Temperature of the winter cluster

View answer

Correct answer: 2 (consumes less energy at 2 °C than at an internal hive temperature of 15 °C.)

As soon as the outside temperature drops below 15 °C, the winter cluster begins to form and is fully established at −7 °C, involving all the bees of the colony. The lower the temperature, the more the cluster contracts, in accordance with the principle of heat transfer by convection, which increases with surface area. At very low temperatures, the cluster eventually retreats deep between the comb spaces and becomes invisible.

The ability of the winter cluster to produce heat depends strongly on the number of bees composing it. Moreover, the smaller the cluster, the greater the heat losses, according to the well-known mathematical surface-to-volume ratio described in the annexes below (Southwick, 1983). Consequently, small clusters with few bees and proportionally large heat-loss surfaces have little chance of surviving the winter. The overall metabolism of a cluster increases with the number of bees, but not linearly; around 17,000 bees represent the inflection point of this curve (see the definition of an inflection point in the annexes). At low temperatures, this effect is less pronounced. As a result, a small cluster of less than approximately 1.7 kg (< 17,000 bees) will expend more energy to survive at 2 °C than at 15 °C, which is easy to understand.

However, this relationship is reversed for a larger bee mass: a cluster of more than 1.7 kg will consume less energy at 2 °C than at 15 °C. At moderate ambient temperatures (10–14 °C), the cluster gradually disperses, leading to a massive increase in total surface area for heat loss and a strong concomitant increase in metabolism (E. Southwick, 1983).

To explore further:

•  Sense and nonsense of thermal insulation of beehives