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).