Parental care behaviour in bumble bee queens is tightly regulated by the number of helpers in incipient nests

Parents that provide parental care must balance the conservation of resources with the benefits of expenditure in support of existing offspring. There is added complexity in social systems in which siblings also contribute to brood care, wherein dynamic social interactions exist, and brood care can influence group fitness. Most bumble bees are annually eusocial insects in which queens undergo a stark transition from living a solitary, to subsocial, to eusocial lifestyle over the course of several weeks as they initiate new nests. We used detailed behavioural analyses of queen bumble bee (Bombus impatiens) parental care behaviour to examine how performance of five parental care-related tasks differs when queens are living in either subsocial conditions (with brood but no adult workers) or as part of a eusocial group with either three or five workers. These three groups reflect the ontogeny of sociality across the early nesting stage. We show for the first time that queen bumble bee parental care behaviour is tightly regulated by the number of helpers in the nest. With three workers in the nest, queens exhibited ∼60% reduction in brood feeding and ∼30% reduction in incubation but no reduction in food collection. However, the addition of just two additional workers resulted in nearly complete cessation of pollen and nectar collection behaviours, whereas in-nest parental care behaviours persisted. We propose that this sequence of behavioural changes might reflect queen avoidance of threats such as predators, parasites or harsh weather. Queens may cease performing tasks outside the nest early in colony development, but only when a threshold number of helpers in the nest is met. More broadly, our results suggest that bumble bee queen parental care behaviour is heavily impacted by the number of helpers in the nest, who may alleviate some of the constraints queens face during nest initiation.