Metabolic rate and thermal stability during honeybee foraging at different reward rates.

During honeybee foraging, the stabilization of thoracic temperature (Tth) at elevated values is necessary to meet the power requirements of flight at different air temperatures (T(a)). To understand how the bee achieves thermal stability at different reward rates, the metabolic rates of undisturbed foraging bees were measured at different T(a) values and different sucrose solution flow rates. Metabolic heat production, calculated from the rate of carbon dioxide production, decreased linearly from 49.7 to 23.4 mW as T(a) increased from 19 to 29 degrees C (sucrose flow rate 1.75 microl x min(-1), 50 % w/w). In contrast, crop load and inspection rate remained constant. Metabolic rate displayed a linear relationship with both T(a) and the logarithm of the flow rate of sucrose solution (range analyzed 0.44-13.1 microl x min(-1), 50 % w/w). Metabolic rate decreased by 3.13+/-0.52 mW (mean +/- s.e.m., N=37) for every 1 degrees C increase in T(a) and increased by 4.36+/-1.13 mW for a doubling in flow rate. These changes in metabolic power output might be used to achieve thermal stability during foraging. It is suggested that the foraging bee might increase its Tth in accordance with the reward rate.