Plant volatiles emitted in response to herbivory have been suggested to function as signals to attract natural enemies of herbivores. Most known examples of induced plant volatiles used by natural enemies involve parasitoids that locate caterpillars by means of odours emitted by plants after caterpillar attack. We study the tritrophic system that comprises the parasitoid Cotesia marginiventris, host caterpillars from the genus Spodoptera, and maize plants. Among the volatiles emitted by caterpillar-damaged maize plants, sesquiterpenes and indole are particularly attractive to the parasitoid. The usefulness of these plant volatiles for parasitoids is obvious. Less clear is their benefit to plants that emit them, as in most cases parasitization does not immediately stop caterpillars from damaging plants. However, plants appear to benefit directly from attracting C. marginiventris, as parasitized caterpillars consume considerably less plant tissue than unparasitized caterpillars. It is expected that in systems where parasitoids significantly reduce herbivory, they have contributed to selective pressures that have shaped the phenomenon of herbivore-induced volatile emissions by plants.
