Glossary
The tight, spherical formation that honey bees create inside the hive during cold weather to conserve heat and protect the colony, especially the queen. Worker bees on the outer shell of the cluster vibrate their flight muscles to generate heat, maintaining a core temperature of approximately 92 to 95 degrees Fahrenheit regardless of outside temperature.
When ambient temperatures drop below approximately 57 degrees Fahrenheit, honey bees stop flying and begin contracting into a cluster. The cluster has two distinct zones: an active outer shell of bees that generate heat through muscle vibration and insulate the core, and a warm inner core where the queen rests and where bees rotate in from the shell to warm up and feed.
As temperatures drop further, the cluster contracts tighter, reducing its surface area to minimize heat loss. On warmer winter days, the cluster may expand to access honey stores on adjacent frames. This ability to thermoregulate allows honey bees to survive winters that would kill other insect colonies, with interior temperatures maintained near 93 degrees even when it is well below freezing outside.
A colony's ability to survive winter depends on three factors: adequate honey stores (typically 60 to 90 pounds depending on climate), a sufficient population of healthy young bees (10,000 to 20,000 minimum), and a healthy, mated queen. Beekeepers assess these factors in fall and supplement as needed.
Ventilation is critical during winter. The cluster generates moisture as a byproduct of metabolism, and if this moisture cannot escape, it condenses on cold hive surfaces and drips back onto the bees. Wet bees lose body heat rapidly and die. Proper ventilation, often achieved with a small upper entrance or moisture board, allows excess moisture to escape without creating drafts that chill the cluster.
The cluster slowly moves as a unit across the frames to access stored honey. On warmer days, the cluster may expand, allowing bees to reach more distant food stores. If the cluster moves to the top of the frames and runs out of accessible honey, the colony can starve even with honey remaining in lower parts of the hive.
Brief inspections on mild winter days (above 45 degrees Fahrenheit) are possible but should be kept to under 30 seconds. Extended exposure to cold air can chill brood and stress the cluster. Most winter monitoring should be done externally: checking entrance activity, hefting the hive for weight, and looking for signs of life.
Varroa mites and the viruses they transmit are the leading cause of winter colony losses. Mites that fed on bees throughout fall produce a population of weakened, virus-laden winter bees that cannot sustain the cluster for the full winter period. Effective fall mite treatment is the single most important factor in winter survival.
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