Age-dependent shift from high lipid reserves to high haemolymph sugar supply underlies worker maturation in Bombus terrestris (Hymenoptera: Apidae)

Division of labour is key to the success of eusocial insects, including ants, termites, social wasps, and bees. While the well-studied honeybees are the primary example of age-based polyethism, the physiological transitions and underlying mechanisms mediating task allocation remain poorly understood in its sister taxon, the bumblebees. To gain a better understanding of the energy homeostasis in bumblebees carrying out different tasks, we quantified the haemolymph sugar titres (fructose, glucose, and trehalose) and lipid reserves across five age classes (0-28 days) and varying body sizes in Bombus terrestris (Hymenoptera: Apidae). Our results reveal novel insights into the physiological maturation of bumblebees, characterised by a drastic metabolic shift during the first week of adult life. While newly emerged workers (≤ 1 day) show high lipid reserves and low circulating sugar levels, by the age of 7 days, lipid reserves are depleted by half, while circulating glucose and trehalose levels increase 4-to 6-fold and remain relatively stable thereafter. Although body size (i.e. dry mass) positively correlated with absolute lipid content in young workers and with trehalose in 14- and 21-day-old workers, age was the primary variable explaining the variation in our data. Our findings provide a mechanistic basis for age-based polyethism in bumblebees and highlight the importance of considering age as a fundamental factor in social insect research.