What do bees do in Winter?

Did you know that honey bees are experts at managing the temperature inside their colony? There are even “heater bees” who act like little radiators, applying their heat as required. They vibrate their wing muscles vigorously in order to generate the heat, which, as you can see from the above thermal image, is concentrated in the thorax and head. This whole “thermoregulation” thing is elevated to a fine art by honey bees who use various strategies, both active and passive, to keep things tickety-boo. There is some amazing scientific research into this area. The image above comes from this research paper by Stabentheiner, Kovac & Brodschneider which is worth reading for those of you who like scientific research articles.

I often get asked about what bees do in the winter. Hopefully the answer is “survive and prosper”, but that will depend on a number of factors, many of which can be influenced by the beekeeper. Winters vary significantly from year to year and from region to region. Here in England we tend to have wet weather and only sometimes do we get prolonged periods of snow and ice. In Scandinavia and parts of North America it always freezes over for many months. The bees seem to cope.

There is an excellent book by Jurgen Tautz called “The Buzz about Bees: Biology of a Superorganism” which does bee science in a very accessible way. He talks about how honeybees survive the winter as a complete colony. They collect together in a dense cluster, and keep themselves warm by vibrating their wing muscles. They use honey stores as energy for this activity.

Bees get the energy for heating from honey. Honey is used mostly to warm the brood nest in summer, and to keep the bees clustered in the hive from freezing during winter. The large reserves of honey of a bee colony are therefore not food, but fuel.

In Canada they have proper winters, so I was interested to read “The Honey Bee, A Guide for Beekeepers” by V R Vickery, who was a professor at McGill University, Montreal, and who taught apiculture for many years. He led the way in wintering honeybee colonies in Eastern Canada and showed how it can be done very successfully, despite the harsh weather.

He says that a single bee is just able to function at a temperature of 6.1 degrees C. As outside temperature falls the cluster contracts until all of the bees are involved, and at 6 – 8 degrees C a shell, or insulating layer, of bees 3 to 7 cm thick is formed. Bees crawl into any empty cells within the shell and crowd together in the spaces between the combs (Moeller, 1977). Bees in the shell are not very active but they do change positions slowly so that any single bee does not stay long enough on the outside to become chilled and die.

At the beginning of winter the bee cluster usually forms over the lower part of the honey and pollen stores. They move upward as winter progresses. This cluster can move, but mostly this happens in warmer periods (above freezing). The temperatures in different parts of the cluster are about 7 degrees C on the outside of the cluster, 13 degrees C in the shell, and at least 24 degrees C in the centre. During broodless periods (mid October to late January) maximum temperatures in the heat generating area fluctuate between 26 and 36 degrees C. Brood rearing usually starts early in February as a response to increasing day length. Then the interior of the cluster is maintained at around 34 degrees C, which is what is needed to raise baby bees.

The cluster size changes in response to fluctuations in external temperatures. When the ambient temperature drops below 7 degrees C the bees shrink the cluster to reduce the surface area from which heat is radiated and lost. At the same time the bees in the centre become more active and generate more heat. If the external temperature rises above 7 degrees C the cluster expands. There is a balance between cluster size, the thickness of the insulating shell and the amount of heat generated in the centre.

A cluster of bees can withstand long periods of cold weather if they have access to enough honey so that they can continue to generate heat. A colony that was placed in a refrigerated room, at an average temperature of minus 24 degrees C for 84 days, maintained a minimum temperature of 27 degrees C in the centre of the cluster for the entire period (Owens, 1971).

Cold starvation can and does kill honey bee colonies (Greve, 1973). The cluster can move vertically or horizontally on the combs but this movement is very slow and takes place mainly during warmer periods. Prolonged cold weather can prevent movement from comb to comb and cold starvation can occur, even with plenty of honey stored in combs that the bees are unable to reach. A single warm day, with temperatures above freezing, may be enough to allow the bees to move to honey stores which previously had not been available to them.

Here is another amazing research article on how bees generate heat in the winter cluster. It concludes that the cluster temperature is regulated through a combination of the actions of “heater bees” in the middle together with the insulating properties of the bees in the shell on the outside.

None of this explains what the beekeeper must do to ensure that his or her bees survive the winter, but that can be for another time. Hopefully now you know, if you didn’t already, what bees do in the winter – they cluster.