Investigating the impact of deploying commercial Bombus terrestris for crop pollination on pathogen dynamics in wild bumble bees

Abstract

The use of commercial bumble bees for crop pollination has been implicated in the decline of wild bumble bees through the spread of pathogens. This study investigates whether diseases from commercial bumble bees threaten native species in the UK. We sampled bumble bees from ten soft fruit farms: five that deploy commercial Bombus terrestris and five that do not. Each farm was visited monthly throughout the summer and workers of B. terrestris, B. pratorum, B. pascuorum and B. lapidarius were captured. The faeces of these bees were inspected for the gut microparasites Crithidia spp., Nosema bombi and Apicystis bombi. Prevalence was defined as the proportion of individuals infected and abundance was defined as the number of pathogen cells per volume of bumble bee faeces. The prevalence of A. bombi and N. bombi was too low to analyse. The prevalence and abundance of Crithidia spp. was significantly different among bumble bee species. Overall, the prevalence of Crithidia spp. was initially lower on farms deploying commercial bumble bees, possibly due to a dilution effect caused by the high density of imported bees. Crithidia spp. prevalence in Bombus terrestris, however, rose sharply on commercial farms at the end of the season. One potential explanation is that commercial bumble bees contract the local pathogen, which is then rapidly transmitted among them due to the high bee density. Whilst our data provide no evidence of pathogen spillover to wild species, it would be premature to conclude with certainty that commercial colonies do not represent a disease risk to native bees in the UK and we urge further studies into this phenomenon.