Thermal investigations associated with the behaviour patterns of resting workers of Bombus atratus (Hymenoptera: Apidae)

Abstract

The Neotropical bumblebee Bombus (Fervidobombus) atratus Franklin is widely distributed in South America ranging from tropical and subtropical lowlands to high altitudes in the Andes. Thermal investigations of the Neotropical bumblebee Bombus atratus at rest were performed by means of thermometry. Most social insects are diurnal foragers. When they are inside their nests during the night, they carry out some tasks such as cleaning the nest, brood care or nest thermoregulation. However, forager bees generally pause at night in a sleep-like state. A bumblebee nest was transferred to a brood chamber which was maintained at approximately constant temperature (28 ± 1 °C) to prevent the workers from building the protecting involucrum so that one could mark them at birth. To provide a temperature gradient, two chambers were installed adjacent to the brood chamber, so that the workers had the chance to choose between 3 temperatures: the first one kept at 22 ± 1 °C, the second at 19 ± 1 °C and 28 ± 1 °C. The temperatures of the chambers were recorded continuously by a four channel data logger. Three categories of tasks performed by workers were registered in this investigation: warming brood (task 1), feeding brood (task 2) and foragers (task 3). During the experiment 357 workers were born and 252 (71%) of them showed sleep-like behaviour. The total sleep time in hours according to the work done by the workers was task 1 (240.6 h) > task 3 (236.0 h) > task 2 (227.3 h). The workers who performed tasks 1 and 2 preferred to sleep at the highest temperature (28 ± 1 °C) and the forager workers at 22 ± 1 °C.