Managed bumble bees acquire parasites from their foraging environment: A case study on parasite spillback

The use of commercially reared bumble bees in agricultural environments has been recognized as a potential threat to wild pollinators due to competition, genetic contamination, and most notably, disease transmission. Higher parasite prevalence near greenhouses where managed bumble bees are used has been linked to parasite spillover from managed to wild bees. However, pathogen transmission is not unidirectional, and can also flow from wild to managed bees. These newly infected managed bees can subsequently re-infect (other) wild bees, in a process known as spillback, which is an alternative explanation for the increased parasite prevalence near greenhouses. Reducing parasite prevalence in managed bees is key to controlling host-parasite dynamics in cases of spillover; in spillback, producing managed bees that are resilient to infection is important. Here we establish that the managed bumble bee Bombus terrestris can acquire parasites from their foraging environment, which is the major infection route for Apicystis spp. and Crithidia spp., but not for Nosema spp.. Managed B. terrestris were found to have a higher prevalence of Crithdia and a higher load of Apicystis than local wild conspecifics, showing that for these parasites, spillback is a possible risk scenario.