Individual and colony factors in bumble bee division of labor (Bombus bifarius nearcticus Handl; Hymenoptera, Apidae)

We quantified the relationships of colony-level factors (number of workers and colony age) with task performance by workers of the bumble bee Bombus bifarius. Worker's age of first foraging decreased as their colonies aged, which may have caused the observed decrease in worker longevity with colony age. Daily variation in colony nectar foraging rates was related to the number of active foragers, while pollen foraging rates corresponded more strongly with variation in numbers of adult workers. Rates of brood care behavior decreased as the colonies aged, but rates of thermoregulatory behavior (incubation and fanning) did not decrease over time. We conclude that rates of performing some tasks (pollen foraging, brood care) were largely determined by changes in colony demography or correlated variables, while rates of performance of other tasks (nectar foraging, thermoregulation) depended more on external environmental conditions. Most workers switched between two foraging tasks (pollen and nectar collection) and among several in-nest tasks. However, some foragers specialized by focusing their effort on either nectar or pollen. Other workers specialized on in-nest tasks by performing thermoregulatory behavior (incubation and fanning) at significantly higher rates than their nestmates. The task specialists contributed disproportionate amounts of labor to their colonies. Task specialization indicates that workers were not identical in their responses to variation in colony need.