Bumble bee queen egg laying is facilitated by both social conditions and nest cavity size

Abstract Bumble bees are essential pollinators that have an annual eusocial lifestyle where new cohorts of queens establish colonies each season. The early stages of colony establishment are critical for colony success and are heavily impacted by how readily queens activate their ovaries and initiate egg laying at the start of the season. Understanding the social and nesting factors that influence queen reproduction may provide key insights into early colony establishment. Here, we examine how social conditions and nest cavity size affect the onset of egg laying in early nesting Bombus impatiens queens. In the first experiment, queens were assigned to a social condition where they were placed into nesting boxes that either completely restricted contact with nestmates (and were smaller in size) or allowed direct physical contact and then housed either alone or with nestmates (males or workers). We found that contact with either workers or males accelerated egg laying in queens relative to being housed alone. Unexpectedly, queens housed in the restricted cages, regardless of whether alone or with nestmates, also initiated egg laying earlier. This led us to a second experiment, where we examined ovarian activation and hemolymph juvenile hormone (JH-III) concentrations in early queens housed in different nesting cavity sizes. Here, we found that smaller nest cavity sizes lead to a greater degree of ovarian activation in gynes (young, unmated queens), but juvenile hormone concentrations did not differ. Our findings demonstrate that both social conditions and nest cavity size influence reproductive timing in queens in early-stage bumble bee colonies.