How honey bees survive winter

The cluster

As temperatures fall below about 14 °C the colony forms a ball around the queen. The outer bees pack tightly, forming an insulating mantle several bees thick; the inner bees are looser and generate heat by shivering their flight muscles.

The cluster's outer surface is held at around 8-10 °C. The centre sits at about 20-25 °C when there is no brood, rising to 34-35 °C as soon as brood rearing resumes in late winter. Bees rotate between the cold mantle and the warm interior.

Winter bees

Bees reared in late summer and autumn are physiologically distinct from summer bees. They carry enlarged fat bodies and high levels of vitellogenin, a storage protein, and they live five to six months instead of six weeks. A colony's ability to overwinter depends almost entirely on producing enough of them.

This is the mechanism by which varroa kills colonies. Mites feeding on autumn brood produce winter bees with reduced fat bodies and shortened lives, and the colony dwindles through winter and collapses in late February or March — frequently with stores still in the hive.

What can go wrong

What other bees do instead

The honey bee's strategy is unusual. Almost all other bees do not overwinter as a colony at all.

• Bumblebees — the colony dies entirely. Only mated new queens survive, hibernating alone underground.
• Solitary bees — the offspring overwinter in the sealed cell as a prepupa or an adult, and the parents are long dead.
• Stingless bees — perennial colonies in the tropics, with no winter to survive.
• Arctic bumblebees — the same annual cycle as temperate species, compressed into an eight-week summer.

This is why honey bees store honey and other bees do not. An annual colony has no reason to store more than a few days' buffer; a perennial one in a cold climate must store several months of food.