Intrinsic and extrinsic competitive interactions between two larval parasitoids of Heliothis virescens

1. Competition between parasitoid species may be a key factor in the community dynamics of plant-herbivore-parasitoid systems and is an important consideration in the selection and management of effective biological control agents. 2. Interspecific competition can occur between adult parasitoids searching for hosts (extrinsic competition) and between multiple parasitoid larvae developing within a single host individual (intrinsic competition). A model system comprising the lepidopteran pest Heliothis virescens and two key hymenopteran endoparasitoids, Microplitis croceipes and Cardiochiles nigriceps, was employed to explore parasitoid host-location strategies and the consequences of intrinsic and extrinsic competitive interactions between parasitoid species. 3. The less specialised of the two parasitoids, M. croceipes, was found to have a shorter hatching time and to dominate intrinsic competition, except when its oviposition followed that of the more specialised parasitoid, C. nigriceps, by 16h or more. This interval corresponded to the differential in hatching time between the two species. 4. Cardiochiles nigriceps, however, displayed superior host-searching efficiency that may compensate for its disadvantage in intrinsic competition. This parasitoid was more effective at detecting host infestation sites via airborne odours and at locating and attacking early instar host larvae than was M. croceipes.