The organisation of larval feeding in bumblebees (Hymenoptera, Apidae) and its significance to caste differentiation

Summary.

In pollen-storing bumblebees, the rate at which workers nourish larvae has been proposed to be the main factor influencing caste differentiation since workers feed prospective queens more frequently and longer than worker larvae during the last instars. In order to determine how the frequency of feedings is established small groups of Bombus terrestris larvae were either subjected to starvation or nourished regularly by workers. Experimentally starved larvae were fed significantly earlier and more often than control larvae. Behavioural observations provide evidence for the existence of a stimulus of larval origin that releases feeding behaviour in workers. Intentional inspections in the sense of a recognisable and functional behaviour intended to organise the feeding process were never observed. We argue that worker inspections are not required for the adequate maintenance of larvae and that a simple stimulus-response chain appears to be sufficient to regulate feeding behaviour at the individual and the colony level in bumblebees. Furthermore, hand-rearing experiments with female larvae indicate that queen rearing is not dependent on a high frequency of feedings in itself, nor is worker development induced by larval deprivation. This makes it unlikely that workers impose a caste-specific frequency of feedings on larvae in order to actively control or manipulate caste development. Since caste fate seems to be predetermined early in larval life, we propose that early caste-specific differences in development are reflected in the physiology of the larvae and the associated nutritional needs to which workers respond accordingly. Consequently, caste-specific differences in feeding frequencies are a result, but not the cause of differences in development.