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

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Abstract
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.

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