Winter can be a difficult time for bees. After fall management, the colonies, left alone, try to survive through the cold months while the beekeeper waits for survival news come spring.
Through the winter, I keep watch on winter clusters with thermal cameras in the apiaries or study them in the bee house with observation hives. The bee house holds 30 single-comb top-bar observation hives. While these small observation hives are mostly for studies during the active bee season (see Figures 1a and 1b), I study the small clusters while trying to winter them, a problematic proposition. It does force one to confront, up close and in detail, the thermal mechanics of bee clusters surviving in the cold.
For comparison to upcoming examples, here is a single-comb observation hive, originally a small nuc, just outside of the bee house under its shed roof (see Figure 2). Normally, I pipe the bees to the outside of the bee house from the floor of the hive. For this hive, I just raised the glass to form an entrance slit at the floor. Duct tape or metal clips hold the glass to the hive.
In warmer temperatures, the bees are dispersed throughout the hive. As the temperature drops to about 57ºF (13.9ºC), the bees form a well-defined cluster, roughly a spherical mass of bees in a full-size hive. Many of the bees occupy the interior of the cluster and are not directly exposed to the cold, except for the bees on the surface of the cluster. While in a winter cluster, the bees can keep their core cluster warm, well above the frozen ambient temperature inside the hive. In that regard the bees as a group function more like a warm-blooded organism.
On a cold morning on November 9, 2015, Figure 3 shows the heat radiating through the insulating boards of the unwrapped observation hive, appearing as a warm-yellow glow. That heat loss seems too much, although I did not have a quantitative analysis of it.
Figure 4 shows the colony with the insulating boards removed, exposing the glass and revealing more of the heat structure of the cluster. In this heat scale, the warmest is white then cooling to red, yellow, green, and finally a cold blue. The cluster remained large, covering the central area of the comb. However as mentioned above, the glass is in contact with the bees, slowly draining away their body heat. The glass, warm to the touch at the center of the cluster, revealed this heat loss. Plexiglass gave similar results, warm to the touch.
Before proceeding into the cold time of the winter, let’s review some biology of winter clusters, beginning with heat retention. Within the space limits between the bees, the cluster can contract, as it becomes colder with the arrival of the worst of winter. This contraction causes a further decrease in the surface area of the entire cluster. With less surface area to lose heat through, the cluster reduces its heat loss. In addition, some bees crawl into empty cells deep within the cluster. The cell-bound bees make the cluster more compact, which helps the cluster to retain more heat. The lower area of the brood comb should have empty cells for the cluster to form properly. Therefore a winter cluster should not form completely on sealed honey cells.
In regulating the temperature of the cluster, the bees have different functions depending on their location in the cluster. The bees in the interior of the cluster produce the heat, and the bees on the periphery of the cluster, at or near the surface, function as an insulating layer. The interior bees actively produce this heat by micro-shivering their flight muscles without moving their wings.1 The temperature within the cluster partially depends on whether the bees are rearing brood. When brood is present, the bees maintain that region of the cluster at the brood-rearing temperature of 90-97˚F (32-36˚C).2
Under broodless conditions, the bees maintain a cooler interior cluster temperature. However, they do not let this interior temperature fall below about 64˚F (18˚C), which is their lower limit for heat production. The rate of heat production in the winter cluster approximates the warmth emanating from a small incandescent light bulb with a brightness of 20-40 watts (not an LED bulb).2
Several layers of bees in close contact form an insulating shell around the heat-producing bees. The bees in the shell of the cluster orient themselves with their heads pointing into the cluster. This orientation leaves the outermost layer of bees with their abdomens exposed to the cold air on the surface of the cluster. (Notice that the colony does not heat the entire interior of the hive. Rather, the cluster only warms the region that it occupies.)
The hair on the bees comprising the insulating shell enhances their effectiveness as insulators. Instead of having hair follicles each with a simple unbranched shaft, as found with our hair, the shaft on a bee hair has many branches, increasing its insulating value. Bee hair, referred to as plumose hair, has a structure similar to goose down.1 With many tightly packed bees comprising the shell of the cluster, their plumose hair, in a collective sense, has a blanketing effect that helps to retain heat and keep the cluster warm. The heat-producing bees maintain the bees in the shell of the cluster at a cooler temperature, a little above 50˚F (10˚C). Just a few degrees less and a bee will slip into a chill coma and die.2 (Note, honey bees also have simple-shaft, non-plumose hair, like the “hair” that grows from between the ommatidia on their compound eyes.)
In the fall of 2021, I made up four observation hives (see Figure 5). I heavily insulated these hives before the cold set in. Covering the glass sides, first came a thick 2-inch (5.08-cm) insulating board (see Figures 6 and 7). A thick reflecting space blanket (with a red backing) covered the top of each hive (see Figure 8). A thin space blanket (reflective on both sides) was loosely wrapped around each hive (see Figure 9).
The initial health status of the four colonies, and probably the inherent natural variation among them, greatly influenced their survivorship. When the weather began to cool, keeping the bees in the hive, and the nights turned cold, scattering frost over the fields, the bees began dropping out of the clusters. Figure 10 shows Hive 1 deep into the winter.
All through the winter I had to remove the dead bees accumulating under the clusters, usually when the weather warmed. If not, I did it when the temperatures were not that cold. I preferred working at night. From one side of the hive, I removed the insulation and cut the duct tape holding the glass to the hive. After breaking the propolis seal, I slid up the glass, forming a crack at the hive floor. With a set of large tweezers, I pulled out the dead bees and saved them in containers matched to each hive (see Figure 11). I had to keep up with the dead bee removal or they would block the entrance pipes, and the bees could not fly during warm spells.
In typical beekeeping, the winter bee death may be somewhat cryptic. In some scenarios the bees, presumably sensing they are sick, self-evict, flying into the winter cold to chill and perish. The winter cluster dwindles away, its dead bees mostly invisible. If dead bees are scattered over the bottom board from a ….