GEOGRAPHICAL VARIATION IN RESISTANCE OF THE PARASITOID ASOBARA-TABIDAAGAINST ENCAPSULATION BY DROSOPHILA-MELANOGASTER LARVAE - THE MECHANISM EXPLORED

The braconid parasitoid Asobara tabida Nees attacks larvae of several Drosophila species in fermenting substrates. Northwestern and central European populations of the parasitoid attack mainly D.subobscura Coll in. Southern European parasitoids attack mainly D.melanogaster Meigen. Larvae of this last species can defend themselves against parasitoids by encapsulating the parasitoid egg. Parasitoids from southern Europe an populations are better able to resist encapsulation of their eggs t han their northwestern and central European conspecifics. The eggs of southern European parasitoids appear to have a 'sticky' egg chorion. A s a result of this 'stickiness' the eggs become embedded in host tissu e where they are not completely covered by the host's blood cells. Thi s leads to, at most, partial encapsulation of the egg. Parasitoid larv ae can escape from partially closed capsules.