Bees, like many insects, rely on external warmth to power their bodies and initiate activity. While a bee colony exhibits a remarkable ability to generate its own heat, individual bees venturing out for forage are still subject to ambient conditions. They must actively warm their flight muscles to a functional temperature, which becomes more difficult as the air cools.
The Threshold for Flight Inactivity
The temperature at which most foraging bees cease flight is the point where the cost of generating heat outweighs the benefit of foraging. For the western honeybee (Apis mellifera), this threshold is 50°F to 55°F (10°C to 13°C) ambient air temperature. Below this, the individual bee cannot maintain the necessary flight muscle temperature, which must be 85°F (30°C) or higher, for sustained flight.
Flight requires the bee to shiver its flight muscles, uncoupled from the wings, to generate heat. When the air temperature dips low, the rate of heat loss from the thorax becomes too great to overcome, leading to flight cessation. Bees stranded in cold weather are victims of this thermal limit, becoming immobile before they can return to the colony. This inactivity is a state of cold-induced immobility that conserves energy until temperatures rise.
Species-Specific Thermal Tolerance
The minimum temperature for activity varies across different bee species. Bumblebees (Bombus species) are far more cold-tolerant than honeybees due to their larger body mass and superior ability to shiver. Their larger size provides a better surface area-to-volume ratio, which reduces heat loss. They can begin foraging at temperatures as low as 40°F (4°C), several degrees below the honeybee’s minimum.
A bumblebee can raise its thoracic temperature from ambient to over 86°F (30°C) even when the air is cool. This intense shivering capability allows them to be active much earlier in the spring and later in the year than honeybees. Solitary bee species also show variation. The ability to maintain a high muscle temperature dictates the cold-weather foraging range for any individual bee.
Thermoregulation Within the Colony
When individual bees stop foraging due to low ambient temperatures, the honeybee colony forms a dense, insulating structure called the winter cluster. This social behavior transforms the colony into a single, heat-generating entity.
The cluster has a tightly packed outer mantle layer that acts like insulation, surrounding an inner core of bees actively generating heat. Core bees shiver their flight muscles rapidly, remaining stationary while generating warmth. This muscle activity maintains the core temperature near 93°F (34°C), even without brood. Bees cycle from the cold outer edge to the warm inner core, preventing any single bee from freezing. This mechanism allows the colony to survive extended periods of sub-freezing weather.
Lethal Temperature Limits
The ultimate lethal temperature for a bee colony is the point at which the colony’s combined heat generation can no longer counteract heat loss. If the external temperature drops severely, such as below 18°F (-8°C), the cluster contracts tightly, increasing metabolic demand. While the core remains warm, the outer mantle temperature can drop to about 63°F (17°C). If the temperature falls much lower, the bees on the outside will lose function.
Colony death during winter most often results from starvation. The cluster must move slowly across the frames to access stored honey reserves. If the ambient temperature is too low, the cluster cannot move to a new area of food, and the bees starve near their honey. An individual bee becomes immobile and loses its ability to shiver when its body temperature drops to about 50°F (10°C). If a bee is isolated from the cluster and its internal temperature drops below this, it will rapidly freeze and die.