Using two closely related moths, Heliothis subflexa (HS), a specialist, and Heliothis virescens (HV), a generalist, we investigated the importance of host specialization in determining larval parasitism rates. HS feeds only on plants in the Genus Physalis, while HV has extremely generalized feeding habits. In North America, these species serve as the only known hosts for the parasitoid Cardiochiles nigriceps Vierick (CN). HS uses the calyx of Physalis as enemy free space, and has much lower parasitism rates than HV. We explored the impact of (1) parasitoid attraction to herbivore-induced plant volatiles (HIPVs), and (2) specialized behaviors for the use of enemy free space on larval parasitism. We found that both larval behavior and parasitoid attraction to HIPVs affect larval parasitism rates on Physalis. Substantial differences exist between the larval behavior of the specialist and the generalist, such that HS larvae are more efficient than HV in using Physalis as a refuge. We tested the importance of these behaviors in the field by comparing rates of attraction and parasitism by CN. CN displayed no preference between HV and HS Physalis. Within herbivore-plant combinations, plants infested with the typical species of herbivore were most attractive (HV on tobacco > HS on tobacco; HS on Physalis > HV on Physalis). Rates of parasitism, however, did not depend solely on parasitoid attraction. While HV on tobacco was most attractive (mean visits/2hr ± SEM=43 ± 7) and most parasitised (43 ± 4%), HS on Physalis, the second most attractive combination (24 ± 4), had the lowest parasitism (1 ± 4%). HV on Physalis, the least attractive combination (15 ± 3), had 13 ± 4% parasitism. We attribute this incongruity to specialized larval behaviors for the exploitation of Physalis as enemy-free space.
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA