Cuticular hydrocarbons as host recognition cues in specialist and generalist endoparasitoids

The chemical composition of cuticular hydrocarbons differs qualitatively and quantitatively among insect species. These differences convey species-specific information about host suitability to foraging parasitoids, enabling them to discriminate between the host and non-host species. Specialist parasitoids that parasitize fewer host species are predicted to use host-specific cues compared to generalists that have evolved to use common cues present in multiple potential host species. Here, we tested the above hypothesis by evaluating two parasitoids with varying levels of host specificity, Microplitis croceipes (specialist) and Cotesia marginiventris (generalist), for their responses to cuticular extracts of three caterpillar species, Chloridea virescens, Helicoverpa zea, and Spodoptera exigua. First, we compared the cuticular profiles of the three caterpillar species and found that C. virescens and H. zea were qualitatively similar in cuticular composition, whereas S. exigua differed qualitatively and quantitatively from the other two. In contact bioassays, both parasitoid species were behaviorally arrested by the host cuticular extracts, with specialist M. croceipes able to discriminate between the cuticular extracts of its host and non-host caterpillar species. Assessment with the coupled gas chromatography-electroantennogram detection (GC-EAD) revealed qualitative and quantitative differences in parasitoids' antennal responses to the components of cuticular extracts of host species, with M. croceipes showing greater antennal response than generalist C. marginiventris. The data implicated 13-methylhentriacontane as a probable host-specific kairomone used by specialist M. croceipes. These results suggest that specialist parasitoids like M. croceipes may exploit the differences in the composition of cuticular chemicals of caterpillars as cues for host recognition and discrimination.