Is attraction fatal? The effects of herbivore-induced plant volatiles on herbivore parasitism

We investigated the relationship between parasitoid attraction to herbivore-induced plant volatiles and larval parasitism rates of two closely related heliothine; noctuid moths. Heliothis subflexa Guenee is a specialist on plants in the genus Physalis, while Heliothis virescens Fabricius. is an extreme generalist. In North America, these species serve as the only known hosts for the specialist parasitoid Cardiochiles nigriceps Vierick; oviposition into Helicoverpa zea, a non-host, does occur but results in lethal encapsulation, of C. nigriceps' eggs,. Heliothis virescens larvae are parasitised by C. nigriceps far more frequently than are H. subflexa larvae. Parasitoid attraction to volatiles emitted by tobacco in response to herbivory by H. virescens has previously been demonstrated. Using field experiments, we examined the possibility that pre-detection defenses against parasitoid attraction to herbivore-induced plant volatiles are responsible for H. subflexa's relatively low rates of parasitism by C. nigriceps. Herbivore-damaged plants were significantly more attractive to C. nigriceps than were larvae, larval frass, larval saliva, or damaged leaves alone. Plant species affected parasitoid attraction: tobacco was the most preferred plant species, followed by Physalis angulata, and then cotton. The parasitoid was also more attracted to host species (H. subflexa and H. virescens) than to the non-host species, H. zea. There was an interaction between plant species and herbivore species: each plant species was most attractive when infested by its typical herbivore (e.g., H. virescens on tobacco). We compared these data with those of a previously published experiment on field parasitism of H. virescens and H. subflexa, conducted at the same time and place. Our results indicate that differences in parasitoid attraction to herbivore-induced plant volatiles do not contribute to H. subflexa's relatively low parasitism: rate. For the generalist, H. virescens, rates of attraction correspond with rates-of parasitism; for the. specialist, H. subflexa, they do not. Attraction to H: subflexa on P. angulata is greater than to H. virescens on P. angulata, yet parasitism of H. subflexa is much less than parasitism of H. virescens. These results. indicate that pre-detection defenses are not responsible for H. subflexa's low parasitism on P. angulata.