GEOGRAPHICAL VARIATION IN RESISTANCE OF THE PARASITOID ASOBARA-TABIDA AGAINST 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 Collin. 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 European populations are better able to resist encapsulation of their eggs than their northwestern and central European conspecifics. The eggs of southern European parasitoids appear to have a 'sticky' egg chorion. As a result of this 'stickiness' the eggs become embedded in host tissue where they are not completely covered by the host's blood cells. This leads to, at most, partial encapsulation of the egg. Parasitoid larvae can escape from partially closed capsules.