Why is there no interspecific host discrimination in the two coexisting larval parasitoids of Drosophila species; Lepyopilina heterotoma (Thomson) and Asobara tabida (Nees).

(1) The parasitic wasps Leptopilina heterotoma (Thomson) and Asobara tabida (Nees) usually refrain from laying eggs in hosts that have already been parasitized by themselves or by a conspecific (=intraspecific host discrimination). (2) In contrast to intraspecific host discrimination, interspecific host discrimination, in which females refrain from laying eggs in hosts that have already been parasitized by the other species, is not found in these sympatric parasitid species. (3) Because we expected that development of interspecific host discrimination would increase the fitness of the wasps, we wondered why this strategy had not evolved in L. heterotoma and A. tabida. (4) We therefore developed a simulation model of the parasitization process, in which interspecific host discrimination can be included. (5) By varying the time needed for host location, survival chances and the proportions of hosts parasitized, we obtained estimates for the number of offspring in situations with and without interspecific host discrimination. (6) The results imply that, assuming that female wasps carry an ample supply of eggs, the development of interspecific host discrimination by L. heterotoma or A. tabida will not lead to increased fitness, even under extreme circumstances. Hence, interspecific host discrimination will not evolve.