Microsatellite genotyping
After filtering to include only individuals with genotype scores from ≥ 20 microsatellite loci, 1,588 samples were retained for subsequent analyses. When the North American samples were analyzed using the 12 polymorphic microsatellite loci that co-amplify between Bruce spanworm and winter moth, four individuals were classified as being Bruce spanworm and four individuals were identified as probable hybrids. The final dataset therefore included 1,580 pure winter moth individuals.