Variation in the specificity of plant volatiles and their use by a specialist and a generalist parasitoid

Herbivore-induced plant volatiles (HIPV) provide important information that influences host location behaviour for insect natural enemies, such as parasitoid wasps, that develop in the bodies of herbivorous insects. The dietary breadth of both the parasitoid and its host may affect the extent to which a searching parasitoid relies on HIPV. Specialist species are expected to rely on specific volatile cues to which they respond innately, whereas generalists are expected to show a higher degree of phenotypic plasticity that depends on foraging experience in the parasitoid. We compared the response to HIPV emitted by different plant species damaged by host and nonhost caterpillars for two congeneric parasitoid species, the specialist Diadegma semiclausum and the generalist Diadegma fenestrale, attacking caterpillars of the diamondback moth, Plutella xylostella. For the three tested plant species, Brassica oleracea, a feral Brassica population and Sinapis alba, both parasitoid species preferred volatiles from host-infested plants over those produced by undamaged plants. However, both parasitoid species only distinguished between volatiles induced by host and nonhosts when the caterpillars had been feeding on B. oleracea, the plant on which they had been reared. Chemical analysis of the volatile blends could not explain volatile preferences of the parasitoids. Despite the difference in their dietary breadth, the two parasitoids responded similarly to HIPV and experience treatments. A flexible response to a wide array of volatile blends by parasitoids is probably important in nature, given that different generations of the host and the parasitoid probably develop on different food plants.