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

1. Competition between parasitoid species may be a key factor in the commun ity dynamics of plant-herbivore-parasitoid systems and is an important cons ideration in the selection and management of effective biological control a gents. 2. Interspecific competition can occur between adult parasitoids searching fur hosts (extrinsic competition) and between multiple parasitoid larvae de veloping within a single host individual (intrinsic competition). A model s ystem comprising the lepidopteran pest Heliothis virescens and two key hyme nopteran endoparasitoids, Microplitis craceipes and Cardiochiles nigriceps, was employed to explore parasitoid host-location strategies and the conseq uences of intrinsic and extrinsic competitive interactions between parasito id 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. n igriceps, 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 effic iency that may compensate for its disadvantage in intrinsic competition. Th is parasitoid was more effective at detecting host infestation sites via ai rborne odours and at locating and attacking early instar host larvae than w as M. craceipes.