The influence of the social environment on larval development and resulting caste in Bombus impatiens

The ability of a single genome to express multiple phenotypes is key to understanding social systems. In social insects, caste and task-specific morphologies are typically determined by the social environment during larval development. Here, we used the bumble bee Bombus impatiens to examine the social regulation of body size and caste by manipulating the identity (queen/worker) and the number of caretakers tending for the brood. Eggs of females and males were kept in cages with (1) a queen, (2) one worker, (3) three workers or (4) 10 workers. We measured larval weight, developmental duration and the resulting caste (in females) throughout the development of >2000 individuals. We found differences in larval mass when reared by varying number of workers, but not when reared by a single worker compared to a queen. Additionally, in contrast with previous findings in Bombus terrestris, none of the female eggs reared by workers developed into queens, indicating the existence of multiple mechanisms for caste determination in bumble bees. Adult males were slightly larger than workers and took longer to develop in the presence of the queen. The hypothesis that the queen's presence manipulates female development and caste was not supported by our data. Overall, we suggest that body size in B. impatiens is determined by the number of caretakers and is likely mediated by the amount of provision.