Conclusion
The study of invasion, and of accidental or intentional introductions of species are central to much of contemporary population and community ecology (Davis, 2009). Parasitoids are used as insect biological control agents (Wang, Liu, Shi, Huang, & Chen, 2019). Consequently, they are intentional introduced into both agricultural and natural environments (Grangirard, Hoddle, Petit, Roderick, & Davies, 2009) but often these introductions do not persist in the long term (Goldson et al., 2014), and if they do, their genetic structures might differ (Hufbauer et al., 2004). Brought along during the introduction events, associated symbionts can impact the success story of their host populations, by affecting the phenotypes and genotypes of their hosts (Charlat et al., 2009; Hornett et al., 2006). We use genetic evidence to document the trajectory of a successful introduced parasitoid population over 22 years. We show persistence of the wasp on the previously uninhabited island, and spread of the transplanted genotypes to surrounding island populations, along with the associated symbiotic bacteriumWolbachia. The shuffling of the nuclear and matrilineally inherited markers among islands suggests differential selection for the infection-associated genotypes in the local host genetic pools under relaxed predation pressures in an island where the hyperparasitoid species is absent (van Nouhuys et al., 2016). The establishment of newly introduced parasitoid genotypes might contribute to either the persistence of the isolated island wasp populations, or the replacement of the local wasp populations, or both. It may have also changed environment for the pre-existing nearby isolated butterfly host population, which previously did not have the parasitoid. The Glanville fritillary butterfly is vulnerable to loss of genetic variability and extinction in the Baltic Sea region due to habitat fragmentation, and has gone extinct from the Turku archipelago in southwest Finland (Fountain et al., 2016). The spread of an introduced parasitoid population can represent another pressure for the host population, as well as any competing local parasitoid populations (Benson, Van Driesche, Pasquale, & Elkinton, 2003; Benvenuto, Cheyppe-Buchmann, Bermond, Ris, & Fauvergue, 2012). This is especially true if newly introduced parasitoid genotypes show greater fitness than native genotypes (Dupas, Dubuffet, Carton, & Poirie, 2009; Kraaijeveld, Van Alphen, & Godfray, 1998; van Nouhuys, Niemikapee, & Hanski, 2012).