Thresholds of response in nest thermoregulation by worker bumble bees, Bombus bifarius nearcticus (Hymenoptera : Apidae)

Regulation of nest temperature is important to the fitness of eusocial inse ct colonies. To maintain appropriate conditions for the developing brood, w orkers must exhibit thermoregulatory responses to ambient temperature. Beca use nestmate workers differ in task performance, thermoregulatory behavior provides an opportunity to test threshold of response models for the regula tion of division of labor. We found that worker bumble bees (Bombus bifariu s nearcticus) responded to changes in ambient temperature by altering their rates of performing two tasks - wing fanning and brood cell incubation. At the colony level, the rate of incubating decreased, and the rate of fannin g increased, with increasing temperature. Changes in the number of workers performing these tasks were more important to the colony response than chan ges in workers' task performance rates. At the individual level, workers' l ifetime rates of incubation and fanning were positively correlated, and mos t individuals did not specialize exclusively on either of these temperature -sensitive tasks. However, workers differed in the maximum temperature at w hich they incubated and in the minimum temperature at which they fanned. Mo re individuals fanned at high and incubated at low temperatures. Most of th e workers that began incubating at higher temperatures continued performing this task at lower temperatures, when additional nest-mates became active. The converse was true for fanning behavior. These data are consistent with a threshold of response model for thermoregulatory behavior of B. bifarius workers.