Winter is coming. The bitter-cold time for bees brings an intriguing sight, a rainbow beauty within the hive, but there’s a hitch. One needs to see heat in the dark hive, and to detect its subtle variations, from frozen to cool to warm and so on to more intense temperatures, even to hot. That requires peering inside the warmth of the winter cluster, calling for a special beehive arrangement.
My bee house holds 30 single-comb observation hives. In the active season, these hives can be used for all sorts of experiments and observations. I have used them for studying usurpation, queen introduction, comb construction, and swarming, just to name a few. I have even let some small colonies attempt to over winter in these single-comb hives. While our winters are generally mild in Piedmont Virginia, we do get cold intervals of a few days at or near 0ºF (-17.8ºC). Small clusters with already limited heat production and retention have difficulty surviving such cold. And there’s another problem. The cluster is in contact with the glass panes. As we will see in more detail, glass is a poor insulator and drains the cluster’s heat away.
(Generally, those are the reasons why it’s not worth over wintering these single-comb observation hives. Even if the bees survive, for public showing, the hives would probably need to be disassembled and cleaned anyway, especially the glass. It would be easier and more attractive to start these observation hives over in the spring.)
For this thermal investigation, I used a single-comb observation hive, originally a small nuc, just outside of the bee house under its shed roof (see Figure 1). Normally, I pipe the bees to the outside of the bee house from the floor of the hive. Here I just raised the glass to form an entrance slit at the floor. Duct tape or metal clips hold the glass to the hive. To ventilate the hive, I made a small crack by separating the upper edge of the glass from the wood support. (This observation hive design is definitely not for public viewing. I can open the hive in about 30 seconds, one of its design features.)
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 its core warmth well above the frozen ambient temperature inside the hive. In that regard the bees function more like a warm-blooded organism.
On a cold morning on Nov. 9, 2015, Figure 2 shows the heat radiating through the insulating boards of the unwrapped observation hive, appearing as a warm-yellow glow. Figure 3 shows the colony with the boards removed, exposing the glass and more structure to the heat from 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, draining away their body heat. The glass, warm to the touch at the center of the cluster, revealed this heat loss. I have tried Plexiglass with the same result, warm to the touch.
Figure 4 shows the cluster in a different color scale taken on November 25, 2015, about two weeks later. Since converting the heat levels to colors is arbitrary, other scales are possible and may bring out different details. The photograph is on the entrance side of the hive and closer to the cluster. The capped honey is visible all around the upper half of the cluster, with a strange appearance though, the caps in eerie green.
This observation hive cluster is abnormal because it is a slice of a spherical cluster with multiple intervening combs. We can nonetheless learn much from it. First some more 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, during the most severe cold. 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 (i.e. not a honey bound colony).
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). 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 producing heat. The rate of heat production in the winter cluster approximates the warmth emanating from a small appliance light bulb with a brightness of 20-40 Watts.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 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
With the cluster in the observation hive, keeping the bees on the shell of the cluster warm enough apparently became a difficulty. The outermost bees, the ones on the surface of the cluster, would be the most at peril. The first sign of a problem …