The winter cluster

The art of economy (Janine Kevits)

Winter represents a formidable challenge for fauna, as it must cope both with cold temperatures and with food scarcity. Some insects have “chosen” to avoid it by migrating to warmer regions; this is the case, for example, of the painted lady butterfly. Others concentrate their chances of survival on a few individuals—reproductives that are abundantly nourished during the favorable season and whose task is to found a new colony on their own the following spring; this is the strategy of wasps, hornets, and other solitary bees. The honey bee, by contrast, has found a different path: it is the powerful organization of the colonies it forms that enables it to meet this challenge, by implementing two means that are entirely original in the insect world—on the one hand, the storage of reserves, and on the other, the reorganization of the colony to form the winter cluster, a system characterized by the absence of brood and by modes of functioning that differ fundamentally from those of the summer colony.

The winter cluster – the art of energy economy

Understanding how the colony reduces its heat losses in winter while keeping the bees warm enough to remain active.

The honey bee faces winter collectively. As temperatures fall, the workers gradually move closer together and form a cluster around the combs. There is, however, no single temperature at which this cluster "begins" or becomes "complete": its formation depends in particular on the temperature, on colony strength, on the presence of brood and on the organisation of the nest.

A living structure, not a motionless ball

The winter cluster is a dynamic structure. Its periphery generally becomes denser as the cold intensifies, which reduces heat exchange with the environment. Inside, workers actively produce heat using their flight muscles, activated without any visible wingbeat (Stabentheiner et al., 2003).

The bees change position continuously, and the cluster contracts or expands according to conditions. It is therefore too simple to describe it as a succession of fixed layers of bees each with a permanent function.

Nor is the temperature uniform. The centre can remain markedly warmer than the periphery, and the air elsewhere in the hive can be colder still. The colony therefore does not seek to keep the whole cavity at a constant temperature.

Producing heat costs honey

The heat produced by the bees ultimately comes from the chemical energy contained in their stores. The greater the heat losses, the more food the colony must consume, all other things being equal, in order to compensate for them.

This relationship explains why the accessibility of the stores is as important as their total quantity. During a cold spell, the mobility of the bees decreases. A colony can therefore run into difficulty while honey is still present on combs that have become hard to reach.

The presence of brood also changes the problem considerably. Without brood, a large part of the cluster can remain relatively cool. With brood, the colony must locally maintain a much narrower temperature, mainly around 34–36 °C. Energy demand may then increase.

And CO₂?

The CO₂ concentration is naturally higher in an overwintering colony than in the outside air, and it varies over the course of the day. The bees are able to maintain these regimes despite changes in hive ventilation (Meikle et al., 2022; Meikle & Weiss, 2025).

It has not, however, been demonstrated that the colony deliberately creates hypoxia in order to limit its metabolism, or that oxygen acts as a "thermostat" allowing honey to be saved. This appealing explanation goes beyond the data currently available.

Colony size matters, but without a magic threshold

A small cluster has proportionally a larger exchange surface and fewer bees available to produce heat. Colony strength before winter is indeed an important factor in successful overwintering.

Experimental work does not, however, allow a universal limit to be set at 17,000 bees, 400 g, 1.7 kg or a given number of combs. A colony's capacity to overwinter depends simultaneously on its size, on the physiological quality of the winter bees, on its stores, on the presence of brood, on the climate and on its health status.

Nor is the relationship between outdoor temperature and consumption linear. Southwick (1983) showed that metabolic expenditure can increase when the cluster expands at milder temperatures. This result does not, however, mean that a particular temperature would be optimal for all colonies, nor that a hive should deliberately stay cold.

What this means for the beekeeper

To come through the winter well, a colony must above all have:

a sufficient population and winter bees in good physiological condition;
an infestation by Varroa destructor that is properly controlled;
sufficient and accessible stores;
a hive protected from water ingress and uncontrolled draughts;
an envelope that does not needlessly impose heavy heat losses on it.

Good wind protection and a properly insulated top cover are therefore reasonable. Neither maximum ventilation nor minimum insulation, by contrast, constitutes an objective in itself.

The right question is not how to keep the bees clustered, but how to enable them to manage their own energy balance with as few unnecessary constraints as possible.

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References used for the update

Meikle, W. G., Barg, A., & Weiss, M. (2022). Honey bee colonies maintain CO₂ and temperature regimes in spite of change in hive ventilation characteristics. Apidologie, 53, 51. https://doi.org/10.1007/s13592-022-00954-1

Meikle, W. G., & Weiss, M. (2025). Temperature and CO₂ concentration in honey bee hives exhibit circadian rhythms. Scientific Reports, 15, 22042. https://doi.org/10.1038/s41598-025-03614-3

Southwick, E. E. (1983). The honey bee cluster as a homeothermic superorganism. Comparative Biochemistry and Physiology Part A: Physiology, 75(4), 641–645. https://doi.org/10.1016/0300-9629(83)90434-6

Stabentheiner, A., Pressl, H., Papst, T., Hrassnigg, N., & Crailsheim, K. (2003). Endothermic heat production in honeybee winter clusters. Journal of Experimental Biology, 206(2), 353–358. https://doi.org/10.1242/jeb.00082

Stabentheiner, A., Kovac, H., Mandl, M., & Käfer, H. (2021). Coping with the cold and fighting the heat: Thermal homeostasis of a superorganism, the honeybee colony. Journal of Comparative Physiology A, 207(3), 337–351. https://doi.org/10.1007/s00359-021-01464-8

Author

Janine Kievitz

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