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 homogenous 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.