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 1a), 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 have been irregularly collected since the 90’s, and systematically collected as part of the annual survey between 2008 and 2014 (Fountain et al., 2016; I. A. Hanski, 2011). A fraction 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, which does not have a direct impact on several individual wasp fitness traits, including dispersal, longevity, and egg production (Duplouy et al., 2015), but which increases the susceptibility of the wasp to its hyperparasitoidMesochorus cf. stigmaticus (van Nouhuys et al., 2016).
Before 1991, 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). The caterpillars introduced on to Sottunga in 1991 originated from the mainland Åland in northern Finström (Hanski pers. Comm.), where the parasitoid was common (G. Lei & Hanski, 1998). Since 1991, the butterfly (Fountain et al., 2016) and the parasitoid wasp populations (Couchoux, Seppa, & van Nouhuys, 2016) have persisted despite going through occasional strong local population bottlenecks (Figure 2). The butterfly and its parasitoid wasps are not classified as endangered or protected and hence no permits are required for their collection in the Åland Islands.
We selected 324 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). 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), 96 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 1). See Table 1 for sample size for each year at the different populations. The chance of collecting full-siblings in this sample is low (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.