When the temperature of the air directly surrounding the colony drops below 14 ° C, the colony groups together in the form of a cluster which approaches a sphere, because it seeks to save heat. The bees in the center produce heat, while those on the surface form an insulating layer varying in thickness between 2 and 8 cm.
The bees live in society and must produce, by their muscular activity and their metabolism, enough heat to maintain a minimum temperature at the periphery of the group, and to remain in contact with the provisions accumulated during the previous season.
When the temperature reaches 7 °, all the bees are integrated into the cluster. This temperature must be maintained at the surface, otherwise the bees there will go numb and may become detached.
Above 7 °, the cluster expands, below it contracts, thus reducing the heat loss surface.
The main mechanism that allows the colony to fight against the cold lies in the contraction and expansion of the cluster, whose functioning depends on permanent contact with honey, a source of energy.
The insulating casing of the cluster is very efficient, because the radiated heat is very low. It is not even sufficient to make a layer of warmer air appear on the ceiling of a beehive. A few centimeters from the cluster, temperatures are close, to within a few degrees, to that of the outside air under shelter, whether the hive is well insulated or not.
The bees that form the insulating layer are calm, with their heads pointing towards the center, the wings slightly raised at an angle of about 45 °. The empty cells, released by the last brood, are occupied by bees.
The bees which are in the center of the cluster are less tight and produce heat by their metabolism and their muscular activity, in sufficient quantity so that the temperature on the surface exceeds 7 °. The core temperature can drop to 15 °. It oscillates between 20 and 35 °. A normal cluster emits a slight rustle, similar to that produced by pronouncing "ch" very weakly. An unusual disturbance results in an increase in intensity, which corresponds to a reaction of the bunch, followed by a rise in internal temperature.
A very low temperature, -30 ° for example, for a prolonged period, can cause a strong contraction of the bunch, causing it to lose contact with the provisions and thus causing fatal numbness. Weak colonies are the most vulnerable because they have an unfavorable ratio between the number of bees and the area of wastage. They must therefore maintain a higher internal temperature than a strong colony and consume more per bee.
Optimal use of provisions is achieved when the temperature around the bunch is around 7 °. The theory that the consumption of honey is higher the lower the temperature is unfounded. It has in fact been observed that a colony can consume more at -2 ° than at -9 °. It is above all sudden changes in temperature and various disturbances that cause significant increases in the internal temperature of the bunch, and therefore increased consumption. In winter, bees need calm.
Too much insulation can be harmful to bees. For example, on a beautiful winter day, the warming of the outside air is not even noticed by the colony and thus loses a precious opportunity to perform a cleansing flight.
The cleansing flight
Bees are hygienic insects: the waste they produce accumulates in their rectal bulb and they can go several weeks without defecating, in order to prevent the development of pathogens in the hive.
When temperatures in the shade are close to 10 - 12 ° C, the temperature inside the hive rises very quickly and there is an extraordinary explosion in the activity of the hive. A large number of bees that have been confined for a long time take the opportunity to go out, empty their rectal bulbs and sunbathe, others are already setting out to scout the surroundings, and still others take the opportunity to search for minerals in the damp soils. But the bees know it's not yet spring and inside the queen needs warmth. After a brief outing, they come back in again to give way to those who remained to watch over the interior temperature. The stronger the colony, the more bees will be out.
After the cleansing flight, the different glands of the bees come into operation again. The hypopharyngeal and mandibular glands in nurses, the wax glands in constructor bees and the Dufour and Koshevnikov glands in the queen. All these transformations cause the colony to be reborn and the queen to start laying eggs again. It's only a few dozen eggs first, but it gets the machine going. Only when the temperature rises will the colony truly come back to life. If the queen is in good shape and the colony is strong enough, then the queen will actually begin to lay. The first supplies of pollen will come in and it will be up to the winter bees to make the transition go smoothly.