How Population Structure and Nest Membership Shape Pathogen Patterns in Bumble Bees

Host density, genetic diversity and social groups are key factors influencing pathogen transmission in wildlife populations, but their interactions remain poorly understood in insects. Islands can provide natural laboratories with distinct populations that vary in density and genetic diversity, whereby dense, genetically homogeneous populations are expected to facilitate pathogen transmission. We used bumble bees to test these predictions, assessing the population structure of the two common species Bombus pascuorum and B. terrestris across island and mainland sites in the British Isles and France and testing bees for five micro-parasitic and four viral pathogens. B. pascuorum formed distinct genetic clusters on islands, with varying levels of heterozygosity and only the Isle of Arran clustered with mainland populations. B. terrestris populations were less structured, but populations on the Isle of Man and the Scilly Isles were genetically separated from other island and mainland populations while showing low heterozygosity. Colony density was similar between species and not linked to genetic diversity but had a positive effect on the prevalence of some pathogens. Contrary to expectations, there was no protective effect of high genetic diversity, suggesting that generalist bumble bee pathogens could be more affected by host species diversity and density. Yet, within B. terrestris populations, we found that nestmates showed more similar pathogen profiles than unrelated individuals, suggesting that genetic similarity and high contact rates within nests affect pathogen prevalence in wild bees.