The road to sociality: brood regulation of worker reproduction in the simple eusocial bee Bombus impatiens

Abstract

Eusocial societies, in which egg laying is monopolized by one or a few females, have evolved multiple times during the evolution of insects but were always rooted in a simple family structure. Female reproduction in such families is often characterized by a trade-off between reproduction and brood care, yet most work on the regulation of reproduction in social insects has focused on signals and traits exhibited by adults: particularly on the behaviour and chemical signals produced by the queen and nestmate workers. Here we examined the role of brood in regulating worker reproduction in Bombus impatiens , an annual eusocial species whose reproduction is monopolized by the queen via an unknown mechanism. We found that the presence of young larvae reduced workers' egg laying, whereas the presence of pupae stimulated egg laying. The effect of young larvae was quantity dependent, with nearly complete suppression of egg laying in cages containing a pair of workers and more than 10 young larvae, and replicable regardless of worker age, relatedness to brood or brood parentage/sex. The findings that any larvae can regulate worker reproduction in this simple, yet eusocial, species highlight the role of brood in the evolution of advanced eusocial insects as a mechanism for regulating worker sterility. These findings also provide the first holistic explanation for the regulation of worker reproduction in B. impatiens , suggesting that the queen inhibits worker reproduction through her brood. • Presence of young larvae inhibited worker egg laying in B. impatiens . • Presence of pupae stimulated worker egg laying. • Larval effects were independent of relatedness, brood parentage or its sex. • Larval effect was quantity dependent. • Brood care plays an important role in the evolution of sociality.