INTERSPECIFIC COMPETITIONBETWEEN TWOENDOPARASITOIDS Cotesiavestalis (HYMENOPTERA:BRACONIDAE) ANDOomyzus sokolowskii(HYMENOPTERA: EULOPHIDAE)

摘要

Two endoparasitoids, Cotesia vestalis and Oomyzus sokolowskii, parasitizethe same host, larvae of Plutella xylostella. These two species have evolveddifferent parasitization strategies. O. sokolowskii expresses a single factor,venom, and exerts virtually no detrimental effects on the development of itshost. C. vestalis, on the other hand, injects polydnavirus (PDV) and venomduring oviposition, and teratocytes are released into the host’s hemolymphafter egg hatching. Parasitization suppresses host immune reactions andredirects its developmental program. Because both these species parasitize the same stage of their hosts, there is the possibility of multiparasitism innature. Only one species survives multiparasitism and because of itsparasitic strategy, we hypothesized that C. vestalis would invariably be thestronger competitor. We designed competition experiments which revealedthat C. vestalis is a stronger competitor than O. sokolowskii. We also showthat C. vestalis survives intrinsic competition with O. sokolowskii throughtwo mechanisms: physical attack and physiological suppression. Wediscovered melanized wounds on O. sokolowskii eggs and larvae, which isstrong evidence of physical attacks. The physiological suppression is due toPDV and venom injected by C. vestalis. To test this idea more rigorously,we designed a pseudoparasitization experiment which revealed that noO. sokolowskii emerged from multiparasitized hosts when infertile C. vestlaiseggs and normal O. sokolowskii larvae are both present inside the same host.These results support our hypothesis that C. vestalis is the strongercompetitor and demonstrate two mechanisms that account for the outcome ofintrinsic competition between these two endoparasitoids.