Do you remember last spring?
Here we are, in the winter, 2023. Last spring is only a faint memory now. Remember those early warm days when the bees — and you — were busily producing and storing honey? While you and I were relishing future surplus honey crops, the bees, as early as last spring, were preparing for swarming events and had begun to gather future winter stores. The bees’ annual life cycle is circular. Last spring, the bees were already storing food for this winter season.
For beekeepers, the winter season is the quiet season. It’s the season when we read, prepare, and reflect, but for the bees, it’s The Big Dearth. Their survival depends on the honey stores they have set aside.
Cluster dynamics
In anticipation of the upcoming winter season, it is the beekeeper’s management goal to assist, as much as possible, in getting the colony prepared for winter. Though honey bee wintering biology is a surprisingly complicated system of temperature and humidity regulatory procedures, the basic procedure is nothing more than a group of cold-blooded animals (bees) huddling together to pool their warmth (clustering). They are not hibernating.
The wintering honey bee cluster works actively to control its environment when the ambient temperature is lower than they can tolerate as individual bees. At about 50°F, individual bees have limited capacity to respond to low temperatures and become increasingly unable to function. Though they can live for quite a long while, single bees will die if held at temperatures much lower than 45-50°F. The honey bee relies on sugar from honey as its energy source. As the temperature drops, sugar absorption in the bee’s gut is increasingly restricted. Ultimately, the chilled bee dies of starvation.
The winter cluster generates heat by micro-vibrations of muscles or flexing of individual bees’ flight muscles, movements which are so small as to be invisible. As beekeepers, we frequently think that bees have a special attribute when it comes to heat generation, but in fact, many insects can generate heat as readily as honey bees. The honey bees’ talent is in controlling the heat production over long periods of time. So long as they are well-fed, developing honey bee larvae are also excellent producers of cluster heat. The clustering behavior of adult bees and developing bees fundamentally makes the bee colony essentially become a single larger animal rather than a collection of many smaller ones.
The cluster of bees in an average colony forms a generalized spherical shape. As a colony is cooled, clustering becomes evident at about 64°F. A distinct cluster is formed at 56°F, but there may still be small satellite clusters near the main cluster. At about 32°F, most bees should be in the main cluster. The temperature at the edge of the outer cluster is maintained at about 55°F while the brood nest in the center of the cluster is maintained at about 95°F.
As the ambient temperature drops, a cluster has two ways to maintain its temperature: cluster contraction and increased heat generation. Contracting the cluster results in decreased surface area and increases the insulating capabilities of the outer shell of bees. To maintain controlled temperatures, the cluster expands or contracts depending on environmental temperatures. As falling temperatures drop below 41°F, cluster compaction is at its maximum. Increasingly, bees within the interior of the cluster must generate more heat by flexing their flight muscles. Healthy colonies with good supplies of honey have been reported to withstand temperatures as low as -40°F for extended periods.
Food stores
Established recommendations are for 90-100 pounds of honey, or ripened sugar syrup, for strong colony survival from October to April in the northern tier of the U.S. and parts of Canada. Smaller colonies can winter on smaller honey stores (about 27 pounds) but will expend more energy on heat production and will build up much more slowly in the spring than stronger colonies. Generally, a three-deep colony, with a telescoping outer cover, should have a gross weight of approximately 175 pounds in October.
Strong wintering colonies can exist on surprisingly small quantities of honey, about 3-4 pounds per month until brood rearing starts, after which, due to brood production demands, honey consumption increases dramatically. Occasionally, strong colonies that wintered well die in early spring because of stores depletion caused by increased larval food requirements.
Honey in the comb is the best winter feed for a colony, though colonies can pass winter seasons very well on well-ripened sugar syrup. Since they can take more cleansing flights, bees in colonies located in warmer climates can frequently survive on sugar syrup. If extra food must be supplied, the feed should be given early enough for the bees to process and store in combs. The tendency is for the wintering cluster is move upward or sideways, but never downward. Food stores should be on the sides of the cluster in the bottom deep and directly above the cluster in upper deeps.
After brood rearing is initiated in late winter and early spring, the cluster is forced to stay in a fixed position — usually within upper hive equipment. Warm weather breaks are necessary for bees to relocate stores from surrounding combs to nearer the brood nest.
Having high-quality stores readily available and properly positioned not only ensures more successful wintering, but will assist in lowering incidence of disease. Nosema is probably the most serious brood disease of the wintering colony. An infected colony at best results in a weakened colony and, at worst, results in the colony’s death. Naturally, standard mite treatments should be practiced as recommended.
Internal colony moisture
Internal colony moisture places stress on wintering hives that may facilitate the onset of diseases such as nosema and chalkbrood. For every 10 pounds of honey that a colony consumes, about one gallon of water is produced.1 Traditional recommendations are to ventilate colonies to allow the escape of moisture-laden air.
A common ventilation procedure, that I don’t particularly like, is the drilling of a single ¾” auger hole just below the handhold on one end of the brood chamber. Depending on the climate, both ends may be drilled. Though many beekeepers prefer these procedures, it will require additional work closing entrances when the colony is relocated in the future.
A second common procedure is to raise one end of the inner cover approximately ¼” with stones, wooden spacers, or twigs and replace the outer cover over the inner cover. If flat board covers, also called “migratory covers,” are used, raise them the same as inner covers. Such upper openings allow escape of air and allow bees a higher entrance should the lower entrance become blocked with dead bees, ice, snow, or leaves.
Internal frost is an indicator of ventilation adequacy. Some light frost within the colony is not undesirable, but excessive frost and ice is unacceptable. During warm periods, ice melts and cold water drips on the cluster. However, in cold climates, the opposite extreme is probably as bad. Excessive ventilation would result in wider temperature fluctuations with increased cluster temperatures and food consumption.
Winter management procedures
Inner covers
As a management procedure, inner covers should be turned deep-side-down during winter months. Note that not all inner covers have this reversible feature. The extra space beneath the inner cover will give the bees more space to cluster on frame top bars and to move food stores to different locations. However, the inner cover should promptly be returned to its normal position before the nectar flow begins during the following spring. There is risk of burr comb being put in the extra space.
Entrance reduction
Even if lower entrances are expected to be blocked by snow, ice, or leaves, they should still be reduced. Though hive entrance dimensions vary slightly, generally, the entrance height should not exceed ⅜”. This height will restrict …
