No abstract

In choice bioassays, Rhopalosiphum padi L. nonviruliferous apterae preferentially locate near volatile organic compounds (VOCs) emitted from Barley yellow dwarf virus (BYDV)-infected wheat plants compared with VOCs from noninfected plants. However, the specific VOCs responsible for R. padi responses are unknown. It is unclear also if R. padi responses to BYDV-infected wheat are caused by arrestment or attraction. Additionally, the responses of viruliferous apterae and nonviruliferous alate to BYDV-infected wheat have not been examined. R. padi responses were studied through emigration, immigration, and settling laboratory bioassays using BYDV-infected and noninfected wheat plants. Two wheat genotypes, virus-susceptible Lambert and virus-resistant Lambert-derived transgenic 103.1J expressing the BYDV-PAV coat protein gene, were evaluated. In a settling bioassay, alates preferentially settled on noninfected 103.1J. Responses of viruliferous and nonviruliferous R. padi to virus-infected, noninfected, and sham-inoculated (exposed to nonviruliferous aphids) Lambert and 103.1J were examined in separate bioassays. A paper leaf model served as a control. Immigration by viruliferous apterae was significantly lower toward the paper leaf model, but no significant differences were observed among plant treatments. Nonviruliferous apterae exhibited no significant differences in emigration among treatments, suggesting no arrestment occurred. Nonviruliferous apterae significantly preferred to immigrate toward BYDV-infected Lambert. Immigration toward the paper leaf model was significantly lower compared with plant treatments. Responses of R. padi to VOCs were tested by applying compounds to paper leaf models at concentrations designed to mimic those present in headspace of wheat plants. Nonviruliferous apterae immigrated in significantly greater numbers toward paper leaf models individually treated with nonanal, (Z)-3-hexenyl acetate, decanal, caryophyllene, and undecane than toward paper leaf models that served as controls and toward leaf models treated with synthetic blends made to mimic headspace of BYDV-infected compared with blends made to mimic headspace of noninfected wheat plants. Results suggest responses of R. padi to BYDV-infected plants are caused by attraction rather than arrestment.

Volatiles from potato plants (Solanum tuberosum L.) infected with Potato leaf roll virus (PLRV) attract and arrest the principal vector of PLRV, the green peach aphid, Myzus persicae (Sulzer), more strongly than volatiles from non-infected plants. The total concentration of volatiles detectable in the headspace of PLRV-infected plants is greater than that in the headspace of non-infected controls, and the relative composition is altered. To determine the basis of the aphid response to PLRV-infection-induced volatiles from potato, behavioral bioassays were conducted. We measured arrestment of aphids by individual components, by synthetic blends of these, and by a naturally occurring blend by using an emigration rate bioassay, and quantified observations of the behavior of individual aphids. The components tested were those elevated at least twofold in response to PLRV infection. Before conducting the behavioral bioassays, electroantennograms confirmed the electrophysiological responses of aphids to the components of the blend. For bioassays, individual compounds or blends were tested by applying them in solution to paper strips at concentrations designed to mimic those present in the headspace of the plants. All bioassays were conducted by placing aphids on fine-mesh screening positioned above treated paper strips. Arrestment was measured by placing groups of 30 aphids directly over the treated strips and counting the number moving away at 10-min intervals for 50 min. Among the individual compounds tested, only beta-pinene was a mild arrestant. The other compounds did not elicit significant changes in arrestment or behavior at a range of physiologically relevant concentrations. In contrast, synthetic blends that mimicked the concentration and composition present in headspace of PLRV-infected potato plants arrested aphids significantly more strongly than blends mimicking volatiles from the headspace of non-infected plants. The naturally occurring blend collected from headspace of PLRV-infected plants also arrested M. persicae more strongly than the blend collected from headspace of non-infected plants. Aphid behavior was quantified by directly observing individual aphids and recording their activities during a 5-min period on screening above strips treated with test materials. Few differences in time budgets were observed among aphids exposed to individual components, but synthetic blends and trapped headspace volatiles from PLRV-infected plants resulted in significantly less time spent walking by aphids than synthetic blends and trapped headspace from non-infected controls. Our results indicate that arrestment of M. persicae by PLRV-infected plants requires the blend of volatile organic compounds released by these plants and is not produced in response to a single compound.

Plants emit volatile blends that may be quantitatively and/or qualitatively different in response to attack by different herbivores. These differences may convey herbivore-specific information to parasitoids, and are predicted to play a role in mediating host specificity in specialist parasitoids. Here, we tested the above prediction by using as models two parasitoids (Hymenoptera: Braconidae) of cotton caterpillars with different degree of host specificity: Microplitis croceipes, a specialist parasitoid of Heliothis spp., and Cotesia marginiventris, a generalist parasitoid of caterpillars of several genera including Heliothis spp. and Spodoptera spp. We compared GC-EAD (coupled gas chromatography electroantennogram detection) responses of both parasitoid species to headspace volatiles of cotton plants damaged by H. virescens (a host species for both parasitoids) vs. S. exigua (a host species for C. marginiventris). Based on a recent study in which we reported differences in the EAG responses of both parasitoids to different types of host related volatiles, we hypothesized that M. croceipes (specialist) would show relatively greater GC-EAD responses to the herbivore-induced plant volatile (HIPV) components of cotton headspace, whereas C. marginiventris (generalist) would show greater response to the green leaf volatile (GLV) components. Thirty volatile components were emitted by cotton plants in response to feeding by either of the two caterpillars, however, 18 components were significantly elevated in the headspace of H. virescens damaged plants. Sixteen consistently elicited GC-EAD responses in both parasitoids. As predicted, C. marginiventris showed significantly greater GC-EAD responses than M. croceipes to most GLV components, whereas several HIPV components elicited comparatively greater responses in M. croceipes. These results suggest that differences in the ratios of identical volatile compounds between similar volatile blends may be used by specialist parasitoids to discriminate between host-plant and non-host-plant complexes.