Response of Predatory Insect Scolothrips takahashii Toward Herbivore-Induced Plant Volatiles Under Laboratory and Field Conditions

We studied the response of a predatory thrips, Scolothrips takahashii, towards herbivore-induced plant volatiles emitted by Lima bean plants infested by two-spotted spider mites Tetranychus urticae (green form). Tests were conducted with a Y-tube olfactometer in the laboratory and with traps under field conditions. The odor of artificially damaged and uninfested Lima bean leaves was not more attractive than clean air in the Y-tube olfactometer. The predatory insects showed a greater preference for Lima bean leaves infested by the two-spotted spider mites than for either clean air or uninfested bean leaves. They showed the same preference towards infested leaves from which all spider mites and their visible products had been removed. Neither the spider mites themselves nor their products attracted the predators. In a satsuma mandarin grove, two traps with infested Lima bean plants as an odor source attracted 42 adult S. takahashii in 55 days, whereas no S. takahashii were trapped in two control traps with uninfested Lima bean plants during the same period. No S. takahashii were found during this period in the vicinity of either the sample traps or the control traps (5-m radius of each trap). These data showed that S. takahashii use herbivore-induced plant volatiles in their foraging behavior in natural ecosystems.     

The Involvement of Volatile Infochemicals from Spider Mites and from Food-Plants in Prey Location of the Generalist Predatory Mite <Emphasis Type="Italic">Neoseiulus californicus</Emphasis>

We investigated volatile infochemicals possibly involved in location of the generalist predatory mite Neoseiulus californicus to plants infested with spider mites in a Y-tube olfactometer. The predators significantly preferred volatiles from lima bean leaves infested with Tetranychus urticae to uninfested lima bean leaves. Likewise, they were attracted to volatiles from artificially damaged lima bean leaves and those from T. urticae plus their visible products. Significantly more predators chose infested lima bean leaves from which T. urticae plus their visible products had been removed than artificially damaged leaves, T. urticae, and their visible products. These results suggest that N. californicus is capable of exploiting a variety of volatile infochemicals originating from their prey, from the prey-foodplants themselves, and from the complex of the prey and the host plants (e.g., herbivore-induced volatiles). We also investigated predator response to some of the synthetic samples identified as volatile components emitted from T. urticae-infested lima bean leaves and/or artificially damaged lima bean leaves. The predators were attracted to each of the five synthetic volatile components: linalool, methyl salicylate, (Z)-3-hexen-1-ol, (E)-2-hexenal, and (Z)-3-hexenyl acetate. The role of each volatile compound in prey-searching behavior is discussed.     

Olfactory Responses of the Predatory Mites (N eoseiulus cucumeris) and Insects (O rius strigicollis ) to Two Different Plant Species Infested with Onion Thrips (T hrips tabaci)

Responses of Neoseiulus cucumeris (a predatory mite) and the predatory insect Orius strigicollis to volatiles associated with two different plant species infested with onion thrips, Thrips tabaci, were examined in a Y-tube olfactometer. Both predators species showed a significant preference for volatiles from infested cucumber leaves without T. tabaci over clean air. However, they were not attracted to volatiles from uninfested cucumber leaves, artificially damaged cucumber leaves, or volatiles from T. tabaci plus their visible products collected from cucumber leaves. These results suggest that both predator species are capable of exploiting herbivore-induced volatiles from T. tabaci-infested cucumber leaves as a foraging cue. Neither predator was attracted to volatiles from uninfested spring onion leaves, infested spring onion leaves without T. tabaci, or volatiles from T. tabaci plus their visible products collected from spring onion leaves. Interestingly, they avoided volatiles from artificially damaged spring onion leaves. A possible explanation for the non-significant olfactory responses of the predator species to spring onion plants with infestation damage of T. tabaci is discussed.     

Herbivory induces systemic production of plant volatiles that attract predators of the herbivore: Extraction of endogenous elicitor

It was previously shown that in response to infestation by spider mites (Tetranychus urticae), lima bean plants produce a volatile herbivoreinduced synomone that attracts phytoseiid mites (Phytoseiulus persimilis) that are predators of the spider mites. The production of predator-attracting infochemicals was established to occur systemically throughout the spider mitein-fested plant. Here we describe the extraction of a water-soluble endogenous elicitor from spider mite-infested lima bean leaves. This elicitor was shown to be transported out of infested leaves and was collected in water in which the petiole of the infested leaf was placed. When the petioles of uninfested lima bean leaves were placed in water in which infested leaves had been present for the previous seven days, these uninfested lima bean leaves became highly attractive to predatory mites in an olfactometer when an appropriate control of uninfested lima bean leaves was offered as alternative. The strength of this effect was dependent on the number of spider mites infesting the elicitor-producing leaves. Higher numbers of spider mites resulted in an elicitor solution with a stronger effect. In addition, spider mite density was important. The elicitor obtained from one leaf with 50 spider mites had a stronger effect on the attractiveness of uninfested leaves than the elicitor obtained from three leaves with 17 spider mites each. This suggests that the stress intensity imposed on a plant is an important determinant of the elicitor quantity. While the elicitor has a strong effect on the attractiveness of uninfested leaves, spider mite-infested leaves are still much more attractive to predatory mites than elicitor-exposed leaves. The data are discussed in the context of systemic effects in plant defense and the biosynthesis of herbivore-induced terpenoids in plants.     

Exposure of Lima Bean Leaves to Volatiles from Herbivore-Induced Conspecific Plants Results in Emission of Carnivore Attractants: Active or Passive Process?

There is increasing evidence that volatiles emitted by herbivore-damaged plants can cause responses in downwind undamaged neighboring plants, such as the attraction of carnivorous enemies of herbivores. One of the open questions is whether this involves an active (production of volatiles) or passive (adsorption of volatiles) response of the uninfested downwind plant. This issue is addressed in the present study. Uninfested lima bean leaves that were exposed to volatiles from conspecific leaves infested with the spider mite Tetranychus urticae, emitted very similar blends of volatiles to those emitted from infested leaves themselves. Treating leaves with a protein-synthesis inhibitor prior to infesting them with spider mites completely suppressed the production of herbivore-induced volatiles in the infested leaves. Conversely, inhibitor treatment to uninfested leaves prior to exposure to volatiles from infested leaves did not affect the emission of volatiles from the exposed, uninfested leaves. This evidence supports the hypothesis that response of the exposed downwind plant is passive. T. urticae-infested leaves that had been previously exposed to volatiles from infested leaves emitted more herbivore-induced volatiles than T. urticae-infested leaves previously exposed to volatiles from uninfested leaves. The former leaves were also more attractive to the predatory mite, Phytoseiulus persimilis, than the latter. This shows that previous exposure of plants to volatiles from herbivore-infested neighbors results in a stronger response of plants in terms of predator attraction when herbivores damage the plant. This supports the hypothesis that the downwind uninfested plant is actively involved. Both adsorption and production of volatiles can mediate the attraction of carnivorous mites to plants that have been exposed to volatiles from infested neighbors.     

Induction of Direct and Indirect Plant Responses by Jasmonic Acid, Low Spider Mite Densities, or a Combination of Jasmonic Acid Treatment and Spider Mite Infestation

Jasmonic acid (JA) and the octadecanoid pathway are involved in both induced direct and induced indirect plant responses. In this study, the herbivorous mite, Tetranychus urticae, and its predator, Phytoseiulus persimilis, were given a choice between Lima bean plants induced by JA or spider mites and uninduced control plants. Infestation densities resulting in the induction of predator attractants were much lower than thus far assumed, i.e., predatory mites were significantly attracted to plants that were infested for 2 days with only one or four spider mites per plant. Phytoseiulus persimilis showed a density-dependent response to volatiles from plants that were infested with different numbers of spider mites. Similarly, treating plants with increasing concentrations of JA also led to increased attraction of P. persimilis. Moreover, the duration of spider mite infestation was positively correlated with the proportion of predators that were attracted to mite-infested plants. A pretreatment of the plants with JA followed by a spider mite infestation enhanced the attraction of P. persimilis to plant volatiles compared to attraction to volatiles from plants that were only infested with spider mites and did not receive a pretreatment with JA. The herbivore, T. urticae preferred leaf tissue that previously had been infested with conspecifics to uninfested leaf tissue. In the case of choice tests with JA-induced and control leaf tissue, spider mites slightly preferred control leaf tissue. When spider mites were given a choice between leaf discs induced by JA and leaf discs damaged by spider mite feeding, they preferred the latter. The presence of herbivore induced chemicals and/or spider mite products enhanced settlement of the mites, whereas treatment with JA seemed to impede settlement.     

Change in Behavioral Response to Herbivore-induced Plant Volatiles in a Predatory Mite Population

Damage by herbivorous spider mites induces plants to produce volatiles that attract predatory mites that consume the spider mites. A clear attraction to volatiles from Lima bean plants infested with the spider mite Tetranychus urticae has been consistently reported during more than 15 years for the predatory mite Phytoseiulus persimilis. We have monitored the response to volatiles from spider-mite infested Lima bean plants for a laboratory population of the predatory mite from 1991 to 1995 on a regular basis. A reduction in the level of attraction in the laboratory population of P. persimilis was recorded in mid-1992. The attraction of the laboratory population was weaker than that of a commercial population in the latter part of 1992, but the responses of these two populations were similarly weak in 1994 and 1995. Therefore, a behavioral change has also occurred in this commercial population. Experiments were carried out to address the potential causes of this change in attraction. The attraction of predators from a commercial population with a strong response decreased after being reared in our laboratory. Within a predator population with a low degree of attraction, strongly responding predators were present and they could be isolated on the basis of their behavior: predators that stayed on spider-mite infested plants in the rearing set-up had a strong attraction, while predators that had dispersed from the rearing set-up were not attracted to prey-infested bean plants. From our laboratory population with a low degree of attraction, isofemale lines were initiated and maintained for more than 20 generations. All isofemale lines exhibited a consistently strong attraction to spider mite-induced plant volatiles, similar to the attraction recorded for several populations in the past 15 years. Neither in a population with a strong attraction nor in two with a weak attraction was the response of the predators affected by a starvation period of 1–3 hr. Based on these results, possible causes for the observed reduction in predator attraction to spider mite-induced bean volatiles are discussed. The predatory mite P. persimilis is a cornerstone of biological control in many crops worldwide. Therefore, the change in foraging behavior recorded in this predator may have serious consequences for biological control of spider mites.     

Induction of parasitoid attracting synomone in brussels sprouts plants by feeding ofPieris brassicae larvae: Role of mechanical damage and herbivore elicitor

Induction of plant defense in response to herbivory includes the emission of synomones that attract the natural enemies of herbivores. We investigated whether mechanical damage to Brussels sprouts leaves (Brassica oleracea var.gemmifera) is sufficient to obtain attraction of the parasitoidCotesia glomerata or whether feeding byPieris brassicae caterpillars elicits the release of synomones not produced by mechanically damaged leaves. The response of the parasitoidCotesia glomerata to different types of simulated herbivory was observed. Flight-chamber dual-choice tests showed that mechanically damaged cabbage leaves were less attractive than herbivore-damaged leaves and mechanically damaged leaves treated with larval regurgitant. Chemical analysis of the headspace of undamaged, artificially damaged, caterpillar-infested, and caterpillar regurgitant-treated leaves showed that the plant responds to damage with an increased release of volatiles. Greenleaf volatiles and several terpenoids are the major components of cabbage leaf headspace. Terpenoids are emitted in analogous amounts in all treatments, including undamaged leaves. On the other hand, if the plant is infested by caterpillars or if caterpillar regurgitant is applied to damaged leaves, the emission of green-leaf volatiles is highly enhanced. Our data are in contrast with the induction of more specific synomones in other plant species, such as Lima bean and corn.     

Leaf age affects composition of herbivore-induced synomones and attraction of predatory mites

We investigated the olfactory response of the predatory mitePhytoseiulus persimilis to cucumber leaves infested with prey, the herbivorous spider miteTetranychus urticae. The predators responded to volatiles from young rather than old infested cucumber leaves. GC-MS analysis of the head-space of spider mite-infested, artificially damaged and undamaged cucumber plants showed that herbivore-induced plant volatiles were present among the volatiles of both old and young infested cucumber leaves. The major components of the herbivore-induced plant volatiles were (3E)-4,8-dimethyl-1,3,7-nonatriene and (E)--ocimene: these compounds are known to attract the predatory mites. In addition, we found three oximes (2-methylbutanalO-methyloxime, 3-methylbutanalO-methyloxime, and an unknown oxime) in the headspace of both old and young infested cucumber leaves. 3-MethylbutanalO-methyloxime and the unknown oxime were much more abundant in the headspace of infested old cucumber leaves. The potential adaptive value of differential attractiveness of cucumber plant leaves of different age is discussed.     

The Response of Phytoseiulus persimilis to Spider Mite-Induced Volatiles from Gerbera: Influence of Starvation and Experience

When leaves of the ornamental crop Gerbera jamesonii are damaged by the spider mite Tetranychus urticae, they produce many volatile compounds in large quantities. Undamaged gerbera leaves produce only a few volatiles in very small quantities. In the headspace of spider mite-damaged gerbera leaves many terpenoids are present, comprising 65% of the volatile blend. In addition, a number of nitrogen containing compounds, such as oximes and nitriles, are produced.We studied the attraction of P. persimilis to the volatiles from spider mite-damaged gerbera leaves and how attraction is affected by starvation and previous experience. Phytoseiulus persimilis that were reared on spider mites (T. urticae) on Lima bean were not attracted to spider mite-induced volatiles from gerbera. Starvation did not influence the predator's response to these volatiles. In contrast, predators that were reared on spider mites on gerbera leaves were strongly attracted to volatiles from spider mite-infested gerbera. This was found also for predators that originated from a culture on spider mite-infested bean and were offered six days of experience with spider mites on gerbera leaves.     

Developmental stage of herbivorePseudaletia separata affects production of herbivore-induced synomone by corn plants

The female parasitic waspCotesia kariyai discriminated between the volatiles of corn leaves infested by younger host larvaePseudaletia separata (first to fourth instar) and uninfested leaves in a Y-tube olfactometer; the wasps were attracted to the infested leaves. In contrast, when corn plants were infested by the later stages (fifth and sixth instar) of the armyworm, the wasps did not distinguish between infested corn leaves and uninfested corn leaves in the olfactometer. Mechanically damaged leaves were no more attractive than undamaged leaves, and host larvae or their feces were not attractive to the parasitoid. Through chemical analysis, the herbivore-induced plant volatiles were identified in the headspace of infested corn leaves. The herbivore-induced volatiles (HIVs) constituted a larger proportion of the headspace of corn leaves infested by early instar armyworms than of corn leaves infested by late instar armyworms. Application of third-instar larval regurgitant onto artificially damaged sites of leaves resulted in emission of parasitoid attractants from the leaf, whereas leaves treated with sixth-instar regurgitant did not. The function of this herbivore-stage related specificity of herbivore-induced synomones is discussed in a tritrophic context.     

Systemic release of herbivore-induced plant volatiles by turnips infested by concealed root-feeding larvae Delia radicum L

When attacked by herbivorous insects, many plants emit volatile compounds that are used as cues by predators and parasitoids foraging for prey or hosts. While such interactions have been demonstrated in several host–plant complexes, in most studies, the herbivores involved are leaf-feeding arthropods. We studied the long-range plant volatiles involved in host location in a system based on a very different interaction since the herbivore is a fly whose larvae feed on the roots of cole plants in the cabbage root fly, Delia radicum L. (Diptera: Anthomyiidae). The parasitoid studied is Trybliographa rapae Westwood (Hymenoptera: Figitidae), a specialist larval endoparasitoid of D. radicum. Using a four-arm olfactometer, the attraction of naive T. rapae females toward uninfested and infested turnip plants was investigated. T. rapae females were not attracted to volatiles emanating from uninfested plants, whether presented as whole plants, roots, or leaves. In contrast, they were highly attracted to volatiles emitted by roots infested with D. radicum larvae, by undamaged parts of infested roots, and by undamaged leaves of infested plants. The production of parasitoid-attracting volatiles appeared to be systemic in this particular tritrophic system. The possible factors triggering this volatile emission were also investigated. Volatiles from leaves of water-stressed plants and artificially damaged plants were not attractive to T. rapae females, while volatiles emitted by leaves of artificially damaged plants treated with crushed D. radicum larvae were highly attractive. However, T. rapae females were not attracted to volatiles emitted by artificially damaged plants treated only with crushed salivary glands from D. radicum larvae. These results demonstrate the systemic production of herbivore-induced volatiles in this host-plant complex. Although the emission of parasitoid attracting volatiles is induced by factors present in the herbivorous host, their exact origin remains unclear. The probable nature of the volatiles involved and the possible origin of the elicitor of volatiles release are discussed.     

Identification of Volatiles That Are Used in Discrimination Between Plants Infested with Prey or Nonprey Herbivores by a Predatory Mite

Carnivorous arthropods can use herbivore-induced plant volatiles to locate their herbivorous prey. In the field, carnivores are confronted with information from plants infested with herbivores that may differ in their suitability as prey. Discrimination by the predatory mite Phytoseiulus persimilis between volatiles from lima bean plants infested with the prey herbivore Tetranychus urticae, or plants infested with the nonprey caterpillar Spodoptera exigua, depends on spider mite density. In this article, we analyzed the chemical composition of the volatile blends from T. urticae-infested lima bean plants at different densities of spider mites, and from S. exigua-infested plants. Based on the behavioral preferences of P. persimilis and the volatile profiles, we selected compounds that potentially enable the mite to discriminate between T. urticae-induced and S. exigua-induced volatiles. Subsequently, we demonstrated in Y-tube olfactometer assays that the relatively large amounts of methyl salicylate and (3E, 7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene emitted by T. urticae-infested bean plants compared to S. exigua-infested plants enable the predators to discriminate. Our data show that specific compounds from complex herbivore-induced volatile blends can play an important role in the selective foraging behavior of natural enemies of herbivorous arthropods.     

Effect of the Presence of a Nonhost Herbivore on the Response of the Aphid Parasitoid <Emphasis Type="Italic">Diaeretiella rapae</Emphasis> to Host-infested Cabbage Plants

The vast majority of studies of plant indirect defense strategies have considered simple tritrophic systems that involve plant responses to attack by a single herbivore species. However, responses by predators and parasitoids to specific, herbivore-induced, volatile blends could be compromised when two or more different herbivores are feeding on the same plant. In Y-tube olfactometer studies, we investigated the responses of an aphid parasitoid, Diaeretiella rapae (McIntosh) (Hymenoptera: Braconidae), to odors from cabbage plants infested with the peach-potato aphid Myzus persicae (Sulzer) (Homoptera: Aphididae), in both the presence and absence of a lepidopteran caterpillar, Plutella xylostella L. (Lepidoptera: Plutellidae). Female parasitoids chose aphid-infested plants over uninfested plants but did not distinguish between caterpillar-infested and uninfested plants. When given a choice between odors from an aphid-infested plant and those from a plant infested with diamondback moth larvae, they significantly chose the former. Furthermore, the parasitoids responded equally to odors from a plant infested with aphids only and those from a plant infested with both aphids and caterpillars. The results support the hypothesis that the aphid and the caterpillar induce different changes in the volatile profile of cabbage plants and that D. rapae females readily distinguish between the two. Furthermore, the changes to the plant volatile profile induced by the caterpillar damage did not hinder the responses of the parasitoid to aphid-induced signals.     

Effects of molasses grass,Melinis minutiflora volatiles on the foraging behavior of the cereal stemborer parasitoid,Cotesia sesamiae

Olfactory responses of the cereal stemborer parasitoid Cotesia sesamiae to volatiles emitted by gramineous host and nonhost plants of the stemborers were studied in a Y-tube olfactometer. The host plants were maize (Zea mays) and sorghum ( Sorghum bicolor), while the nonhost plant was molasses grass (Melinis minutiflora). In single-choice tests, females of C. sesamiae chose volatiles from infested and uninfested host plants and molasses grass over volatiles from the control (soil). In dual-choice tests, the wasp preferred volatiles from infested host plants to those from uninfested host plants. There was no discrimination between molasses grass volatiles and those of uninfested maize, uninfested sorghum, or infested maize. The wasp preferred sorghum volatiles over maize. Combining uninfested maize or sorghum with molasses grass did not make volatiles from the combination more attractive as compared to only uninfested host plants. Infested maize alone was as attractive as when combined with molasses grass. Infested sorghum was preferred over its combination with molasses grass. Local growth conditions of the molasses grasses influenced attractiveness to the parasitoids. Volatiles from Thika molasses grass were attractive, while those from Mbita molasses grass were not. Growing the Thika molasses grass in Mbita rendered it unattractive and vice versa with the Mbita molasses grass. This is a case of the same genotype expressing different phenotypes due to environmental factors.     

Predatory Mite Attraction to Herbivore-induced Plant Odors is not a Consequence of Attraction to Individual Herbivore-induced Plant Volatiles

Predatory mites locate herbivorous mites, their prey, by the aid of herbivore-induced plant volatiles (HIPV). These HIPV differ with plant and/or herbivore species, and it is not well understood how predators cope with this variation. We hypothesized that predators are attracted to specific compounds in HIPV, and that they can identify these compounds in odor mixtures not previously experienced. To test this, we assessed the olfactory response of Phytoseiulus persimilis, a predatory mite that preys on the highly polyphagous herbivore Tetranychus urticae. The responses of the predatory mite to a dilution series of each of 30 structurally different compounds were tested. They mites responded to most of these compounds, but usually in an aversive way. Individual HIPV were no more attractive (or less repellent) than out-group compounds, i.e., volatiles not induced in plants fed upon by spider-mites. Only three samples were significantly attractive to the mites: octan-1-ol, not involved in indirect defense, and cis-3-hexen-1-ol and methyl salicylate, which are both induced by herbivory, but not specific for the herbivore that infests the plant. Attraction to individual compounds was low compared to the full HIPV blend from Lima bean. These results indicate that individual HIPV have no a priori meaning to the mites. Hence, there is no reason why they could profit from an ability to identify individual compounds in odor mixtures. Subsequent experiments confirmed that naive predatory mites do not prefer tomato HIPV, which included the attractive compound methyl salicylate, over the odor of an uninfested bean. However, upon associating each of these odors with food over a period of 15 min, both are preferred. The memory to this association wanes within 24 hr. We conclude that P. persimilis possesses a limited ability to identify individual spider mite-induced plant volatiles in odor mixtures. We suggest that predatory mites instead learn to respond to prey-associated mixtures of volatiles and, thus, to odor blends as a whole.     

Intraspecies Variation in the Kanzawa Spider Mite Differentially Affects Induced Defensive Response in Lima Bean Plants

The Kanzawa spider mite, Tetranychus kanzawai, is a polyphagous herbivore that feeds on various plant families, including the Leguminacae. Scars made by the mite on lima bean leaves (Phaseolus lunatus) were classified into two types: white and red. We obtained two strains of mites—“White” and “Red”—by selecting individual mites based on the color of the scars. Damage made by the Red strain induced the expression of genes for both basic chitinase, which was downstream of the jasmonic acid (JA) signaling pathway, and acidic chitinase, which was downstream of the salicylic acid (SA) signaling pathway. White strain mites also induced the expression of the basic chitinase gene in infested leaves but they only slightly induced the acidic chitinase gene. The Red genotype was dominant over the White for the induction of the acidic chitinase gene. The amount of endogenous salicylates in leaves increased significantly when infested by Red strain mites but did not increase when infested by White strain mites. JA and SA are known to be involved in the production of lima bean leaf volatiles induced by T. urticae. The blend of volatiles emitted from leaves infested by the Red strain were qualitatively different from those infested by the White strain, suggesting that the SA and JA signaling pathways are differently involved in the production of lima bean leaf volatiles induced by T. kanzawai of different strains.Ryo Matsushima and Rika Ozawa contributed equally to this work.     

Induction and Systemic Release of Herbivore-Induced Plant Volatiles Mediating In-Flight Orientation of Aphidius ervi

In-flight orientation of the braconid Aphidius ervi in response to volatiles released from broad bean plants infested by the pea aphid, Acyrthosiphon pisum, was studied in a no-choice wind-tunnel bioassay. The role of aphid infestation level and duration, systemic production of volatiles by insect-free parts of the plant, and the specificity of aphid-induced volatiles on the flight behavior of the foraging female parasitoids were investigated. The upper insect-free part of a three-leaved broad bean plant, which was basally infested by a population of 40 A. pisum, released synomones detectable by A. ervi females after at least 48–72 hr of infestation, resulting in both significant increases in oriented flights and landings on the source compared with uninfested control plants. This suggests that volatiles involved in host-location by A. ervi are systemically released by broad bean plants either in response to circulation of aphid saliva, circulation of saliva-induced bioactive elicitors, or circulation of the synomones themselves. Air entrainment extracts of volatiles collected from a broad bean plant infested by the nonhost Aphis fabae or an uninfested broad bean plant elicited few oriented flights and landing responses by female parasitoids. These extracts were significantly less attractive than extracts collected from a broad bean plant infested by the host A. pisum, indicating the specificity of synomones elicited by different aphid species on the same plant species.     

Host microhabitat location by stem-borer parasitoidCotesia flavipes: the role of herbivore volatiles and locally and systemically induced plant volatiles

The origin of olfactory stimuli involved in the host microhabitat location inCotesia flavipes, a parasitoid of stem-borer larvae, was investigated in a Y-tube olfactometer. The response of femaleC. flavipes towards different components of the plant-host complex, consisting of a maize plant infested with two or more larvae of the stem borerChilo partellus, was tested in dualchoice tests. The concealed lifestyle of the stem-borer larvae did not limit the emission of volatiles attractive to a parasitoid. A major source of the attractive volatiles from the plant-host complex was the stem-borer-injured stem, including the frass produced by the feeding larvae. Moreover, the production of volatiles attractive to a parasitoid was not restricted to the infested stem part but occurs systemically throughout the plant. The uninfested leaves of a stem-borer-infested plant were found to emit volatiles that attract femaleC. flavipes. We further demonstrate that an exogenous elicitor of this systemic plant response is situated in the regurgitate of a stem-borer larva. When a minor amount of regurgitate is inoculated into the stem of an uninfested plant, the leaves of the treated plant emit volatiles that attract femaleC. flavipes.     

Systemically Induced Plant Volatiles Emitted at the Time of “Danger”

Feeding by Pieris brassicae caterpillars on the lower leaves of Brussels sprouts (Brassica oleracea var. gemmifera) plants triggers the release of volatiles from upper leaves. The volatiles are attractive for a natural antagonist of the herbivore, the parasitoid Cotesia glomerata. Parasitoids are attracted only if additional damage is inflicted on the systemically induced upper leaves and only after at least three days of herbivore feeding on the lower leaves. Upon termination of caterpillar feeding, the systemic signal is emitted for a maximum of one more day. Systemic induction did not occur at low levels of herbivore infestation. Systemically induced leaves emitted green leaf volatiles, cyclic monoterpenoids, and sesquiterpenes. GC-MS profiles of systemically induced and herbivore-infested leaves did not differ for most compounds, although herbivore infested plants did emit higher amounts of green leaf volatiles. Emission of systemically induced volatiles in Brussels sprouts might function as an induced defense that is activated only when needed, i.e., at the time of caterpillar attack. This way, plants may adopt a flexible management of inducible defensive resources to minimize costs of defense and to maximize fitness in response to unpredictable herbivore attack.