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.