We investigated the relationship between parasitoid attraction to herbivore-induced plant volatiles and larval parasitism rates of two closely related moths. Heliothis subflexa specialises on plants in the genus Physalis, while Heliothis virescens is an extreme generalist. In North America, these species are the only known hosts for the specialist parasitoid Cardiochiles nigriceps. Heliothis virescens larvae are parasitised by C. nigriceps far more frequently than H. subflexa larvae. Using field experiments, we examined whether pre-detection defenses against parasitoid attraction to herbivore-induced plant volatiles are responsible for H. subflexa's relatively low parasitism.
Herbivore damaged plants were more attractive to C. nigriceps than larvae, larval products, or damaged leaves. Tobacco was the most preferred plant species, followed by Physalis angulata and cotton. Parasitoids were more attracted to host species than to a non-host species, Helicoverpa zea. Plant species interacted with herbivore species: each plants were most attractive when infested their typical herbivore. We compared these data with a previous experiment on field parasitism. We found that differences in attraction to herbivore-induced volatiles do not explain H. subflexa's low parasitism. For H. virescens, rates of attraction correspond with rates of parasitism, but for the specialist, H. subflexa, they do not. Attraction to H. subflexa on P. angulata exceeds attraction to H. virescens on P. angulata, yet parasitism of H. subflexa is lower than parasitism of H. virescens. These results indicate that pre-detection defenses are not responsible for H. subflexa’s low parasitism on P. angulata.
Species 1: Lepidoptera Noctuidae Heliothis subflexa (Physalis lanternworm)
Species 2: Lepidoptera Noctuidae Heliothis virescens (tobacco budworm)
Species 3: Hymenoptera Braconidae Cardiochiles nigriceps (redtailed wormstinger)
Keywords: Herbivore-induced plant volatiles, herbivore host range evolution
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