Interspecific competition between two endoparasitoids Cotesia vestalis (Hymenoptera: Braconidae) and Oomyzus sokolowskii (Hymenoptera: Eulophidae).

Two endoparasitoids, Cotesia vestalis and Oomyzus sokolowskii, parasitize the same host, larvae of Plutella xylostella. These two species have evolved different parasitization strategies. O. sokolowskii expresses a single factor, venom, and exerts virtually no detrimental effects on the development of its host. C. vestalis, on the other hand, injects polydnavirus (PDV) and venom during oviposition, and teratocytes are released into the host's hemolymph after egg hatching. Parasitization suppresses host immune reactions and redirects its developmental program. Because both these species parasitize the same stage of their hosts, there is the possibility of multiparasitism in nature. Only one species survives multiparasitism and because of its parasitic strategy, we hypothesized that C. vestalis would invariably be the stronger competitor. We designed competition experiments which revealed that C. vestalis is a stronger competitor than O. sokolowskii. We also show that C. vestalis survives intrinsic competition with O. sokolowskii through two mechanisms: physical attack and physiological suppression. We discovered melanized wounds on O. sokolowskii eggs and larvae, which is strong evidence of physical attacks. The physiological suppression is due to PDV and venom injected by C. vestalis. To test this idea more rigorously, we designed a pseudoparasitization experiment which revealed that no O. sokolowskii emerged from multiparasitized hosts when infertile C. vestlais eggs and normal O. sokolowskii larvae are both present inside the same host. These results support our hypothesis that C. vestalis is the stronger competitor and demonstrate two mechanisms that account for the outcome of intrinsic competition between these two endoparasitoids.