Insect material
The entire metapopulation of the Glanville fritillary butterfly (Melitaea cinxia , L.) in the Åland archipelago, Southwest Finland (60°13’N 19°55’E, Figure 1), has been surveyed for more than two decades as part of a long-term ecological study of its metapopulation dynamics (Ojanen, Nieminen, Meyke, Poyry, & Hanski, 2013). Caterpillars and the larval parasitoids in them were collected occasionally from each of the regions in the 1990’s, and early 2000’s, and were systematically collected as part of the annual survey starting in 2008 (Fountain et al., 2016; I. A. Hanski, 2011). About a third of the field collected caterpillars are naturally parasitized by the solitary endoparasitoid wasp H. horticola, some of which in turn are hyperparasitized byM. stigmaticus (Montovan et al., 2015; Shaw et al., 2009; van Nouhuys & Hanski, 2005). About half the H. horticola in Åland are infected by Wolbachia (Duplouy et al., 2015; van Nouhuys et al., 2016).
Before 1991 (Figure 1c), the Glanville fritillary butterfly, the parasitoid and the hyperparasitoid did not inhabit Sottunga, but the butterfly at least was known to inhabit nearby islands of Föglö, Seglinge and Kumlinge (I. Hanski et al., 2004; Murphy, Wahlberg, Hanski, & Ehrlich, 2004) (G. Lei & Hanski, 1998). Since 1991, the butterfly (Fountain et al., 2016) the parasitoid (Couchoux, Seppa, & van Nouhuys, 2016), and the hyperparasitoid populations (Nair et al., 2016) have persisted despite going through occasional strong local population bottlenecks (Figure 2).
We selected 323 H. horticola parasitoid individuals, including both males and females, reared from different butterfly host nests sampled from five localities in the Åland archipelago between 1992 and 2013 (Ojanen et al., 2013). The butterfly and its parasitoids are not classified as endangered or protected and hence no permits are required for their collection in the Åland Islands. In total, we used 40 wasps from the island of Sottunga (60°07’N 20°40’E), 43 from the northern islands of Föglö (60°03’N 20°32’E, in areas called Jyddö, Nötö, and Överö), 44 from the closely adjacent islands of Seglinge-Kumlinge (60°14’N 20°46’E, 14 samples from Kumlinge, others from Seglinge), 95 from northern Finström (60°32’N 19°95’E) and 101 from Saltvik (60°16’N 20°03’E) on the main Åland island (Figure 1a). See Table 1 for sample size for each year for different populations. The chance of collecting full-siblings in this sample is low, as a previous study designed to detect siblings using samples from the same collections found very low incidence of siblings outside of those within a gregarious host group (Couchoux, Seppä, & van Nouhuys, 2015a). The coast-to-coast distances between Sottunga and Seglinge, and between Sottunga and Föglö, are of 8.5km and 6.5km, respectively, while the closest distance between the two suitable habitat patches on different islands is 12km and 13km, respectively. The distance from a mainland area to a suitable patch on Sottunga is about 30km.