Specificity of Systemically Released Cotton Volatiles as Attractants for Specialist and Generalist Parasitic Wasps

Cotton plants under herbivore attack release volatile semiochemicals that attract natural enemies of the herbivores to the damaged plant. The volatiles released in response to herbivory are not only released from the damaged leaves but from the entire cotton plant. We found that cotton plants that released myrcene, (Z)-3-hexenyl acetate, (E)--ocimene, linalool, (E)-4,8-dimethyl-1,3,7-nonatriene, (E)--farnesene, and (E, E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene systemically from undamaged leaves of caterpillar damaged plants were attractive to the generalist parasitoid Cotesia marginiventris and the specialist parasitoid Microplitis croceipes. Plants from which the caterpillar damaged leaves were removed and that released those compounds systemically were significantly preferred over undamaged control plants in two-choice experiments in a flight tunnel. Artificially damaged cotton plants that released green leafy volatiles and constitutive terpenoids were less attractive for M. croceipes and C. marginiventris. Only C. marginiventris preferred artificially damaged plants over undamaged control plants, whereas M. croceipes showed no preference. The apparent lack of specificity of systemically released compounds in response to different herbivores feeding on the lower leaves is discussed.     

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.     

Isolation and identification of allelochemicals that attract the larval parasitoid,Cotesia marginiventris (Cresson), to the microhabitat of one of its hosts

Volatiles released from corn seedlings on which beet armyworm larvae were feeding were attractive to females of the parasitoid,Cotesia marginiventris (Cresson), in flight tunnel bioassays. Analyses of the collected volatiles revealed the consistent presence of 11 compounds in significant amounts. They were: (Z)-3-hexenal, (E)-2-hexenal, (Z)-3-hexen-1-ol, (Z)- 3-hexen-1-yl acetate, linalool, (3E)-4,8-dimethyl-1,3,7-nonatriene, indole, -trans-bergamotene, (E)--farnesene, (E)-nerolidol, and (3E,7E)-4,8,12-trimethyl-1, 3,7,ll-tridecatetraene. A synthetic blend of all 11 compounds was slightly less attractive to parasitoid females than an equivalent natural blend. However, preflight experience with the synthetic blend instead of experience with a regular plant-host complex significantly improved the response to the synthetic blend. Our results suggest thatC. marginiventris females, in their search for hosts, use a blend of airborne semiochemicals emitted by plants on which their hosts feed. The response to a particular odor blend dramatically increases after a parasitoid experiences it in association with contacting host by-products.     

Distribution and Differential Expression of (E)-4,8-Dimethyl-1,3,7-Nonatriene in Leaf and Floral Volatiles of Magnolia and Liriodendron Taxa

Analyses of volatiles emitted from artificially damaged leaves attached to branches of seven Magnolia taxa revealed the presence of (Z)-3-hexenyl acetate, (Z)-3-hexenol (the green odor compounds), and several mono- and sesquiterpenes, e.g., (Z)- and (E)-β-ocimene and caryophyllene. An herbivore-induced leaf volatile, (E)-4,8-dimethyl-1,3,7-nonatriene, known as a predator attractant in agricultural plants, was emitted 4–6 hr after leaves were damaged in M. hypoleuca. The damaged leaves of M. grandiflora, however, immediately released (E)-4,8-dimethyl-1,3,7-nonatriene. Undamaged leaves of Magnolia species examined did not emit volatile compounds. In addition, detached flowers of six Magnolia taxa and Liriodendron tulipifera also emit (E)-4,8-dimemyl-1,3,7-nonatriene as a floral volatile (up to 30% in some species); the chemical was also emitted from the intact flowers of M. heptapeta and M. salicifolia.     

Attraction to Herbivore-induced Plant Volatiles by the Host-foraging Parasitoid Fly <Emphasis Type="Italic">Exorista japonica</Emphasis>

Responses of the tachinid fly Exorista japonica Townsend to odors from corn plants infested with the fly’s host, the larvae of the noctuid moth Mythimna separata (Walker), were examined in a wind tunnel. Naïve female flies showed a higher rate of landing on M. separata-infested corn plants from which the host larvae had been removed than on artificially damaged or intact corn plants. When paper impregnated with a solution of headspace volatiles collected from host-infested plants was attached to intact plants, females landed on the plants at a high rate. Females also responded to intact plants to which had been attached with paper impregnated with a synthetic blend of nine chemicals identified previously in host-infested plants. There was an optimum concentration of the synthetic blend for the females’ landing. Of the nine chemicals identified previously, four [(E)-4,8-dimethyl-1,3,7-nonatriene, indole, 3-hydroxy-2-butanone, and 2-methyl-1-propanol] released only by host-infested plants were classified as a host-induced blend. The other five [(Z)-3-hexen-1-yl acetate, (E)-2-hexenal, hexanal, (Z)-3-hexen-1-ol, and linalool] were classified as a non-specific blend released not only by infested plants but also by artificially damaged or intact plants. In the wind tunnel, E. japonica females did not respond to intact plants to which paper containing a solution of non-specific blend or host-induced blend was attached. However, they showed a high level of response to a mixture of the non-specific and host-induced blends. These results indicate that naïve E. japonica use a combination of non-specific and host-induced blends as an olfactory cue for locating host-infested plants.     

Herbivore-induced volatile emissions from cotton (Gossypium hirsutum L.) seedlings

The effect of herbivory on the composition of the volatile blends released by cotton seedlings was investigated by collecting volatiles from undamaged, freshly damaged (0–2 hr after initiation of feeding), and old damaged (16–19 hr after initiation of feeding) plants on which corn earworm caterpillars (Helicoverpa zea Boddie) were actively feeding. A blend of 22 compounds was consistently observed to be emitted by the old damaged plants with nine occurring either only in, or in significantly greater amounts in old damaged, as compared with freshly damaged plants. These were (Z)-3-hexenyl acetate, hexyl acetate, (E)--ocimene, (3E)-4,8-dimethyl-1,3,7-nonatriene, (Z)-3-hexenyl butyrate, (E)-2-hexenyl butyrate, (Z)-3-hexenyl 2-methylbutyrate, (E)-2-hexenyl 2-methylbutyrate, and indole. The nature of this response is compared with other studies where herbivore-induced volatile responses are also known. The presence of large amounts of terpenes and aldehydes seen at the onset of feeding and the appearance of other compounds hours later suggest that cotton defense mechanisms may consist of a constitutive repertoire that is augmented by an induced mechanism mobilized in response to attack. A number of the induced compounds are common to many plants where, in addition to an immediate defensive function, they are known to be involved in the attraction of natural enemies.     

Electroantennogram Responses of a Predator, Perillus bioculatus, and its Prey, Leptinotarsa decemlineata, to Plant Volatiles

The two-spotted stinkbug, Perillus bioculatus, is a predator of the Colorado potato beetle (CPB), Leptinotarsa decemlineata. Behavioral tests revealed that P. bioculatus is attracted to potato plants, Solanum tuberosum L. (Solanaceae), infested by the CPB. Electroantennograms from the antennae of P. bioculatus were recorded in response to compounds present in the headspace of CPB-infested potato plants. (Z)-3-Hexen-1-ol and 2-phenylethanol elicited the highest EAG amplitudes. Linalool, 4,8-dimethyl-1,3(E),7-nonatriene, nonanal, decanal, and (R)-(+)-limonene evoked lower EAG amplitudes. The major headspace components -caryophyllene and -selinene produced only weak EAG responses. Antennal sensitivity of the CPB to (Z)-3-hexen-1-ol was higher than that of P. bioculatus, whereas the stinkburg was more sensitive to 2-phenylethanol, -caryophyllene, (R)-(+)-limonene, and decanal. Among these compounds, 2-phenylethanol is of special interest since it was observed to be emitted by potato foliage only after being damaged by CPBs.     

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.     

Essential Compounds in Herbivore-Induced Plant Volatiles that Attract the Predatory Mite <Emphasis Type="Italic">Neoseiulus womersleyi</Emphasis>

Carnivorous arthropods use volatile infochemicals emitted from prey-infested plants in their foraging behavior. Although several volatile components are common among plant species, the compositions differ among prey–plant complexes. Studies showed that the predatory mite Neoseiulus womersleyi is attracted only to previously experienced plant volatiles. In this study, we identified the attractant components in prey-induced plant volatiles of two prey–plant complexes. N. womersleyi reared on Tetranychus kanzawai-infested tea leaves showed significant preference for a mixture of three synthetic compounds [mimics of the T. kanzawai-induced tea leaves volatiles: (E)-β-ocimene, (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), and (E,E)-α-farnesene] at a level comparable to that for T. kanzawai-induced tea plant volatiles. However, mixtures lacking any of these compounds did not attract the predatory mites. Likewise, N. womersleyi reared on T. urticae-infested kidney bean plants showed a significant preference for a mixture of four synthetic compounds [mimics of the T. urticae-induced kidney bean volatiles: DMNT, methyl salicylate (MeSA), β-caryophyllene, and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene] at a level comparable to that for T. urticae-induced kidney bean volatiles. The absence of any of the four compounds resulted in no attraction. These results indicate that N. womersleyi can use at least four volatile components to identify prey-infested plants.     

Sexual Maturation and Temporal Variation of Neural Responses in Adult Colorado Potato Beetles to Volatiles Emitted by Potato Plants

Neural responses of the Colorado potato beetle (CPB), Leptinotarsa decemlineata to volatiles emitted by potato plants, Solanum tuberosum L were investigated. Amplitudes of electroantennograms to measured amounts of a standard odorant, (Z)-3-hexenyl acetate, increased from day of emergence through at least six to eight days of adulthood. Among 20 potato volatiles examined, several constitutive compounds, e.g., the green leaf volatiles (Z)-3-hexen-1-ol, (E)-2-hexen-1-ol, and (Z)-3-hexenyl butyrate, and systemic volatiles released primarily in response to insect feeding, e.g., (±)-linalool, nonanal, methyl salicylate, and indole, were the most effective stimuli. A statistic called linear age-skew (linear orthogonal polynomial) was used to examine differences in responses to potato volatiles between young and mature CPB. Based on plots of linear age-skew and overall neural responsiveness, 10 volatiles could be identified for which responses increased at a rate similar to or greater than the standard. The results are discussed with regard to the relationship of the CPB to its host plant and developmental studies of insect sensory responses to chemical signals.     

The Tea Weevil, <Emphasis Type="Italic">Myllocerinus aurolineatus</Emphasis>, is Attracted to Volatiles Induced by Conspecifics

The tea weevil, Myllocerinus aurolineatus (Voss) (Coleoptera: Curculionidae), is a leaf-feeding pest of Camellia sinensis (O.Ktze.) with aggregative behaviors that can seriously reduce tea yield and quality. Although herbivore-induced host plant volatiles have been shown to attract conspecific individuals of some beetle pests, especially members of the Chrysomelidae family, little is known about the volatiles emitted from tea plants infested by M. aurolineatus adults and their roles in mediating interactions between conspecifics. The results of behavioral bioassays revealed that volatile compounds emitted from tea plants infested by M. aurolineatus were attractive to conspecific weevils. Volatile analyses showed that infestations dramatically increased the emission of volatiles, (Z)-3-hexenal, (Z)-3-hexenol, (E)-β-ocimene, linalool, phenylethyl alcohol, benzyl nitrile, indole, (E, E)-α-farnesene, (E)-nerolidol, and 31 other compounds. Among the induced volatiles, 12 chemicals, including γ-terpinene, benzyl alcohol, (Z)-3-hexenyl acetate, myrcene, benzaldehyde, (Z)-3-hexenal, and (E, E)-α-farnesene, elicited antennal responses from both sexes of the herbivore, whereas (E)-β-ocimene elicited antennal responses only from males. Using a Y-tube olfactometer, we found that six of the 13 chemicals, γ-terpinene, benzyl alcohol, (Z)-3-hexenyl acetate, myrcene, benzaldehyde, and (Z)-3-hexenal, were attractive to both males and females; two chemicals, (E/Z)-β-ocimene and (E, E)-α-farnesene, were attractive only to males; and four chemicals, (E)-4,8-dimethyl-1,3,7-nonatriene, phenylethyl alcohol, linalool, and (Z)-3-hexenol, were attractive only to females. The findings provide new insights into the interactions between tea plants and their herbivores, and may help scientists develop new strategies for controlling the herbivore.     

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 Host and Nonhost Semiochemicals of Eucalyptus Woodborer Phoracantha semipunctata by Gas Chromatography–Electroantennography

The host range of the eucalyptus woodborer, Phoracantha semipunctata, is restricted mainly to species of Eucalyptus (Myrtaceae). Volatile semiochemicals possibly involved in host selection and nonhost rejection were identified by high-resolution gas chromatography–electroantennography on samples obtained by air entrainment of foliage from host and nonhost trees. Compounds from the main host E. globulus, active at physiologically significant levels, included 3-hydroxy-2-butanone, 3-methyl1-butanol, a branched epoxyalkane, ethyl 3-methylbutanoate, (Z)-3-hexen1-ol, -pinene, -pinene, p-cymene, 1,8-cineole, limonene, and guaiene. E. camaldulensis gave a similar spectrum of volatiles that also included -terpinene and linalool. The volatiles from E. tereticornis were similar to E. globulus but without ethyl 3-methylbutanoate, (Z)-3-hexen-1-ol, 1,8-cineole, or limonene. The nonhost Pinus pinaster (Pinaceae) yielded active compounds common to the host species, including ethyl 3-methylbutanoate, (Z)-3-hexen-1-ol, -pinene, -pinene, p-cymene, 1,8-cineole, limonene, and linalool but, in addition, myrcene, (E)--ocimene, and -cubebene as candidate nonhost cues. The nonhost Olea europeae (Oleaceae) also shared some active compounds in common with the host species, including 3-hydroxy-2-butanone, 3-methyl-1-butanol, the branched epoxyalkane, ethyl 3-methylbutanoate, (Z)-3-hexen-1-ol, -pinene, but an apparent nonhost cue from this species was the homomonoterpene (E)-4,8-dimethylnona-1,3,7triene, plus other compounds so far unidentified.     

Green leaf volatiles as antiaggregants for the mountain pine beetle,Dendroctonus ponderosae Hopkins (Coleoptera: Scolytidae)

We tested the hypothesis that green leaf volatiles act as antiaggregants for the mountain pine beetle (MPB),Dendroctonus ponderosac Hopkins. In coupled gas chromatographic-electroantennographic detection (GC-EAD) analysis MPB antennae responded to 30 ng doses of all six-carbon green leaf alcohols tested [1-hexanol, (E)-2-hexen-1-ol, (Z)-2-hexen-1-ol, (E)-3-hexen-1-ol, and (Z)-3-hexen-1-ol], but not to the aldehydes, hexanal or (E)-2-hexenal, or to alcohol or aldehyde homologues with more or fewer than six carbon atoms. In field trapping experiments a blend of green leaf alcohols [1-hexanol, (Z)-2-hexen-1-ol, (E)-3-hexen-1-ol and (Z)-3-hexen-1-ol] effectively disrupted the response to attractive semiochemicals; a blend of the aldehydes hexanal and (E)-2-hexenal was inactive. The two best disruptants. (E)-2-hexen-1-ol and (Z)-3-hexen-1-ol, reduced catches of both sexes to levels not significantly different from catches in unbaited control traps. They also reduced the attack on trees baited with attractive MBP pheromones to a level not significantly different from that on unbaited control trees. Neither of the clerid predators captured,Enoclerus sphegeus (F.) norThanasimus undatulus (Say), was repelled by green leaf volatiles. Our results suggest that green leaf alcohols are promising disruptants which may be used to supplement the antiaggregation pheromone, verbenone, in protecting single high-value trees as well as carefully selected stands with low-level populations of MPBs.     

Isolation and identification of volatile kairomone that affects acarine predatorprey interactions Involvement of host plant in its production

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 [¹H]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)--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.     

Genetic Variation in Jasmonic Acid- and Spider Mite-Induced Plant Volatile Emission of Cucumber Accessions and Attraction of the Predator <Emphasis Type="Italic">Phytoseiulus persimilis</Emphasis>

Cucumber plants (Cucumis sativus L.) respond to spider–mite (Tetranychus urticae) damage with the release of specific volatiles that are exploited by predatory mites, the natural enemies of the spider mites, to locate their prey. The production of volatiles also can be induced by exposing plants to the plant hormone jasmonic acid. We analyzed volatile emissions from 15 cucumber accessions upon herbivory by spider mites and upon exposure to jasmonic acid using gas chromatography—mass spectrometry. Upon induction, cucumber plants emitted over 24 different compounds, and the blend of induced volatiles consisted predominantly of terpenoids. The total amount of volatiles was higher in plants treated with jasmonic acid than in those infested with spider mites, with (E)-4,8-dimethyl-1,3,7-nonatriene, (E,E)-α-farnesene, and (E)-β-ocimene as the most abundant compounds in all accessions in both treatments. Significant variation among the accessions was found for the 24 major volatile compounds. The accessions differed strongly in total amount of volatiles emitted, and displayed very different odor profiles. Principal component analysis performed on the relative quantities of particular compounds within the blend revealed clusters of highly correlated volatiles, which is suggestive of common metabolic pathways. A number of cucumber accessions also were tested for their attractiveness to Phytoseiulus persimilis, a specialist predator of spider mites. Differences in the attraction of predatory mites by the various accessions correlated to differences in the individual chemical profiles of these accessions. The presence of genetic variation in induced plant volatile emission in cucumber shows that it is possible to breed for cucumber varieties that are more attractive to predatory mites and other biological control agents.     

Learning of Herbivore-Induced and Nonspecific Plant Volatiles by a Parasitoid,Cotesia kariyai

Learning of host-induced plant volatiles by Cotesia kariyai females was examined with synthetic chemicals in a wind tunnel. Wasps were preconditioned by exposure to volatiles and feces simultaneously. A blend of four chemicals, geranyl acetate, -caryophyllene, (E)--farnesene, and indole, which are known to be specifically released from plants infested by host larvae Mythimna separata (host-induced blend), elicited a response in naive C. kariyai, but did not enhance the response after conditioning. A blend of five chemicals, (E)-2-hexenal, (Z)-3-hexen-1-ol, (Z)-3-hexen-1-yl acetate, -myrcene, and linalool, which are known to be released not only from plants infested by the host larvae, but also from artificially damaged plants or undamaged ones (unspecific blend), elicited little response in naive wasps, but significantly enhanced the wasps' response after conditioning. With a blend of the above nine chemicals, wasps could learn the blend at lower concentrations than they did in the nonspecific blend. Hence, both the host-induced and nonspecific volatile compounds appear to be important for C. kariyai females to learn the chemical cues in host location.     

In vivo Volatile Emissions from Peanut Plants Induced by Simultaneous Fungal Infection and Insect Damage

Peanut plants, Arachis hypogaea, infected with white mold, Sclerotium rolfsii, emit a blend of organic compounds that differs both quantitatively and qualitatively from the blend emitted from plants damaged by beet armyworm (BAW; Spodoptera exigua) larvae or from uninfected, undamaged plants. Attack by BAW induced release of lipoxygenase products (hexenols, hexenals, and hexenyl esters), terpenoids, and indole. The plant-derived compound methyl salicylate and the fungal-derived compound 3-octanone were found only in headspace samples from white mold infected plants. White mold-infected plants exposed to BAW damage released all the volatiles emitted by healthy plants fed on by BAW in addition to those emitted in response to white mold infection alone. When BAW larvae were given a choice of feeding on leaves from healthy or white mold-infected plants, they consumed larger quantities of the leaves from infected plants. Exposure to commercially available (Z)-3 hexenyl acetate, linalool, and methyl salicylate, compounds emitted by white mold-infected plants, significantly reduced the growth of the white mold in solid-media cultures. Thus, emission of these compounds by infected plants may constitute a direct defense against this pathogen.     

Electrophysiological and behavioral responses of female African rice gall midge, <Emphasis Type="Italic">Orseolia oryzivora</Emphasis> Harris and Gagné, to host plant volatiles

African rice gall midge, Orseolia oryzivora Harris and Gagné, is a major pest of rice in Africa. Depsite its economic importance, its chemical ecology is not well understood. Here, we assessed behavioral and electrophysiological responses of O. oryzivora to host plant volatiles. In olfactometer bioassays, mated female O. oryzivora were attracted to volatiles emitted from intact rice plants but were repelled by volatiles collected from plants infested by conspecifics. In a choice test, there was a preference for volatiles from uninfested plants over those from infested plants. Coupled gas chromatography-electroantennography analyses of panicle volatiles isolated four electrophysiologically active components: (S)-linalool, 4,8-dimethyl-1,3,7-nonatriene, (E)-caryophyllene, and (R/S)-(E)-nerolidol. A synthetic blend of volatiles at the same concentration and ratio as that from an intact plant was attractive to mated females, whereas a blend based on the ratio of volatiles from an infested plant was repellent. This suggests that O. oryzivora uses olfaction for host plant recognition. The identification of blends of volatiles emitted by plants that can both attract and repel O. oryzivora may aid the development of sustainable control measures.     

Leaf Volatiles from Nonhost Deciduous Trees: Variation by Tree Species, Season and Temperature, and Electrophysiological Activity in Ips typographus

The leaf volatiles emitted from four nonhost tree species of Ips typographus, i.e. Betula pendula, B. pubescens, Populus tremula, and Sambucus nigra, were collected outdoors by headspace sampling in situ and analyzed by GC-MS. Three major classes of compounds, aliphatics [mainly green-leaf volatiles (GLVs)], monoterpenes, and sesquiterpenes, existed in all the deciduous tree species investigated. In June, when the bark beetles are searching in flight for host trees, GLVs mainly consisting of (Z)-3-hexenyl acetate and (Z)-3-hexen-1-ol were the dominant constituents in B. pendula and S. nigra. In B. pubescens and P. tremula, sesquiterpenes (and their derivatives) and monoterpenes made up the major part of whole volatile blends, respectively. Surprisingly, sesquiterpene alcohols and other oxides released from B. pubescens in considerable amounts were not found in the closely related species, B. pendula. By August, both the total volatiles and individual compounds significantly decreased, mainly due to the maturation of leaves, since the light intensity and temperatures during sampling were the same as in June. There were almost no volatiles detected from P. tremula and S. nigra leaves in August. The total emissions from these deciduous species were significantly different among the species, with B. pubescens releasing 5–10 times more than other species. Under the conditions of constant light intensity and humidity, emissions of both total volatiles and most individual components of severed B. pendula and S. nigra branches (with fresh leaves) increased according to a saturation curve from 16°C to 40°C. Ips typographus antennae responded strongly to green leaf alcohols: (Z)-3-hexen-1-ol, 1-hexanol, and (E)-2-hexen-1-ol, but not to aldehydes or acetates in GC-EAD analyses of B. pendula and B. pubescens leaf volatiles. No antennal responses to monoterpenes, sesquiterpenes, or sesquiterpene oxides were found. These three antennally active GLVs emitted from nonhost tree leaves might be indicators of a wrong habitat in the host selection of conifer bark beetles.