Sexual production is regulated by the brood in a social bee (Bombus impatiens)

The production of sexuals in social insects often marks the climax of colony development and the ultimate purpose for building the worker force. However, the mechanisms regulating this process are largely unknown. Here we examined the role of the brood in regulating colony development and sexual production using the bumble bee Bombus impatiens. Previous studies in this species demonstrate that the presence of larvae reduces worker egg laying and enhances the inhibitory effect of the queen. However, these studies were conducted using small groups, and the effect of the brood on colony-level events, such as the onset of worker reproduction and sexual production, remained unexplored. To examine these effects, we doubled or removed the brood in full-size, young colonies at the onset of the experiment and monitored colony development thereafter. We show that double-brood colonies produced significantly more gynes, whereas colonies with a reduced amount of brood produced significantly more males. Additionally, worker reproduction started sooner in colonies with a reduced amount of brood and was delayed in double-brood colonies, while aggression overall was the highest in double-brood colonies. Overall, our findings indicate that the brood has significant impacts on colony development and sociobiology. The mechanisms determining female caste development in social species are still mostly unknown and the brood may be key to understanding how demographical changes in colony development shape social behavior. The variation in the amount of brood may also explain the various reproductive strategies across bumble bee colonies and other social species.