Behavioral responses of the parasitoids, Microplitis croceipes (a specialist) and Cotesia marginiventris (a generalist) to single compounds and blends of host related plant volatiles

The responses of both sexes of a specialist parasitoid, Microplitis croceipes (Cresson) and a generalist parasitoid, Cotesia marginiventris (Cresson) to different types of plant volatile organic compounds (VOCs) (single compounds and blends) were compared in four-choice olfactometer bioassays. Both wasp species belong to the family Braconidae and are larval endo-parasitoids of important cotton pests such as Heliothis virescens and Spodoptera exigua. When attacked by such herbivores, plants release a plethora of VOCs which can be exploited by parasitoids for host location. In the present study, we grouped these VOCs broadly as those released passively from undamaged plants (UD-VOC) and herbivore-induced plant volatiles (HIPVs). HIPVs could be further categorized into: i) volatiles released by freshly damaged plants (FD-VOC), and ii) volatiles released by old damaged plants (OD-VOC). Pure synthetic versions of select VOCs were purchased and used in our bioassays. Our main objective was to know if the degree of host specificity affects the use of plant volatiles for host location by parasitoids. Female M. croceipes (specialist) were significantly (χ2= 6.72; P < 0.0096) more attracted than female C. marginiventris (generalist) to FD-VOC. In contrast, female C. marginiventris showed significantly greater (χ2 =13.50; P < 0.0002) attraction to UD-VOC. This suggests that the olfactory system of the specialist can better distinguish host-damaged plant volatiles from undamaged/passively released plant volatiles. Sexual similarities and differences in response to single and blends of VOCs were recorded. In general, M. croceipes showed significantly greater attraction to the two-component blends than to single compounds, while C. marginiventris did not show preference between single compounds and mixtures. The ecological significance and practical implications of these results are discussed.