Thermoregulation of foraging honeybees on flowering plants: seasonal variability and influence of radiative heat gain

1. During nectar and pollen foraging in a temperate climate, honeybees are exposed to a broad range of ambient temperatures, challenging their thermoregulatory ability. The body temperature that the bees exhibit results from endothermic heat production, exogenous heat gain from solar radiation, and heat loss. In addition to profitability of foraging, season was suggested to have a considerable influence on thermoregulation. To assess the relative importance of these factors, the thermoregulatory behaviour of foragers on 33 flowering plants in dependence on season and environmental factors was investigated. 2. The bees ( Apis mellifera carnica Pollman) were always endothermic. On average, the thorax surface temperature ( T th ) was regulated at a high and rather constant level over a broad range of ambient temperatures ( T th = 33.7–35.7°C, T a = 10–27°C). However, at a certain T a , T th showed a strong variation, depending on the plants from which the bees were foraging. At warmer conditions ( T a = 27–32°C) the T th increased nearly linearly with T a to a maximal average level of 42.6 °C. The thorax temperature excess decreased strongly with increasing T a ( T th − T a = 21.6 − 3.6°C). 3. The bees used the heat gain from solar radiation to elevate the temperature excess of thorax, head, and abdomen. Seasonal dependance was reflected in a 2.7 °C higher mean T th in the spring than in the summer. An anova revealed that season had the greatest effect on T th , followed by T a and radiation. 4. It was presumed the foragers' motivational status to be the main factor responsible for the variation of T th between seasons and different plants.