Winter activity unrelated to introgression in British bumblebee Bombus terrestris audax

Bombus terrestris is a bumblebee with a wide geographic range, with subspecies showing a variety of local adaptations. Global export of commercially-reared B. terrestris started in the 1980s; the bees are a mixture of subspecies bred for ease of rearing, bivoltinism and large nests. This paper investigated whether the increase in bivoltinism in UK resident B. terrestris audax populations was related to introgression with imported foreign subspecies. Workers were collected from wild populations in London and Bristol, as well as two commercial suppliers. Fourteen microsatellite loci were used to study population structure, hybridisation, and introgression. No introgression with commercial B. t. dalmatinus was detected in wild populations. Hence, the increase in winter activity appears unrelated to introgression.