A volatile kairomone emitted from lima bean plants (Phaseolus lunatus) infested with the spider miteTetranychus urticae, was collected on Tenax-TA and analyzed with GC-MS. Two components were identified as the methylene monoterpene (3E)-4,8-dimethyl-1,3,7-nonatriene and the methylene sesquiterpene (3E,7E)-4,8,12-dimethyl-1,3,7,11-tridecatetraene, respectively, after purification by preparative GC on a megabore column and recording of UV, IR, and [(1)H]NMR spectra. The response of two species of predatory mites towards the identified chemicals was tested in a Y-tube olfactometer. Four of the compounds tested, linalool (3,7-dimethyl-1,6-octadien-3-ol), (E)-β-ocimene [(3E)-3,7-dimethyl-1,3,6-octatriene], (3E)-4,8-dimethyI-1,3,7-nonatriene, and methyl salicylate attracted females ofPhytoseiulus persimilis. Linalool and methyl salicylate attracted females ofAmblyseius potentillae. The response ofA. potentillae to these two kairomone components was affected by the rearing diet of the predators in the same way as was reported for the response to the natural kairomone blend: when reared on a carotenoid-deficient diet, the predators responded to the volatile kairomone ofT. urticae, but when reared on a carotenoid-containing diet they did not. The identified kairomone components are all known from the plant kingdom. They are not known to be produced by animals de novo. In addition to biological evidence, this chemical evidence suggests that the plant is involved in production of the kairomone. Based on the present study and literature data on the response ofT. urticae to infochemicals, it is concluded that the kairomone component linalool is also a component of a volatile spider-mite dispersing pheromone.

Cabbage plants respond to caterpillar (Pieris brassicae) herbivory by releasing a mixture of volatiles that makes them highly attractive to parasitic wasps (Cotesia glomerata) that attack the herbivores. Cabbage leaves that are artificially damaged and subsequently treated with gut regurgitant of P. brassicae caterpillars release a volatile blend similar to that of herbivore-damaged plants. We demonstrate the presence of 8-glucosidase in P. brassicae regurgitant. Leaves treated with commercial j8-glucosidase (from almonds) release a volatile blend similar to that of leaves treated with P. brassicae regurgitant. In a flight bioassay, leaves treated with almond j8-glucosidase are highly attractive to the parasitic wasp C. glomerata. Furthermore, the wasps do not discriminate between cabbage leaves treated with almond j3-glucosidase and leaves treated with larval regurgitant.13-Glucosidase was also recorded in cabbage leaf extract, but this is not as effective as caterpillar 38-glucosidase in eliciting the volatile production. Caterpillars that feed on a 8-glucosidase-free diet secrete the enzyme, and their regurgitant is an effective elicitor of the plant response. These findings show that 8-glucosidase is a P. brassicae-secreted elicitor of the defense response of cabbage plants to herbivore injury, inducing the emission of volatiles that are used by parasitoids of the herbivore to locate their victims.It is well-known that plants may react to herbivory or to pathogen infestation by phytochemical responses (reviewed in refs. 1-3). The first step in such responses is the recognition of the attack by the plant. How plants recognize infestation of a pathogen has been intensively studied and many pathogenderived exogenous elicitors of phytoalexins have been identified (4). In contrast, knowledge on the recognition of herbivorous arthropods by plants is scarce, being mostly restricted to the involvement of herbivore secretions (e.g., see refs. 5-7). Yet, a wealth of knowledge is available on endogenous elicitors that originate from mechanical damage (8) and on subsequent steps in the signal transduction pathway (8, 9) in responses of plants to herbivores.A phytochemical response that has been studied in the past decade is the production of volatiles that attract carnivorous arthropods such as predators and parasitic wasps (parasitoids) that attack the herbivore (2). For instance, lima bean plants respond to infestation by the spider mite Tetranychus urticae by producing volatile terpenoids and methyl salicylate that attract a predator (Phytoseiulus persimilis) of these herbivores (2, 10-12). Recently, studies have been initiated on exogenous herbivore elicitors of such plant responses. For instance, the response of corn plants to herbivory by fall armyworm caterpillars is similar to the response to administration of caterpillar regurgitant into a mechanical wound or fed through the petiole of an intact corn leaf (13,14). These treatments result in the production of volatile terpenoids and indole that attract t...

Abstract--To understand the role of allelochemicals in predator-prey interactions it is not sufficient to study the behavioral responses of predator and prey, One should elucidate the origin of the allelochemicals and be aware that it may be located at another trophic level. These aspects are reviewed for predator-prey interactions in general and illustrated in detail for interactions between predatory mites and herbivorous mites. In the latter system there is behavioral and chemical evidence for the involvement of the host plant in production of volatile allelochemicals upon damage by the herbivores with the consequence of attracting predators. These volatiles not only influence predator behavior, but also prey behavior and even the attractiveness of nearby plants to predators. Herbivorous mites disperse away from places with high concentrations of the volatiles, and undamaged plants attract more predators when previously exposed to volatites from infested conspecific plants rather than from uninfested plants. The latter phenomenon may well be an example of plant-to-plant communication. The involvement of the host plant is probably not unique to the predator-herbivore-plant system under study. It may well be widespread since it makes sense from an evolutionary point of view. If so, prospects for application in pest control are wide open. These are discussed, and it is concluded that crop protection in the future should include tactics whereby man becomes an ally to plants in their strategies to manipulate predator-prey interactions through allelochemicals.

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.

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Plants may defend themselves against herbivores by enhancing the effectiveness of natural enemies of herbivores. This is termed "indirect defense," which may be induced by herbivore damage. An important aspect of induced indirect defense is the attraction of the herbivore's natural enemies to infested plants by the plant emitting so-called "herbivore-induced synomone" (HIS) in response to herbivore damage. In this paper, we review the role of terpenoids in the induced indirect defense of plants against herbivorous mites. HIS are emitted from both damaged and undamaged areas of infested plants, and the composition of HIS varies among different plant species. The emission of HIS may also vary within a plant species, depending upon: (1) plant cultivar, (2) leaf growth stage, (3) the herbivore species that is attacking, and (4) abiotic conditions (light intensity, time of year, and water stress). Predatory mites cope with this variation of HIS by innate recognition as well as temporary specialization to a certain HIS via learning.

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.

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