Discussion
The introduction and long-term persistence of the parasitoid waspH. horticola in the small island of Sottunga, on the East side of the Ă…land archipelago, offered a unique opportunity to investigate the spatio-temporal changes in the genetic structure, spread, and dynamics of a parasitoid species introduced into a new habitat along with its host butterfly. Because the parasitoid carried along itsWolbachia symbiont, we could also investigate how symbionts might affect the spread of different host genotypes after introduction. Our results suggest that the H. horticola population that established in Sottunga persisted despite strong bottlenecks during population fluctuations across its 22 years of persistence, and potentially in the founding generation. Additionally, the introduction might have affected the genetic pool of the population inhabiting the nearby islands through dispersal of individuals carrying the mainland genotypes, and by the establishment of populations carrying a costly Wolbachia strain.