Sexual production is regulated by the brood in Bombus impatiens colonies

Sexual production in social insects marks the peak of colony development, yet the mechanisms regulating it remain unclear. We investigated the role of brood in colony development, worker reproduction, and sexual production in Bombus impatiens. While larvae are known to reduce worker egg-laying and enhance the queens reproductive inhibition, these effects were previously tested only in small groups. We manipulated brood size in full-sized, young colonies by doubling or removing brood and monitored development. Colonies with doubled brood produced significantly more gynes, independent of the number of workers, while reduced-brood colonies exhibited a non-significant increase in male production that was driven by colony size. Worker ovary activation was lower in double-brood colonies, with no change in aggression. A follow-up experiment directly testing the effect of colony size showed that higher worker density led to higher ovary activation in workers but did not affect sexual production. These results suggest that brood strongly influences colony development and sexual production, possibly reflecting an extended phenotype of the queen, whereas worker ovary activation appears to be a more flexible process influenced by either brood presence or colony size. Understanding brood dynamics may be key to understanding the evolution of female castes in social insects.