Impact of chronic exposure to a pyrethroid pesticide on bumblebees and interactions with a trypanosome parasite

1. Bees are exposed to pesticides when foraging in agricultural areas and growing evidence suggests that such compounds can be harmful to managed and wild populations. Given the economic and ecological importance of bees, and the evidence of widespread population declines, the full impacts of pesticides and their interactions with other stressors in the environment need to be investigated. 2. Here, we focus on the impacts of chronic exposure to the commonly used pyrethroid pesticide lambda (k)-cyhalothrin on the bumblebee Bombus terrestris at both the individual and colony level. Furthermore, we investigated the interactions of pesticide exposure with a highly prevalent trypanosome parasite Crithidia bombi. Colonies were exposed to k-cyhalothrin in the laboratory, and colony growth and reproductive output were monitored for up to 14 weeks. The potential interactions between the pesticide and C. bombi were investigated by quantifying the impact of pesticide treatment on susceptibility to, and success of experimental infections, as well as the survival of workers. Male survival after larval pesticide exposure was also monitored. 3. Pesticide-treated colonies produced workers with a significantly lower body mass. However, out of the twelve variables of colony development measured, this was the only metric that was significantly affected by pesticide treatment and there was no subsequent significant impact on the reproductive output of colonies. 4. Lambda-cyhalothrin had no significant impact on the susceptibility of workers to C. bombi, or intensity of parasitic infection. 5. Pesticide exposure did not cause differential survival in workers or males, even when workers were additionally challenged with C. bombi. 6. Synthesis and applications. Chronic exposure to k-cyhalothrin has a significant impact on worker size, a key aspect of bumblebee colony function, particularly under conditions of limited food resources. This could indicate that under times of resource limitation, colonies exposed to this pesticide in the field may fail. However, the lack of other impacts found in this study indicate that further field trials are needed to elucidate this.