Bumblebee infestation status and interspecific competition between bumblebees and honeybees increase with proximity to honeybee hives

Bumblebees are key pollinators in temperate regions of the northern hemisphere. Niche overlap and competition between bumblebees and honeybees may represent a significant stressor to bumblebees by reducing nutrient acquisition and facilitating spillover of pathogens and parasites from honeybees. One of these parasites is the bumblebee wax moth (Aphomia sociella), whose larvae can be found in bumblebee and wasp nests, as well as weakened commercial honeybee hives. Using Bombus terrestris colonies as experimental models, we expected that young queen and worker bumblebee immunity (measured by encapsulation response) would become weaker under both competitive (i.e., proximity to honeybee hives) and parasite (A. sociella infestation) pressure and, specifically, that the immunity of bumblebees in closest proximity to honeybee hives would be weakest in nests infested by A. sociella. We observed increased infestation, lower reproductive output, and weaker encapsulation response in bumblebee colonies that were near honeybee hives. Our data provide insights on the ecology of A. sociella infestations where honeybees and bumblebees coexist. Our observations of reduced immune response in bumblebees inhabiting colonies nearer to honeybee hives are of critical importance, as this reduction in immune response could facilitate additional infestations of other parasites and pathogens within bumblebee colonies.