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
Honey bees survive winter not as isolated individuals but as a reorganized colony forming a winter cluster. When outside temperatures drop below about 15 °C, bees begin to cluster; at around 7 °C the cluster is fully formed. This winter organization is characterized by the absence of brood, very low activity levels and finely tuned energy management.
The cluster has a functional two-layer structure. At its center lies a warm core, where roughly 15–16 % of the bees actively generate heat. These “heater bees” produce warmth through isometric contractions of their flight muscles without wing movement, converting metabolic energy directly into heat. Because this activity is energetically costly, heater roles are continuously exchanged among individuals.
Surrounding the core is an insulating mantle composed of almost inactive bees with strongly reduced metabolism. By pressing their hairy thoraxes together, they form an effective insulating layer. Temperatures at the periphery may drop to 6–7 °C, while the core temperature can range from about 12 °C to over 30 °C depending on external conditions. Importantly, bees do not heat the entire hive but only the cluster itself.
Thermoregulation is highly dynamic. As external temperatures fall, the core increases heat production to protect the outer bees from reaching their individual collapse temperature, around 10 °C. The goal is not a constant internal temperature but the avoidance of critical thresholds at minimal energy cost.
A key energy-saving mechanism involves controlled respiration. The cluster actively regulates ventilation, maintaining a state of relative hypoxia with elevated CO₂ levels. By limiting oxygen availability, glucose consumption is reduced, conserving honey reserves. Disturbing the cluster increases oxygen influx, accelerates metabolism and immediately raises food consumption.
Cluster size is crucial for survival. Small clusters (below about 400 g or ~17,000 bees) suffer disproportionately high heat losses and are unlikely to survive winter. Larger clusters are energetically more efficient and, under typical Central European winter conditions, may consume less energy at very low temperatures than at milder ones.
In conclusion, the winter cluster is a highly optimized collective thermoregulatory system. For beekeepers, adequate food stores, calm conditions, protection from wind and proper ventilation are far more important than heavy hive insulation.
See also:
- Successful Wintering
- Preserving the Bees' Life Capital
- Winter Survival of Honey Bee Colonies
- The Sense and Nonsense of Hive Thermal Insulation
- Wintering in the Honey Bee: A Unique Phase of Its Biological Cycle
Source: abeilles & cie; 124-2009 No. 131 http://www.cari.be