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.