Age, not body size, influences flight distance in the bumble bee Bombus terrestris L. (Hymenoptera: Apidae)

Bumble bees and honey bees provide crucial pollination service and have become important insect model system. Despite their close relation, they differ in their morphology, physiology, and social organisation. Notably, honey bees workers exhibit age-based polyethism, while bumble bees display considerable body size variation. Though body size is known to affect foraging range, behaviour, and flight ability, the influence of age remains less explored. Here we studied the flight performance (distance and speed) in aging bumble bee workers using tethered flight mills. Additionally, we measured their intertegular distance (ITD), dry mass, and fat content. Flight distance was predominantly influenced by age, challenging assumptions that age does not play a role in foraging and task allocation. Between 7 to 14 days, flight distance increased six-fold before a slight decline at the age of 21 days, aligning with age-dependent flight physiology in honey bees. Larger workers had decreasing mass-specific fat reserves, challenging the notion about their energy preservation for oocyte development. Our findings indicate that age substantially influences flight distance, impacting foraging performance and potentially altering task allocation strategies. This underscores the need to consider individual age and physiological changes alongside body size and mass in experiments involving bumble bee workers.