Local adaptation and evolution of parasitoid interactions in an invasive species, Drosophila subobscura

Background: About 30 years ago, the Palaearctic fly Drosophila subobscura successfully invaded the New World, where parasitoid species diversity was thought to be lower than in the Old World. Hypotheses: Because parasitoids cause major mortality to Drosophila, the invader should benefit from escaping its natural parasitoid enemies and enjoy lower rates of parasitism than in the Old World. Also, if co-evolutionary selection on parasitoids promotes their adaptation to local fly stocks, parasitoids should have enhanced fitness when reared on local flies rather than allopatric ones. Methods: We collected flies and parasitoids from Igé (France) and from near Seattle (USA). In factorial laboratory experiments, we exposed D. subobscura larvae from both sites to parasitoids (Leptopilina heterotoma) from both sites and then scored parasitoid success rate, impact rate, and fecundity. Results: Despite the generally held belief, the parasitoid community in Seattle is the same as that in Igé and not depauperate. Success rate (probability that an infested fly gave rise to an adult wasp) was high (0.83–0.86) and independent of treatment, showing that invasive and native flies were equally vulnerable to both populations of parasitoids. Seattle flies were larger than Igé flies, and parasitoids emerging from Seattle flies were larger than those emerging from Igé flies. Fecundity of parasitoids reared on Seattle flies was greater than those reared on Igé flies, especially when parasitoids were also from Seattle. Overall, the success of D. subobscura in the New World appears to be unrelated to biogeographical escape from parasitoids because these flies have very high mortality in the presence of local L. heterotoma, at least in the laboratory. Yet L. heterotoma does seem to be locally adapted, having higher fecundity when reared on local flies.