shaped by insect taxa (i.e., aphid vs. lepidopteran species), likely due to their different feeding modes. Interestingly, similar patterns of plant responses were induced by the same insect herbivore species in terms of defense signaling (jasmonic acid pathway), aliphatic glucosinolate metabolism (at both the gene expression and phenotypic levels) and genome-wide responses. Furthermore, plant responses to insect herbivores belonging to the same taxon (i.e., four lepidopteran species) were not explained by herbivore specialization or phylogenetic history. Overall, this study suggests that different feeding modes of insect taxa as well as herbivore-specific plant responses, which may result from distinct ecological/evolutionary interactions between A. thaliana (or a close relative) and each of the lepidopteran species, may explain why observed responses deviate from those predicted by the specialist/generalist paradigm.     

Drought changes plant chemistry and causes contrasting responses in lepidopteran herbivores

Drought events are predicted to increase due to climate change, yet consequences for plant–insect interactions are only partially understood. Drought‐mediated interactions between herbivores and their host plants are affected by a combination of factors, including characteristics of the affected plant, its associated herbivore and of the prevailing drought. Studying the effect of these factors in combination may provide important insight into plant and herbivore responses to drought. We studied drought effects on plant resistance to two leaf‐chewing herbivores by considering differing growth conditions, plant chemistry and insect responses in concert. We exposed Alliaria petiolata plants from several wild populations to different intensities of intermittent drought stress and quantified drought‐mediated changes in plant chemistry. Simultaneously, we assessed behavior (feeding preference) and performance of two lepidopteran herbivores: Pieris brassicae, a specialist, and Spodoptera littoralis, a generalist. Drought led to lowest concentrations of secondary defense compounds in severely stressed plants, without affecting total nitrogen content. Additionally, drought evoked opposite patterns in feeding preferences (plant palatability) between the herbivore species. Pieris brassicae consumed most of well‐watered plants, while S. littoralis preferred severely drought‐stressed plants. Hence, feeding preferences of S. littoralis reflected changes in plant secondary chemistry. Contrary to their feeding preference, P. brassicae performed better on drought‐stressed than on well‐watered plants, with faster development and higher attained pupal mass (plant suitability). Spodoptera littoralis showed retarded development in all treatments. In conclusion, drought caused plant secondary defense compounds to decrease consistently across all studied plant populations, which evoked contrasting feeding preferences of two herbivore species of the same feeding guild. These results suggest herbivore specificity as a possible explanation for herbivore responses to drought and emphasize the importance of herbivore characteristics such as feeding specialization in understanding and predicting consequences of future drought events.     

Linking belowground and aboveground phenology in two boreal forests in Northeast China

Plants are frequently attacked by both pathogens and insects, and an attack from one can induce plant responses that affect resistance to the other. However, we currently lack a predictive framework for understanding how pathogens, their vectors, and other herbivores interact. To address this gap, we have investigated the effects of a viral infection in the host plant on both its aphid vector and non-vector herbivores. We tested whether the infection by three different strains of Potato virus Y (PVY^NTN, PVY^NO and PVY^O) on tomato plants affected: (1) the induced plant defense pathways; (2) the abundance and fecundity of the aphid vector (Macrosiphum euphorbiae); and (3) the performance of two non-vector species: a caterpillar (Trichoplusia ni) and a beetle (Leptinotarsa decemlineata). While infection by all three strains of PVY induced the salicylate pathway, PVY^NTN induced a stronger and longer response. Fecundity and density of aphids increased on all PVY-infected plants, suggesting that the aphid response is not negatively associated with salicylate induction. In contrast, the performance of non-vector herbivores positively correlated with the strength of salicylate induction. PVY^NTN infection decreased plant resistance to both non-vector herbivores, increasing their growth rates. We also demonstrated that the impact of host plant viral infection on the caterpillar results from host plant responses and not the effects of aphid vector feeding. We propose that pathogens chemically mediate insect–plant interactions by activating the salicylate pathway and decreasing plant resistance to chewing insects, which has implications for both disease transmission and insect community structure.     

Cyanogenic glucosides in the biological warfare between plants and insects: the Burnet moth-Birdsfoot trefoil model system

Cyanogenic glucosides are important components of plant defense against generalist herbivores due to their bitter taste and the release of toxic hydrogen cyanide upon tissue disruption. Some specialized herbivores, especially insects, preferentially feed on cyanogenic plants. Such herbivores have acquired the ability to metabolize cyanogenic glucosides or to sequester them for use in their own predator defense. Burnet moths (Zygaena) sequester the cyanogenic glucosides linamarin and lotaustralin from their food plants (Fabaceae) and, in parallel, are able to carry out de novo synthesis of the very same compounds. The ratio and content of cyanogenic glucosides is tightly regulated in the different stages of the Zygaena filipendulae lifecycle and the compounds play several important roles in addition to defense. The transfer of a nuptial gift of cyanogenic glucosides during mating of Zygaena has been demonstrated as well as the possible involvement of hydrogen cyanide in male assessment and nitrogen metabolism. As the capacity to de novo synthesize cyanogenic glucosides was developed independently in plants and insects, the great similarities of the pathways between the two kingdoms indicate that cyanogenic glucosides are produced according to a universal route providing recruitment of the enzymes required. Pyrosequencing of Z. filipendulae larvae de novo synthesizing cyanogenic glucosides served to provide a set of good candidate genes, and demonstrated that the genes encoding the pathway in plants and Z. filipendulae are not closely related phylogenetically. Identification of insect genes involved in the biosynthesis and turn-over of cyanogenic glucosides will provide new insights into biological warfare as a determinant of co-evolution between plants and insects.     

How insects overcome two‐component plant chemical defence: plant β‐glucosidases as the main target for herbivore adaptation

Insect herbivory is often restricted by glucosylated plant chemical defence compounds that are activated by plant β‐glucosidases to release toxic aglucones upon plant tissue damage. Such two‐component plant defences are widespread in the plant kingdom and examples of these classes of compounds are alkaloid, benzoxazinoid, cyanogenic and iridoid glucosides as well as glucosinolates and salicinoids. Conversely, many insects have evolved a diversity of counter‐adaptations to overcome this type of constitutive chemical defence. Here we discuss that such counter‐adaptations occur at different time points, before and during feeding as well as during digestion, and at several levels such as the insects' feeding behaviour, physiology and metabolism. Insect adaptations frequently circumvent or counteract the activity of the plant β‐glucosidases, bioactivating enzymes that are a key element in the plant's two‐component chemical defence. These adaptations include host plant choice, non‐disruptive feeding guilds and various physiological adaptations as well as metabolic enzymatic strategies of the insect's digestive system. Furthermore, insect adaptations often act in combination, may exist in both generalists and specialists, and can act on different classes of defence compounds. We discuss how generalist and specialist insects appear to differ in their ability to use these different types of adaptations: in generalists, adaptations are often inducible, whereas in specialists they are often constitutive. Future studies are suggested to investigate in detail how insect adaptations act in combination to overcome plant chemical defences and to allow ecologically relevant conclusions.     

Systematic analysis of rice (Oryza sativa) metabolic responses to herbivory

Plants defend against attack from herbivores by direct and indirect defence mechanisms mediated by the accumulation of phytoalexins and release of volatile signals, respectively. While the defensive arsenals of some plants, such as tobacco and Arabidopsis are well known, most of rice's (Oryza sativa) defence metabolites and their effectiveness against herbivores remain uncharacterized. Here, we used a non‐biassed metabolomics approach to identify many novel herbivory‐regulated metabolic signatures in rice. Most were up‐regulated by herbivore attack while only a few were suppressed. Two of the most prominent up‐regulated signatures were characterized as phenolamides (PAs), p‐coumaroylputrescine and feruloylputrescine. PAs accumulated in response to attack by both chewing insects, i.e. feeding of the lawn armyworm (Spodoptera mauritia) and the rice skipper (Parnara guttata) larvae, and the attack of the sucking insect, the brown planthopper (Nilaparvata lugens, BPH). In bioassays, BPH insects feeding on 15% sugar solution containing p‐coumaroylputrescine or feruloylputrescine, at concentrations similar to those elicited by heavy BPH attack in rice, had a higher mortality compared to those feeding on sugar diet alone. Our results highlight PAs as a rapidly expanding new group of plant defence metabolites that are elicited by herbivore attack, and deter herbivores in rice and other plants.     

Leaf Colour as a Signal of Chemical Defence to Insect Herbivores in Wild Cabbage (Brassica oleracea)

Leaf colour has been proposed to signal levels of host defence to insect herbivores, but we lack data on herbivory, leaf colour and levels of defence for wild host populations necessary to test this hypothesis. Such a test requires measurements of leaf spectra as they would be sensed by herbivore visual systems, as well as simultaneous measurements of chemical defences and herbivore responses to leaf colour in natural host-herbivore populations. In a large-scale field survey of wild cabbage (Brassica oleracea) populations, we show that variation in leaf colour and brightness, measured according to herbivore spectral sensitivities, predicts both levels of chemical defences (glucosinolates) and abundance of specialist lepidopteran (Pieris rapae) and hemipteran (Brevicoryne brassicae) herbivores. In subsequent experiments, P. rapae larvae achieved faster growth and greater pupal mass when feeding on plants with bluer leaves, which contained lower levels of aliphatic glucosinolates. Glucosinolate-mediated effects on larval performance may thus contribute to the association between P. rapae herbivory and leaf colour observed in the field. However, preference tests found no evidence that adult butterflies selected host plants based on leaf coloration. In the field, B. brassicae abundance varied with leaf brightness but greenhouse experiments were unable to identify any effects of brightness on aphid preference or performance. Our findings suggest that although leaf colour reflects both levels of host defences and herbivore abundance in the field, the ability of herbivores to respond to colour signals may be limited, even in species where performance is correlated with leaf colour.     

Do canal‐cutting behaviours facilitate host‐range expansion by insect herbivores?

According to the escalation–radiation model of co-evolution, insect herbivores that acquire the ability to circumvent a plant defence enter a new adaptive zone and increase in species. How herbivore counter-adaptations to plant defences might lead to speciation is poorly understood. Studies of nymphalid butterflies suggest that the evolution of a broadened host range may be a critical step. This paper examines if leaf-feeding insects capable of deactivating defensive plant canals with canal cutting often have broad host ranges. A total of 94 species of canal-cutting insects were identified from the literature, including eight new canal cutters described in this paper. Only 27% of canal cutters with known host ranges are generalists that feed on plants in multiple families. The proportion of generalist canal cutters is similar or lower than estimates of generalists among phytophagous insects overall. Only five species, at most, of the canal-cutting generalists feed exclusively on plants with secretory canals. The paucity of generalists can be attributed in part to the considerable taxonomic distance separating canal-bearing plant families and to their corresponding chemical distinctiveness. The dependence of many canal-cutting species on host chemicals for defence would also favour specialization.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 715–731.     

Community‐wide impact of an exotic aphid on introduced tall goldenrod

1. The aphid Uroleucon nigrotuberculatum Olive, which is specialised to the tall goldenrod, Solidago altissima L., in its native range, has become a dominant species on the introduced tall goldenrod in Japan. How this exotic aphid influenced arthropod communities on the introduced tall goldenrod in aphid‐present (spring) and aphid‐absent (autumn) seasons was examined, using an aphid removal experiment. 2. In spring, aphid presence increased ant abundance because aphid honeydew attracted foraging ant workers. A significant negative correlation was found between the numbers of ants and herbivorous insects other than aphids on the aphid‐exposed plants, but no significant correlation was detected on the aphid‐free plants. Thus, the aphid presence was likely to decrease the abundance of co‐occurring herbivorous insects through removal behaviour of the aphid‐tending ants. There were no significant differences in plant traits between the aphid‐exposed and aphid‐free plants. 3. In autumn, the numbers of lateral shoots and leaves, and the leaf nitrogen content were increased in response to the aphid infestation in spring. Because of the improvement of plant traits by aphid feeding, the abundance of leaf chewers increased on aphid‐exposed plants. In contrast, the abundance of sap feeders decreased on the aphid‐exposed plants. In particular, the dominant scale insect among sap feeders, Parasaissetia nigra Nietner, decreased, followed by a decrease in the abundance of ants attending P. nigra. Thus, aphid feeding may have attenuated the negative impacts of the tending ants on leaf chewers. 4. Aphid presence did not change herbivore species richness but changed the relative density of dominant herbivores, resulting in community‐wide effects on co‐occurring herbivores through ant‐mediated indirect effects, and on temporally separated herbivores through plant‐ and ant‐mediated indirect effects. The aphid also altered predator community composition by increasing and decreasing the relative abundance of aphid‐tending ants in the spring and autumn, respectively.     

Plant‐mediated species networks: the modulating role of herbivore density

1. When herbivores of distinct feeding guilds, such as phloem feeders and leaf chewers, interact, the outcome of these interactions often shows facilitation. However, whether this facilitation turns into competition at stronger herbivory pressure remains unknown. 2. Using an integrative approach that links ecological processes (behavioural choices of insects) with physiological plant mechanisms (nutrient and phytohormone levels) for the wild crucifer Brassica nigra (L.) Koch., this study evaluates preferences of leaf chewers for plants previously infested with several densities of the specialist aphid Brevicoryne brassicae L. (Hemiptera, Aphididae). As leaf chewers, four species of caterpillars (Lepidoptera) were selected that differ in their degree of specialisation in crucifers. 3. These results show that, whereas at low and medium aphid densities caterpillars displayed a preference for aphid‐infested plants or no preference, at high aphid infestation density, all four species of caterpillar preferred uninfested plants, with a significant difference for Pieris rapae and Mamestra brassicae. 4. In contrast to our expectation, the consistent preference for uninfested plants at a high aphid density could not be associated with a decrease in plant nutrition. However, while jasmonate concentrations [i.e. 12‐oxo‐phytodienoic acid and jasmonic acid (JA)] at medium aphid‐density infestation decreased compared with low levels of infestation, at high infestation level, the jasmonates JA as well as JA conjugated with the amino acid isoleucine were present at higher levels compared with low‐infestation treatments. 5. This work provides evidence that positive interactions observed in herbivore communities can be transient, leading to negative interactions mediated by changes in plant defences rather than in plant nutrition.     

Phenotypic plasticity in chemical defence of butterflies allows usage of diverse host plants

Host plant specialization is a major force driving ecological niche partitioning and diversification in insect herbivores. The cyanogenic defences of Passiflora plants keep most herbivores at bay, but not the larvae of Heliconius butterflies, which can both sequester and biosynthesize cyanogenic compounds. Here, we demonstrate that both Heliconius cydno chioneus and H. melpomene rosina have remarkable plasticity in their chemical defences. When feeding on Passiflora species with cyanogenic compounds that they can readily sequester, both species downregulate the biosynthesis of these compounds. By contrast, when fed on Passiflora plants that do not contain cyanogenic glucosides that can be sequestered, both species increase biosynthesis. This biochemical plasticity comes at a fitness cost for the more specialist H. m. rosina, as adult size and weight for this species negatively correlate with biosynthesis levels, but not for the more generalist H. c. chioneus. By contrast, H. m rosina has increased performance when sequestration is possible on its specialized host plant. In summary, phenotypic plasticity in biochemical responses to different host plants offers these butterflies the ability to widen their range of potential hosts within the Passiflora genus, while maintaining their chemical defences.