The basis of honey bee winter survival lies in their ability to form a winter cluster. The winter cluster is a precisely organized, dynamic structure that functions like a biological heating system. It is worker bees instinctively gathering together, centered around the queen and remaining brood.
Ideally the winter cluster will form between boxes (in 2-3 box hive configurations). It moves upward as the season progresses to maintain contact with honey stores. This initial positioning facilitates communication from one frame to another – something difficult at lower temperatures when bees can’t easily pass between full curtain frames.
Clustering bees arrange themselves in layers. The outer layer of bees, termed the insulating shell, is formed by tightly interlocked bees with their heads facing inward. This crush of bodies creates a thick blanket designed to minimize heat loss. There is less congestion deeper within the huddle of bees, where, incredible as it may sound, bees are able to move freely around.
Heat generation within the cluster core is an active, metabolic process produced by isometric vibrating thoracic flight muscles without moving the wings. When developing brood is present, heater bees, workers that can greatly elevate their `body temperature over a capped brood or within an empty cell with brood in immediately adjacent cells, will also contribute to heat production.
The colony transitions from a dispersed state to the beginning of a cohesive, insulated outer and inner heat-generating structure when the ambient temperature drops to about 57 degrees F. As the hive temperature continues to drop, normal movement is reduced inside the hive. The cluster is fully formed around 50 degrees F.
On the coldest days/nights the cluster will contract more tightly – when it is warmer the bees relax, the insulating shell may break down and bees move about. Some bees may even attempt to leave to forage or to excrete excess waste. Outside temperatures may be lower and these individual bees may not be able to keep their body warm enough for a return flight home. If there is brood present, the core temperature will be from 91 to 97 degrees F. ; if the hive lacks brood, the core will be cooler — anywhere from 64 to 85 degrees F.
As the body temperature of the shell bees get colder, bees go into a state of torpor or chill coma – some state that as individual body temperature lowers, they will move inward toward the warmer core. However, in the torpor state, they are sluggish and their nervous system may not be able to make a decision to move. That may mean they must be pushed inward. Is it push or pull? A good contradiction for a student to study.
Chill coma for Individual bees is 50 degrees. Absolute lowest temperature at which a bee will die is a function of both time and temperature. Bees that remain in a chill coma for 48 hours at or below 50 degrees F. will die. The absolute cold death temperature for a honey bee generally falls between 20 and 30 degrees F. Good information on temperatures and behavior can be found in Science Insights article. https://scienceinsights.org/what-temperature-is-too-cold-for-bees/
It is often stated that a greater mass of bees provides superior insulation during winter. Honey stores allow the colony to generate heat with less individual effort, resulting in lower total honey consumption per bee. Conversely, small or weakly populated colonies will survive winter too, although some may lack the biomass needed to create a dense mantle, leading to inefficient heat retention and a higher likelihood of collapse.