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 decides whether a colony survives

Cold on its own kills few colonies. What kills them is a combination: too little food, too few bees, no laying queen, winter bees damaged by mites, and damp, each making the others worse. A small cluster loses heat faster for every bee in it and burns more food per bee; a colony short of food cannot afford the heat a small cluster needs; mite-damaged winter bees die early and leave the cluster smaller still.

Two of these can be measured going into winter. Autumn colony size predicted survival better than the geographic origin of the stock in a study of colonies in the north-eastern United States, and Britain's National Bee Unit treats stores and adult population as the two strongest predictors it has. Its six winter checks — enough food, enough bees, a queen, no disease, no pests, a sound hive — are a fair summary of the whole list.

Moisture, ventilation and insulation

Burning honey makes water. The vapour from a wintering cluster condenses on whatever is coldest in the hive, and if that is the inner cover above the bees it drips back onto them. The National Bee Unit's advice is to keep hives rain-proof, let condensation run out of the entrance by tilting the hive slightly forward, and rely on the ventilation the roof, entrance and floor already give; it says damp, rather than cold, is what kills.

Insulation is where the evidence has moved. Measurements of common hives and of tree cavities found heat leaving the hives roughly four to seven times faster than a typical tree nest, which suggests that wild colonies in thick-walled cavities live with less heat loss and higher humidity than colonies in thin wooden boxes. The two positions are less opposed than they sound: both say water must not fall on the cluster, and the National Bee Unit itself allows insulation in harsh winters, provided ventilation is kept.

Stores, feeding and checking in winter

Checking a wintering colony is a balance between knowing and disturbing. Hefting — lifting one side of the hive, then the other — tracks weight without opening anything, but it cannot show isolation starvation. A look inside should be brief, on a mild day when some bees are flying, and only when there is a reason; the National Bee Unit puts it at around twenty seconds. A clear inner cover lets a beekeeper see the cluster without letting the heat out.

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.