The Greatest Generation: Winter Bees
“Whether a colony survives the winter in good condition is determined more by its make-up than by the kind or amount of protection.” — Farrar, 19441
Beekeepers love to discuss winter hive protection — wrapping vs. insulation, upper entrances, quilt boxes or moisture boards, bee cozies, straw bales, and which way to turn the hole on the entrance reducer. Our hives may be the best protected hives in history! Our colonies, however, have the lowest rates of survival through winter. In the winter of 2018-2019 beekeepers in the U.S. reported losses of 38% — the highest ever reported in the Bee Informed Partnership survey, and much higher than what beekeepers consider sustainable.2 To improve winter survival, we have to pay attention to what Farrar said, turning our conversations away from hive protection, and focusing on the make-up of the colony. Whether a colony survives winter depends very little on what we did to the structure itself, and very much on the health and size of the colony — the precious and wonderfully adapted generation of winter bees.
Winter bees are a caste. In insects, the term “caste” is used to describe a physically distinct group of individuals that is specialized to perform a function in the colony. When we think of honey bee castes, we focus on queens vs. workers — female bees with very different bodies and very different functions. However, the worker bees are not a homogenous group, and we also see specialization among workers: summer bees and winter bees. Winter bees have very differently adapted bodies and very different functions than their summer counterparts. As honey bee colonies expanded their range north, bees were forced to adapt to survive cold temperatures and periods without incoming pollen. They had to figure out how to insulate themselves, create warmth, and store energy to survive long cold winters. Honey bees adapted by developing a special bee (winter bee), and a special behavior (the winter cluster).
Plenty of other small animals are adapted to survive the cold besides honey bees — Canada geese, snowshoe hares, penguins, otters, Arctic foxes, snowy owls, etc. all do just fine in prolonged cold temperatures. The reason all these animals do fine in cold temperatures is not because someone builds them a hive and vents it just perfectly and tilts it just right. Cold adapted animals survive because they keep their bodies insulated, and they store a layer of food. Honey bees are the same. They use the cluster to insulate themselves, and winter bees store a layer of food. As long as a colony is healthy, it can survive very cold temperatures. In fact, it was shown that a cluster of bees could survive minus 112 degrees Fahrenheit (-80 Celsius for 12 hours)!3
How do bees survive extreme cold if not in insulated hives? By acting like penguins! Emperor penguins in Antarctica don’t try to heat the entire frozen continent, they heat themselves by forming a tight cluster, with the outer individuals forming an insulating layer for the penguins on the inside. Honey bees act similarly; bees do not heat the hive — they heat themselves in a cluster. It would be super-inefficient for penguins to heat an iceberg, or for bees to heat a hive. Instead, the bees (and penguins) act as a unified superorganism, creating a warm insulating layer around their whole “body.”
The insulating layer is formed when the bees on the outer edge pack tightly together with their heads toward the center. When the bees are tightly packed, their branched hairs can interlace, trapping air and basically acting like a warm coat of fur. Even more amazingly, bees can prevent heat loss through their exposed abdomens by controlling their body heat using an internal counter-current heat exchange system. This system works by transferring the heat in the hemolymph leaving the thorax to the hemolymph entering the thorax. As hemolymph is pumped from the abdomen to the thorax it has to pass through the narrow, constricted petiole (waist) of the bee, where the aorta makes a series of hairpin turns. As the cool fluid from the abdomen winds through the twists and turns, it is warmed from the heated hemolymph returning from the thorax. The heat transfers to the incoming fluid, and stays in the thorax, so the bees lose very little heat through their exposed abdomens when in the cluster. Because of the interwoven branched hair, tightly packed bodies, and cool abdomens, the efficient outer layer of the cluster is an excellent insulator. In fact, it approaches the insulation factors of goose down or fur.4 We can now picture our winter colony as an animal with a warm insulating layer, designed to withstand cold temperatures.
So they don’t get too cold, the bees in the insulating layer rotate into the warm core of the cluster. If the body temperature of an individual bee falls below 42 F (about 5.5 C), the bee will enter a “chill coma” where it can’t move. Below 29 F (about -2 C), the body tissue of a bee will freeze, and it will die from cold in less than an hour.5 The temperature at the outer edge of the cluster is usually about 46 F (8 C), so the bees in the outer layer are kept right above the chill coma temperature. This means that it is important to keep bees in the insulating layer from getting cold too quickly. If the cluster is exposed to wind, the bees on the outside of the cluster can cool rapidly, fall into a coma, drop from the cluster, and freeze to death at the bottom of the hive.
This was shown in the 1970s by researchers at the UDSA/University of Wisconsin College of Agriculture.6 They performed an experiment where they overwintered four regular hives and four hives with walls made of screen. The temperatures in the clusters were similar in both styles of hive, even when they were brood rearing, and even during the coldest temperatures in January. The screened colonies only died after a storm with high winds. Presumably, the winds caused the outside bees to cool so quickly that they entered a chill coma and dropped from the cluster, rapidly shrinking the cluster, and exposing the next layer of bees. When the screened hives were in sheltered locations, away from piercing winds, the colonies could survive. Go ahead and open the lid to peek at the colony and check food stores — just don’t do it in a blizzard when you could quickly cool the outer layer of bees. Since the bees don’t let much heat out of the cluster, you won’t be letting much heat out of the hive.
Clustering behavior occurs without a centralized controller coordinating behavior, and in the absence of communication; the winter cluster emerges from the collective behavior of thousands of bees that only know their immediately local condition. They enter the cluster formation whenever ….