Chemical-mediated counter defense: attraction of two parasitoid species to the defensive secretion of host larvae

Abstract

Carnivorous animals put selection pressure on their herbivorous preys, leading to the development of defensive traits in prey species and counter defensive traits in carnivores—an evolutionary arms race for survival. For instance, larvae of the moth Heliothis virescens use defensive oral secretions to deter attacking parasitic wasps (parasitoids). However, it is not clear whether parasitoids also use the same secretion for host location. If carnivores follow the herbivore model, evolution should favor specialist over generalist parasitoid species in their ability to manipulate host defensive chemicals. To test this prediction, Microplitis croceipes and Cotesia marginiventris, both larval endoparasitoids of H. virescens, were used as a study system. Microplitis croceipes is relatively specialized on Heliothis/Helicoverpa species while C. marginiventris is a generalist parasitoid on several noctuid host species. Innate behavioral responses of parasitoids to host defensive secretion were tested in Petri dish arena and Y-tube olfactometer bioassays. Heliothis virescens defensive secretion was manually applied to a spot in a sector (secretion zone) of the arena. Tracking data suggests that both parasitoid species are likely to locate the secretion spot once they enter the secretion zone. Comparing the two species, however, M. croceipes (specialist) spent significantly more time in the secretion zone and spot than C. marginiventris (generalist). In Y-tube olfactometer bioassays, the secretion elicited a strong attraction in M. croceipes but not in C. marginiventris, supporting our prediction. We discussed chemical-mediated counter defense in natural enemies of pest insects and the significance of the results to parasitoid-host coevolution.