Acknowledgements:
We thank C. Couchoux for advice on the genotyping work, S. Ikonen for rearing the insects, and S. Ojanen and many students from the University of Helsinki, who conducted the annual survey and collected the samples. We are grateful to I. Hanski and P. Seppä for their interest in and constructive discussions about the study. This work was supported by the Academy of Finland (grant #266021 to AD, and #255553 to SvN).