Electrophysiological and behavioural responses of Aphidius ervi (Hymenoptera: Braconidae) to tomato plant volatiles

Flight responses of the aphid parasitoid Aphidius ervi to tomato volatiles have recently demonstrated that different plant stresses can lead to increases in attractiveness for this parasitoid. For example, infestation of tomato plants by the aphid Macrosiphum euphorbiae results in the overexpression of defensive genes, as well as the release of volatile compounds that attract aphid parasitoids. Here, we determine which of the induced compounds elicit a significant electrophysiological response from parasitoid antennae. Compounds shown to be detected at the antennal level were then tested at a range of doses in a wind tunnel assay. A significant electroantennogram response was demonstrated for three compounds, (8S,9R)-(E)-caryophyllene, methyl salicylate, and (Z)-3-hexen-1-ol, over four concentrations. These compounds proved to be significantly attractive in the wind tunnel at a rate not always proportionally dependent upon the dose. The practical implications of these findings are discussed in the framework of sustainable control for pest aphids in agriculture.     

Aphid alarm pheromone (E)-β-farnesene: A host finding kairomone for the aphid primary parasitoidAphidius uzbekistanicus (Hymenoptera: Aphidiinae)

Summary The present study aimed to test the possible function of the aphid alarm pheromone (E)--farnesene (EBF) as a host finding kairomone for aphid primary parasitoids. Extracts of volatile emissions of undisturbed aphids and of aphids under parasitoid attack were obtained by air entrainment. Extracts of cornicle secretions were gained by disturbing aphids and taking their secretions into solution. Extracts were compared by gas chromatography. Only air entrainments of aphids under attack and solvent extracts of cornicle secretions contained the alarm pheromone. In Y-tube olfactometer bioassays, femaleA. uzbekistanicus were attracted to aphid groups under attack of parasitoids, presumably by released EBF. High concentrations of synthetic EBF (1.4 µg to 5.7 µg) also attractedA. uzbekistanicus females. Females with oviposition experience reacted more readily to lower concentrations of EBF than females without experience. In experiments designed as Petri-dish bioassays, the test animals could contact filter paper discs that were treated with solutions containing EBF. Behavioural effects like antennation or stinging attack were not observed. With computer video analysis of parasitoid movements, some effects onA. uzbekistanicus behaviour were detected, again indicating attraction to EBF.As the volatile aphid alarm pheromone attractedA. uzbekistanicus females, it can be concluded that this chemical stimulus acts as a host finding kairomone for this parasitoid species. However, its effect over long distances seems to be limited due to the relatively high concentrations required for reactions. Of two other parasitoid species examined (P. volucre andL. testaceipes) onlyP. volucre was also significantly attracted to the volatile EBF in the Y-tube olfactometer.     

Experimental evidence for effective and altruistic colony defence against natural predators by soldiers of the gall-forming aphid Pemphigus spyrothecae (Hemiptera : Pemphigidae)

Summary The thick-legged first instar soldiers of the gall-forming aphid Pemphigus spyrothecae Pass. are able to protect the aphids in the gall from being eaten by a range of insect predators. In artificial galls, the soldier aphids were able to kill first instar ladybirds Adalia bipunctata (L.) (Coleoptera: Coccinellidae), early third instar hoverfly larvae Eupeodes (Metasyrphus) corollae (Fab.) (Diptera: Syrphidae), and first-third instar Anthocoris nemoralis (Fab.) (Hemiptera: Cimicidae). Almost all the aphids that attacked the predators were themselves killed. The soldiers were also able to kill predators introduced into natural galls. Experiments were devised in which individuals of Anthocoris minki, which is the most important insect predator of the gall generations of Pemphigus at the study site, were free to enter and leave the gall: the soldiers were effective both in preventing the predator's access to the gall and in killing those predators that did manage to get in. In galls with experimentally manipulated numbers of soldiers and non-soldiers, it was clearly shown that it is the soldiers alone that kill the predators (Anthocoris minki and 1st instar Adalia bipunctata) (see Table 3). Even though many soldiers may die during these encounters, the selective advantage of killing the predators is high, since observations show that individual A. minki can pass through more than one instar inside a gall and kill all the aphids therein. The aphids were not observed to attack conspecific aphids from other galls or the cohabiting aphid Chaitophorus leucomelas Koch. The primary role of the soldier caste is therefore the defence of the aphid colony against predators.     

Testing a new model of aphid abundance with sedentary and non-sedentary predators

Aphid population dynamics has been thoroughly investigated, especially in tree-dwelling aphids. Among the controls of the aphid rate of increase are the negative effects of antagonists, the positive effects of mutualists, the density-dependence of the aphid dynamics, and the non-stationary quality of plant tissues. Here we present a mechanistic model of aphid growth that considers most of these governing factors using a simple formulation. What is new in this model is that it considers two kinds of antagonists. The first kind is a guild of aphid predator specialists that includes ladybirds (Coleoptera: Coccinellidae), but also species of some families of Hemiptera, Diptera, and Neuroptera. The second kind of antagonists consists of omnivores or generalist predators and in this particular setting is exemplified by the European earwig Forficula auricularia (Dermaptera: Forficulidae). The model developed here compared the effects of these two different kinds of aphid predators, the second one always at the site (sedentary predators) and the first one that arrives in important numbers only once the aphid population has already developed to some degree (non-sedentary predators). Multiple model parameter sets, representing different hypotheses about controls on aphid populations, were evaluated within the Generalised Likelihood Uncertainty Estimation (GLUE) methodology. The model correctly reproduced the experimental data obtained in an organic citrus grove showing the important effect that sedentary predators as earwigs can have on the aphid populations. Low densities of sedentary predators or even low predation rates can have a disproportionate effect on the final aphid density, as they prey on small populations, when the per capita effect on the aphid population is higher. During the main spring peak of aphids the role of non-sedentary predators is secondary, as they track the aphid density rather than control it. However, these non-sedentary predators are important within the proposed model to keep the second autumn peak of aphids at low values.     

Plant chemistry and insect sequestration

Most plant families are distinguished by characteristic secondary metabolites, which can function as putative defence against herbivores. However, many herbivorous insects of different orders can make use of these plant-synthesised compounds by ingesting and storing them in their body tissue or integument. Such sequestration of putatively unpalatable or toxic metabolites can enhance the insects’ own defence against enemies and may also be involved in reproductive behaviour. This review gives a comprehensive overview of all groups of secondary plant metabolites for which sequestration by insect herbivores belonging to different orders has been demonstrated. Sequestered compounds include various aromatic compounds, nitrogen-containing metabolites such as alkaloids, cyanogenic glycosides, glucosinolates and other sulphur-containing metabolites, and isoprenoids such as cardiac glycosides, cucurbitacins, iridoid glycosides and others. Sequestration of plant compounds has been investigated most in insects feeding or gathering on Apocynaceae s.l. (Apocynoideae, Asclepiaoideae), Aristolochiaceae, Asteraceae, Boraginaceae, Fabaceae and Plantaginaceae, but it also occurs for some gymnosperms and even lichens. In total, more than 250 insect species have been shown to sequester plant metabolites from at least 40 plant families. Sequestration predominates in the Coleoptera and Lepidoptera, but also occurs frequently in the orders Heteroptera, Hymenoptera, Orthoptera and Sternorrhyncha. Patterns of sequestration mechanisms for various compound classes and common or individual features occurring in different insect orders are highlighted. More research is needed to elucidate the specific transport mechanisms and the physiological processes of sequestration in various insect species.     

Systemically-induced response of cabbage plants against a specialist herbivore, Pieris brassicae

Summary. Plant responses to herbivory might directly affect the herbivore (“direct” defences) or might benefit the plant by promoting the effectiveness of natural antagonists of the herbivores (“indirect” defences). Brussels sprouts attacked by Pieris brassicae larvae release volatiles that attract a natural antagonist of the herbivores, the parasitoid Cotesia glomerata, to the damaged plant. In a previous study, we observed that feeding by caterpillars on the lower leaves of the plant triggers the systemic release of volatiles detectable by the parasitoids from upper leaves of the same plant.?The role of these systemically induced volatiles as indirect defence and the dynamics of their emission were investigated in wind-tunnel dual choice tests with C. glomerata. The systemically induced emission of volatiles varied depending on leaf age and on plant age. Systemic induction affected parasitoid effectiveness, as induced plants could be more easily located by parasitoids than non-induced ones.?The role of the systemic induction as a direct defence was investigated through behavioural and feeding tests with P. brassicae. In dual choice assays, 1st instar larvae preferred to feed and fed more on systemically induced than on non-induced leaves. In single choice assays, the leaf area consumed by caterpillars was larger on systemically induced leaves than on non-induced control leaves. However, caterpillars fed on systemically induced leaves attained the same weight as those feeding on non-induced controls. In addition, P. brassicae pupae whose larvae were fed on systemically induced leaves had longer developmental times than those of larvae fed on non-induced leaves. Adult oviposition behavior was not influenced by systemic induction.?We conclude that systemically induced responses in cabbage might reduce P. brassicae fitness both directly, by affecting their development and feeding behavior and indirectly by making caterpillars and pupae more vulnerable to attack by carnivores. The occurrence of a possible relationship between direct and indirect defence is discussed. Received 24 January 2001; accepted 3 May 2001.     

Increased nitrogen availability influences predator–prey interactions by altering host-plant quality

Little is known about how plant nutritional and defensive qualities interact to influence predator–prey interactions. To address this need, we provided the neo-tropical milkweed, Asclepias curassavica, with two levels of nitrogen availability and examined how altered host-plant quality influenced the responses of a specialist aphid, Aphis nerii, and a coccinellid predator, Harmonia axyridis. Aphis nerii uses A. curassavica for multiple resources, including nutrition and sequestration of cardenolides for defense against natural enemies. Increased nitrogen availability improved A. curassavica quality by decreasing carbon-to-nitrogen (C:N) ratios and cardenolide concentrations, resulting in A. nerii that also had lower C:N ratios and cardenolide concentrations. Aphis nerii population growth was higher on plants with high nitrogen availability, compared with aphids on plants with low nitrogen availability. In no-choice feeding trials, Harmonia axyridis consumed more high C:N ratio aphids, suggesting a potential compensatory response to reduced aphid nutritional quality. Additionally, H. axyridis were able to consume more low-quality aphids at the expense of increasing exposure to increased cardenolide concentrations, suggesting that interactions between H. axyridis and A. nerii may be strongly influenced by prey nutritional quality. This work highlights the need to consider how variation in plant quality influences herbivore nutritional and defensive quality when examining mechanisms that influence predator–prey interactions.     

The effects of feeding by the black bean aphidAphis fabae Scop. (Homoptera: Aphididae) on copper and cadmium accumulation in broad bean (Vicia faba L.)

Broad beans (Vicia faba L.), cultured hydroponically were supplied with 100 g mL^–1 copper or 50 g mL^–1 cadmium in nutrient solution. Samples of plant material from both nutrient regimes were analysed before and after infestation by the black bean aphid (Aphis fabae Scop.). Heavy aphid infestation resulted in a significant reduction in copper content of shoots in comparison with uninfested plants. A similar, but less well- defined, situation occurred in the case of cadmium.Further investigations examined the effects of different levels of aphid infestation on the above phenomena. In all cases the presence of feeding aphids reduced elemental accumulation in plant shoots. Long term infestation with population densities as low as three adult aphids showed a reduction in shoot copper and cadmium content.     

Social feeding in ladybird beetles: adaptive significance and mechanism

Summary. Young larvae of Adalia bipunctata search an area more intensively when exposed to the odour from other larvae feeding on aphids than when exposed only to the odour of aphids. In an olfactometer young larvae were significantly attracted either to the odour of crushed aphids or larvae feeding on aphids, but not to that of aphids, larvae, larvae plus aphids or larvae feeding on an artificial diet. That is, the change in searching behaviour appears in response to a volatile released by aphids when attacked. The odour released by crushed aphids is made up entirely of aphid alarm pheromone, β-farnesene. It is likely that the adaptive significance of this response is that it increases the ability of larvae to locate larvae that have already caught prey. By sharing the aphid kill of another larva it is likely that a first instar ladybird larva greatly increases its probability of surviving to the next instar. It is suggested that this social feeding is facilitated by egg clustering, which also may additionally account for why aphidophagous ladybirds lay their eggs in clusters. Received 28 February 2000; accepted 24 March 2000     

The optimal balance of defence investment strategies in clonal colonies of social aphids

Social aphid species provide ideal systems to study the ecological influences upon the evolution of sociality because they consist of discrete colonies which are entirely clonal and therefore devoid of any genetic conflict over altruistic behaviour. Although selfishness can be discounted as an obstacle preventing the evolution of altruistic defenders, the vast majority of aphid species are not social. To examine the key life-history and ecological characteristics that interact to facilitate social evolution, we designed a matrix population model based on the natural history of one of the unique aphid species with soldiers, Pemphigus spyrothecae. In addressing the life-history factors, our special interest was to examine the optimal trade-off faced by colonies that can increase their defence investment by producing defenders at birth and/or increasing the duration of the defensive stage. The level and period of exposure to predation and a declining colony birth rate were key factors that selected for social defence. The model demonstrated that, in species which have soldiers that can facultatively develop to make a direct contribution to colony fitness, temporal extension of the soldier stage is a key mechanism of increasing defence investment. This extension is particularly favoured when predation is high and the lifetime of a colony is long. An increase in production of defenders at birth was favoured when mortality due to predation was strongly biased towards defenders. The model suggests that, in species which have the defensive flexibility of choosing whether soldiers remain as such, there is little requirement for flexibility in the morph allocations made at birth. All these predictions were found to be fully compatible with the available empirical data.     

How <Emphasis Type="Italic">Doratomyces stemonitis</Emphasis> copes with Benzoxazolin-2(3<Emphasis Type="Italic">H</Emphasis>)-one (BOA), its derivatives and detoxification products

Summary. Doratomyces stemonitis (Hyphomycetales, Dematiaceae) is a saprotrophic fungus belonging to the mycobiota of the cereal rhizosphere. The fungus is able to metabolize benzoxazolin-2-(3H)-one and a variety of its derivatives including higher plant detoxification products, microbial degradation products and the chemically rather stable 2-amino-(3H)-phenoxazin-3-one. D. stemonitis can use all of these compounds as sole C-sources but their utilization, especially that of microbial degradation products and 2-amino-(3H)-phenoxazin-3-one, seems to be highly energy consuming, resulting in slow mycelium growth and a change of colony morphology. Benzoxazolin-2-(3H)-one derived compounds induce the synthesis of different isoforms of a glycosylated protein with sequence homologies to the endo-1,3-β-glucanase Asp f2, an allergen from Aspergillus fumigatus and other Asp f2-like proteins e.g., from Verticillium dahliae or PRA1 antigen from Candida albicans. The induction of the protein is regarded as a stress response.     

Faecal shield chemical defence is not important in larvae of the tortoise beetle Chelymorpha reimoseri (Chrysomelidae: Cassidinae: Stolaini)

The most frequently investigated defence mechanism among larvae of tortoise beetles is protection by faecal shields, which generally present chemicals that are directly sequestered and/or modified from larval host-plants. In the present work we investigate the tortoise beetle Chelymorpha reimoseri that feeds on the leaves of Ipomoea carnea fistulosa (Convolvulaceae), seeking for the importance of this mechanism to their larvae. We show that 4th instar larvae suffer low predation regardless of the presence of shields in field and laboratory bioassays with ants and chicks. Chloroform extract from larvae without shields provided high protection against predation as shown in bioassays in the field, as well as against chicks, suggesting that C. reimoseri does not rely on the shield for protection. The aqueous extract of the shields did not show activity in such bioassays. The compounds responsible for this protection have yet to be identified, and it remains an open question as to whether they are sequestered from the host plant or de novo biosynthesized. This is the first record of chemical defence in cassidine beetles without the need for faecal shields. These findings indicate that more attention should be paid to chemicals present in the tissues of larvae and/or adults of tortoise beetles; the protective compounds sequestered from host plants or de novo biosynthesized can provide an alternative or complementary strategy against predation in these insects.     

A survey of identified kairomones and synomones used by insect parasitoids to locate and accept their hosts

Summary The host utilization process of insect parasitoids can be described by three stages of (1)habitatlocation, (2)host-location, and (3)host-acceptance andoviposition. There are 19 systems in which chemicals used inhabitat-location have been identified, 12 systems in which chemical cues leading tohost-location have been identified, and 16 systems in which chemicals elicitinghost-acceptance andoviposition have been identified. Both the chemical class and the source of the infochemical change with the stage of the host utilization process. Semiochemicals identified in thehabitatlocation stage were predominantly aldehydes, alcohols, sulfur-containing compounds, esters and terpenes, and were equally likely to be from the host-plant of the host, or from the host itself. Semiochemicals identified in thehost-location stage were sugars, alkanes, terpenes and heterocyclic aromatic compounds and 3/4 of them were host-produced cues. In thehost-acceptance andoviposition stage the identified semiochemicals were all produced by the host and were proteins, amino acids, triglycerides and salts. The importance of recognizing specific cues involved in host utilization by parasitoids is discussed, and suggestions for future research are made.     

The semiochemically mediated interactions between bacteria and insects

In natural environment, semiochemicals are involved in many interactions between the different trophic levels involving insects, plants and hosts for parasitoids or prey for predators. These volatile compounds act as messengers within or between insect species, inducing particular behaviours, such as the localisation of a source of food, the orientation to an adequate oviposition site, the selection of a suitable breeding site and the localisation of hosts or prey. In this sense, bacteria have been shown to play an important role in the production of volatile compounds which ones act as semiochemicals. This review, focusing on the semiochemically mediated interactions between bacteria and insects, highlights that bacterial semiochemicals act as important messengers for insects. Indeed, in most of the studies reported here, insects respond to specific volatiles emitted by specific bacteria hosted by the insect itself (gut, mouthparts, etc.) or present in the natural environment where the insect evolves. Particularly, bacteria from the families Enterobacteriaceae, Pseudomonaceae and Bacillaceae are involved in many interactions with insects. Because semiochemicals naturally produced by bacteria could be a very interesting option for pest management, advances in this field are discussed in the context of biological control against insect pests.     

Balancing between mutualism and exploitation: the symbiotic interaction between Lasius ants and aphids

Nests of Lasius niger (L.) ants were given varied food regimens to test whether their behaviour towards an aphid partner, Aphis fabae (Scop.), changed with alternative food supplies. Honeydew collection and predation on aphids were measured by video monitoring the movement of ants between their nest and an aphid aggregation. Data collected from the aphid aggregations enabled comparisons between remaining aphid biomass and between the tending intensities of the ants. I tested how ant behaviour was influenced by their access to alternative prey and sugar. The results showed that ants accepted a honey solution as a substitute for the honeydew produced by aphids. Ants not only attended their aphid partners, but also preyed on them. The average predation rate increased eightfold when ants were offered the alternative of sugar, whereas alternative prey had no significant effect. In contrast, ant-tending intensity decreased with alternative sugar whereas alternative prey elicited no effect.     

Larval chemical defence and evolution of host shifts in Chrysomela leaf beetles

Summary. Larvae of Chrysomela leaf beetles release for defence volatile compounds belonging to various chemical families. This study focuses on the defensive strategy based on the esterification of isobutyric acid and 2-methylbutyric acid with a wide variety of alcohols taken up from the host plant. To date, only two species are known to produce these repellents C. interrupta, which is associated with Betulaceae and C. lapponica which occurs either on Betulaceae or Salicaceae.? In order to know if other species have developed this chemical defence and how the food plant influences the secretion of these toxins, we targeted by mass spectrometry the presence of iso- and 2-methylbutyric acids and esters of them in the defensive secretions of Chrysomela larvae exclusively associated with Betulaceae or Salicaceae. ?Screening analyses reveal that the synthesis of these compounds is a common character restricted to all the members belonging to the C. interrupta group sensu Brown (1956) regardless of the host-plant family. These results suggest that the biochemical mechanism leading to the synthesis of these compounds could be considered as a synapomorphy meaning that the group is probably monophyletic. ?Defensive secretions of the members of the interrupta group are quantitatively assayed for iso- and 2-methylbutyric acids and their (Z)-3-hexenyl esters. Results reveal a chemical plasticity developed by Chrysomela species associated with Salicaceae. The amounts of iso- and 2-methylbutyric acids derivatives and of salicylaldehyde in their larval secretions depend on the food plant and on its content in phenolglucosides. Received 5 October 1998; accepted 25 November 1998.     

Larval behavior of predacious sister-species: orientation,molting site,and survival in Chrysopa

Field and laboratory studies compared two features of larval behavior in a pair of predacious sisterspecies of green lacewings: one (Chrysopa slossonae) a specialist on a single species of colonial aphids (the woolly alder aphid) that occur on branches and trunks of alder trees, the other (C. quadripunctata) a general aphid feeder whose primary prey is dispersed on foliage of diverse types of trees. First, a few hours after hatching, larvae of the two species develop significantly different phototactic responses; the differences correspond well with the spatial distributions of their prey. Most C. slossonae exhibited negative phototaxis, a response that helps move hatchlings inward on alder trees toward the woolly alder aphid colonies, whereas most C. quadripunctata hatchlings showed positive orientation to light, a response that tends to keep them in tree canopies with their prey. Second, in greenhouse experiments, a significantly greater proportion of C. slossonae larvae (second instars) molted within woolly alder aphid colonies and remained with the aphids than did C. quadripunctata larvae. These differences indicate that the specialist larvae have evolved a high degree of behavioral fidelity to their prey. However, larvae (second instars) of the two species that were released near ant-tended woolly alder aphid colonies in the field had similar recovery (= survival) rates. Consequently, natural selection may not act on behavioral traits that influence larval fidelity to prey during the late second and early third instars.     

Hierarchical Bayesian Modelling of plant colonisation by winged aphids: Inferring dispersal processes by linking aerial and field count data

Understanding and modelling insect pest dispersal is an important prerequisite for designing integrated pest management programs. Nevertheless, studies investigating the dispersal of small insects in natural conditions remain scarce mainly because of the difficulty of tracking the movements of these organisms. Here we propose to use Hierarchical Bayesian Modelling (HBM) framework to gain knowledge on hidden processes that cannot be observed directly in natura, such as insect landing and insect mortality, through the definition of latent variables. An HBM describing crop colonization by winged aphids was fitted to a large dataset of field observations issued from a long term survey at a wide scale of both aerial and field densities of the bird cherry-oat aphid Rhopalosiphum padi. This study provides the first evidence that suction trap data are reliable proxies of aphid colonizing rates in cereal fields in autumn and can be a nice alternative to the very time-consuming crop sampling. The proportion of winged aphids landing in cereal fields is shown to vary between regions according to the degree of investment of local R. padi population in sexual reproduction. Results also indicate that under autumnal field conditions, less than 5% of winged aphids survive more than 10 days after landing. This HBM provides the basis of a predictive model for aphid crop colonization that fully accounts for all sources of uncertainty. It should be of great value to improve the trust of users in any decision making systems.     

Allocation of herbivory-induced hydroxamic acids in the wild wheat Triticum uniaristatum

Summary. We characterized the induction of hydroxamic acids (Hx) by aphid infestation in the wild wheat Triticum uniaristatum by addressing the following questions: i) Do different leaves have similar responses to aphid damage?, ii) Is the Hx induction localized or systemic?, iii) How long does the induction last?, and iv) Is the degree of damage related to the magnitude of induced Hx? Based on earlier results on this wheat/aphid system (lack of costs of Hx induction) we expected to find the plant exhibiting cost-saving patterns of response to herbivory. Aphid infestation in the primary leaf led to induced levels of Hx, but no differences in Hx levels were found after infestation of the secondary leaf. Induction of Hx was restricted to the infested leaf (primary leaf). Induced Hx levels exhibited by the primary leaf at the end of aphid infestation were not observed 2 days later. Finally, different aphid densities (between 10 and 40 aphids per leaf) did not produce significant differences in Hx levels in infested primary leaves. Characteristics of Hx induction by aphid infestation in T. uniaristatum partially support the expected cost-saving patterns in the allocation of induced defenses. Received 15 January 1997; accepted 7 July 1997.     

Ontogenetic and spatio-temporal patterns of induced volatiles in <Emphasis Type="Italic">Glycine max</Emphasis> in the light of the optimal defence hypothesis

Summary. Plants attacked by herbivorous insects emit a blend of volatile compounds that serve as important host location cues for parasitoid wasps. Variability in the released blend may exist on the whole-plant and withinplant level and can affect the foraging efficiency of parasitoids. We comprehensively assessed the kinetics of herbivore-induced volatiles in soybean in the context of growth stage, plant organ, leaf age, and direction of signal transport. The observed patterns were used to test the predictions of the optimal defence hypothesis (OD). We found that plants in the vegetative stage emitted 10-fold more volatiles per biomass than reproductive plants and young leaves emitted >2.6 times more volatiles than old leaves. Systemic induction in single leaves was stronger and faster by one day in acropetal than in basipetal direction while no systemic induction was found in pods. Herbivore-damaged leaves had a 200-fold higher release rate than pods. To some extent these findings support the OD: i) indirect defence levels were increased in response to herbivory and ii) young leaves, which are more valuable, emitted more volatiles. However, the fact that reproductive structures emitted no constitutive or very few inducible volatiles is in seeming contrast to the OD predictions. We argue that in case of volatile emission the OD can only partially explain the patterns of defence allocation due to the peculiarity that volatiles act as signals not as toxins or repellents.