Foraging task specialisation and foraging labour allocation in stingless bees

Abstract

Social bees collect nectar and pollen from flowering plants for energy of the adult bees and for feeding the larvae in the colony. The flowering patterns of plants imply that periods of high food availability are often followed by periods of meagre foraging conditions. Being dependent on such a dynamic food source may have serious implications for the survival of adult bees and for a steady production of brood. To adjust foraging behaviour to food conditions in the field, colonies may often have to reallocate the labour that is available for foraging over the different foraging tasks. There are two ways in which social bee colonies can adjust the labour devoted to different foraging tasks; 1) adjust the total foraging force allocated for a particular task and 2) adjust the individual effort of foragers already working on the task. An increase in foraging force for a particular task can be attained in two ways. Firstly, individuals can switch from other foraging tasks to the required task. Switching foraging tasks is physically possible and has been observed in both stingless bee and honeybee foragers. Specialising on one foraging task, however, is thought to result in more efficient foraging and switching tasks might be costly. Apart from forager task switching, increasing the foraging force for a particular task can also be achieved by allocating new foragers to the task. This assumes a reserve supply of labour that can be allocated over foraging tasks in response to stress. The objective of the present study was to systematically investigate the occurrence and plasticity of foraging task specialisation and the dynamics of forager allocation in stingless bees. Stingless bees (Apidae, Meliponini) occur in the tropics and live in colonies of the same complexity as the honeybees. With the data obtained a contribution to the basic understanding of how social insect colonies can respond adaptively to a dynamic environment is aimed. The results of this thesis show that the adaptive tuning of the colony foraging behaviour of stingless bees can involve changes in the total foraging force as well as changes in the individual foraging effort. The importance of each mechanism to adjust colony foraging behaviour seems to depend on the circumstances. Increase in individual foraging effort seems to occur mainly in situations when large amounts of food are available. Even though increase in foraging effort decreases longevity, in periods of high opportunity, when food is available in large quantities close to the hive, this might be worth the offer. Switching between foraging tasks can be mainly observed when foraging success declines. Successfully employed bees probably do not switch easily to other foraging tasks. As switching is costly, switches between tasks can be economically made when the success of the original task declined. Allocation of new foragers occurs mainly in situations of food stress or when new foraging tasks appear. By allocating non-foragers, foraging on other tasks can continue without interruption, avoiding consequent decrease in storage levels of the other commodities. This thesis contributes to the identification of the mechanisms used by stingless bee colonies to adjust colony foraging activity under different conditions and makes clear the complexity of the organisation of foraging in stingless bees.