SUCCESSFUL INVASION OF NORTH-AMERICA BY 2 PALEARCTIC DROSOPHILA SPECIES (DIPTERA, DROSOPHILIDAE) - A MATTER OF IMMUNITY TO LOCAL PARASITOIDS

The palearctic Drosophila subobscura Collin and D. ambigua Pomini rece ntly invaded western North America. One possible explanation for their success is that they have a competitive edge over the native species by being immune to local parasitoids. The braconid Asobara tabida Nees , the dominant parasitoid of larvae of the obscura-group in western No rth America, is used to test this hypothesis. Survival of the parasito id is as high in D. subobscura and D. ambigua as in two native species , D. pseudoobscura Frolova and D. athabasca Sturtevant & Dobzhansky. N either of the four species can substantially survive a parasitoid atta ck. Parasitoids emerging from the palearctic species do not differ in body mass and development time from parasitoids emerging from the near ctic species. It is concluded that the success of the two palearctic D rosophila species in North America is not caused by immunity to local parasitoids.