Wintering in the honey bee: A very specific phase of its biological cycle
The activity of insects, which are cold-blooded animals, is determined by ambient temperature. Their muscles require a minimum level of heat; if this threshold is not met, all activity ceases. To avoid this potentially lethal phenomenon in winter, insects have developed resistance strategies, most commonly through hibernation.
Most insects spend the winter in the egg, larval, or nymphal stage (which are more conducive to survival); others, fewer in number, overwinter as adults after building up body reserves. In all these cases, overwintering generally occurs through isolated individuals.
The winter cluster: “a heating core, an insulating mantle”
The overwintering system of the honey bee is unique due to the combination of three characteristics:
- the storage of reserves,
- the reorganization of the colony into a cluster,
- the formation of winter bees with their specific physiology.
The cluster is the grouping of the colony’s bees during winter. It is characterized by the absence of brood and by operating modes that differ completely from those of the summer colony. It begins to form when the outside temperature falls below 15 °C and becomes complete (including all bees of the colony) below 7 °C. Overall, it has the shape of an ellipsoid (with variable form depending on its position in the hive). It is interrupted by the combs, which the bees cover in multiple layers without entering them (empty cells serve as air reserves and insulation). It is dynamic: its shape changes with the movement of the bees, and its movements are slow and irregular.
Photo: Fred l’apiculteur – Exometeofraiture
The core of the cluster is formed by about 15% “heater” bees, which generate heat through contractions of their wing muscles without wing movement. This very intense activity requires oxygen consumption equal to or greater than that of flight and rarely lasts more than 30 minutes per bee. A relay organization therefore takes place. The temperature within the cluster can vary greatly: from 6 to 12 °C at the periphery to 33.5 °C at the center, which is in no way comparable to the temperature range in a colony with brood (33.8 °C to 37 °C). The temperature of the cluster core depends on the outside temperature: the colder it is outside the hive, the warmer the core becomes. The heat produced is intended to warm the entire cluster to keep the group alive (particularly at the periphery, where it is coldest), but not to heat the hive itself. Heat is delivered to the bees, not to the dwelling!
At the periphery of the cluster, the bees form a kind of insulating mantle designed to prevent heat loss. The superimposed layers of bees, with their heads oriented toward the inside of the cluster, have strong insulating properties, notably due to the interlocking thoracic hairs when the bees are tightly packed. This mantle has a more or less loose structure and tightens as the temperature drops. A temperature gradient is observed within the cluster from the inside outward; in winter, the temperature of the outer mantle ranges between 6 °C (a short-term survival temperature for bees, with a risk of death if prolonged) and 12 °C (bee lethargy).
This efficient system minimizes heat production in order to adjust the overall metabolism to the minimum necessary, thereby avoiding waste of food reserves and preserving the bees’ lifespan.
The consumption of winter reserves by the cluster entails gas exchanges amounting to approximately 10 m3 of O2, an equal volume of CO2, and the production of roughly one bucket of water.
The number of bees forming the cluster is a determining factor. It has been shown that the ability to produce and maintain heat within the cluster depends on the number of bees, because there is a relay among bees (an exhausting task) and because heat losses at the mantle level are greater when there are fewer bees.
A smaller mass produces less heat and loses more of it, and this occurs exponentially (a cluster of 400 g = 4,000 bees is virtually incapable of survival).
Furthermore, overall metabolism increases with the number of individuals, but more slowly at low temperatures. An interesting observation has been made: a cluster of fewer than 17,000 bees expends more energy at 2 °C than at 15 °C, whereas this balance is reversed if the cluster contains more than 17,000 bees (lower energy expenditure at 2 °C). Beyond 18,000 bees, the energy savings are no longer significant.
These aspects of cluster size and energy consumption must be taken into account by the beekeeper for feeding, insulation, and ventilation of the overwintering colony.
As a reminder: 1 dm2 of comb covered with bees corresponds to approximately 140 bees, and one Dadant frame = 10 dm2.
For further information: http://www.parole-apiculteur.fr/wp-content/uploads/2013/01/Synth%C3%A8se-Parole-dapiculteur-Hivernage.pdf
See also:
