Bonus: Bee Science with Dewey — Clustering and Winter Survival (BSD-1)

[Bonus] Short - Hive Debris Analysis with Dr. Dewey Caron

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Dr. Dewey Caron: Hi, I am Dr. Dewey Caron. I come to you from Cochabamba, Bolivia, where I spent part of winter season with my wife's family. I present another audio postcard in my continuing series of once-monthly Beekeeping Today Podcast. This month, I am unveiling a new series, Bee Science with Dr. Dewey Caron, a Beekeeping Today Podcast mini-series. Each episode will blend research, field experience, and seasonal context, focusing on the why behind honey bee biology behavior. I welcome your suggestions for timely topics. Let me start with a short introduction of me.

I currently am part time content and communication specialist with the Oregon master beekeeper program of Oregon State University. I moved to Oregon in 2009 after retiring from the University of Delaware. My faculty appointments at Cornell University, University of Maryland, and since 1981 at University of Delaware I had various responsibilities. I taught large classes of entomology and entomology lab courses, plus not quite as large beekeeping classes at both Maryland and Delaware, as well as the first one at Cornell. I taught large, up to 200 students classes of wildlife ecology at Delaware as well.

I've been extension specialist with duties in pollination and bio control. I have performed research on pollination and bee diseases, as well as in biocontrol, using basically predacious mites and insects to help control serious insect pests of the landscape and greenhouses. I was acting Entomology Department chair at Maryland, and entomology wildlife ecology chair at Delaware. I have even had an initial appointment as chief of apiary inspection in Maryland. My research included somewhat over a million dollars in grants. I averaged over two peer-reviewed publications in my 40 years at University of Maryland and Delaware, and continue to publish both research publications and popular pubs on beekeepers and bee biology.

Well, that's enough about me. For my first topic, one, Bee Science, very apropos for January, is clustering behavior. The basis of honey bee winter survival lies in their ability to form a winter cluster. The winter cluster is a precisely organized, dynamic structure that functions like a biological heating system. It is worker bees instinctively gathering together, and they form a roundish to oval shape formation centered around the Queen and remaining brood. Ideally, it forms between boxes if you are in a two or three-box hive configuration. Initially, mostly in the lower box, and it moves upward as the season progresses.

This initial positioning facilitates communication from one frame to another, something difficult at lower temperatures when bees can't pass between full curtain frames. We also see clustering behavior of bee swarms in bivouac locations and bee bearding. Some want to label swarm bivouacs or bee beards at the colony entrance, merely as bee congregations, not clustering. A question for you is, should cluster be a term restricted merely to the winter cluster? I looked up the meaning of the two terms, and of course, most of the several listed meanings are in reference to human behavior.

I was especially struck by one definition offered by Wiki Diff, D-I-F-F, Wiki Diff as nouns. The difference between congregation and cluster is that congregation is congregation, while cluster is cluster. Mmm, I thought this definition was not too helpful to try to differentiate, but I do believe the term can be correctly used in all three instances because winter clustering, swarm bivouacs, and bee beards are basically formed under similar conditions. They are a response to temperature. Besides looking the same to us and our tendency to use the term clustering in all three instances, both congregating and clustering behavior provide a means for bees to communicate.

The social honey bee has several modalities for communication, and behaviors such as clustering can be seen as socially enabling the bees to exchange this vital information. You don't have to agree with me, but for the rest of the podcast, I will consider all three congregations of worker bees as clustering. How do you feel? Clustering bees arrange themselves in layers. The outer layer bees, termed the insulating shell, is formed by thickly interlocking bees with their heads facing inward. This crush of bodies creates a thick, interlocking mantle designed to minimize heat loss.

Deeper within the huddle of bees, there is less congregation, and incredible as may sound, bees are able to move freely around in the middle. Heat generation within the core is an active metabolic process produced by isometric, vibrating thoracic flight muscles, that is basically the bees activating their muscles without moving their wings. When developing brood is present, heater bees workers that can greatly elevate their body temperature, such as over a cap brood, or within an empty cell with brood in immediately adjacent cells, will also contribute to heat production in the cluster core.

Similarly, the swarm cluster forms an insulating shell, for example, during rain by arranging their wings to act as a shingled roof to keep interior bees dry. An insulated shell also forms as the temperature drops, for example, in the evening, when the interior less dense, as in the winter situation. During rain, or as the temperature drops, bees at bee beards seek shelter to cluster beneath a hive, if that's possible, depending on your hive stands, or the bees move back inside their home.

The colony transitions from a dispersed state to the beginning of a cohesive, insulated, outer and inner heat-generating structure when the ambient temperature drops to about 57 degrees. That's the beginning of bees starting this gathering together. As the hive temperature continues to drop, normal movement ceases inside the hive or in the swarm bivouac, and the cluster is fully formed around 50 degrees.

On the coldest days, nights, the cluster will contract more tightly. When it is warmer, the bees relax, the insulating shell may even disperse, and bees may move. Some bees may even attempt to leave to forge or to excrete excess waste. Outside temperatures may be lower, and these individual bees may not be able to keep their body temperature warm enough for a return flight home. If there is brood present, the core temperature will be from around 91 to around 97 degrees Fahrenheit. If the hive lacks brood, the core will be cooler, anywhere from as low a 64 to as much as 85 degrees.

The body temperature of the shell bees gets colder, bees go into a state of torpor or chill coma. Some state that as an individual bees body temperature lowers, they will move inward toward the warmer core. However, in the torpor stage, they are sluggish, and their nervous system may not be able to make a decision to move. That may mean they must be pushed inward. Another question for you, is it push or pull?

A good contradiction for a student to study. Chill coma for individual bees is right around 50 degrees. The absolute lowest temperature which a bee will die is a function of both time and temperature. Bees that remain in a chill coma for 48 hours at or below 50 degrees will die. The absolutely cold death temperature for a honey bee generally falls between 20 and 30 degrees Fahrenheit. Good information on temperature behavior can be found in a Science Insights article I reference in the endnotes. The size and compactness of the winter cluster inside a hive varies with temperature, so does the size of a bee beard in a swarm cluster.

Exposed swarms can be quite compact, but still with a core more open. Winter clusters occur inside a hive, while swarm clusters and bee beards are behaviors outside the hive. Clustering inside or out is a worker behavior. While drones might be present, they are normally absent in clustering. The winter cluster will slowly move upward, eventually outward as it reaches the top of the hive. This enables it to stay in contact with honey stores. Office stated that a greater mass of bees provide superior insulation during winter, and more populous forms are more likely to survive rain or cold nights.

Honey stores allows the colony to generate heat with less individual effort, resulting in lower total honey consumption per bee. Conversely, smaller, weakly populated colonies will survive winter too, although some may lack the biomass needed to create a dense mantle, depending on the winter temperatures, leading to inefficient heat retention and a higher likelihood of collapse. Some bee races do well with smaller populations, such as Carniolan and Caucasian bees, Africanized bees as well, while classic Italian or German bees overwinter as larger colonies. Nucs with proper stores will certainly survive as well.

If a hive runs out of food, it will starve and succumb to the cold. Starvation as a winter death-out may occur in a well-stocked colony. We need to do a nuc necropsy of the dead-out to determine what happened. One reason might be because the cluster did not move enough to stay in contact with the honey in the hive. Perhaps the honey was in a super, not in the brood area, or there was a cold spell with an expanded brood area, and the bees didn't access enough honey to be able to maintain the temperatures. Even the genetics of the bees might have been a factor as well.

Starvation may occur in a colony comprising the cluster because the bees couldn't reach the honey. In longitudinal and top bar hives, the brood may be central with hot winter stores to the right and left side of that cluster location. Of course, the cluster can only go in one direction, basically away from half the honey they may have needed to survive winter. Conversely, small or weak colonies may lack that biomass. The general wisdom is just as when producing surplus honey. It takes a populous hive for successful wintering and for a successful swarming, i.e, it takes enough bees.

The most challenging time for the wintering colony is in late winter/early spring, when warm spells and the smell of spring, plus the increasing day length, are powerful signals to ramp up brood rearing. At this point, the pressure to expand the brood rearing area needs to be balanced against the need to return to a tight cluster at night or during that cold, rainy winter snap of the spring weather. How they engineer these conflicting needs can be the difference between death and survival. For cold-blooded bees, temperature should, of course, matter. This is why we believe they cluster inside a cold hive, in a swarm bivouac, or a bearding. Clustering helps keep everyone warm.

You may observe features related to temperature. Wintering bees may exhibit their hive at temperatures below 50 degrees. We generally believe they must do so due to accumulation of body waste, but then die outside, unable to return. When snow covers the ground, some dead bodies will be evident, or maybe a pile of bees on the landing board or immediate below the entry on the coldest days. The simple explanation is the body temperature of these bees fell to 50 degrees or below. They were unable to warm up enough to get back inside the hive. For this situation, perhaps you may have had the surprise of dead bees coming back to be alive when you moved the equipment of what you thought a dead-out into warmer storage. They were in that chill coma stage.

There's another contradictory opinion about clustering. Derek Mitchell, a mechanical engineer from Northern England, has challenged the widespread belief that honeybees naturally cluster to insulate against cold. He says his research "appears to contradict the widely accepted theory that the bees' reaction to cold temperatures is to form layers of insulation." He faults the Langstroth hive as potentially subjecting a colony to thermally induced stress due to beekeepers using "inadequately insulated hives." Mitchell believes clustering is a distressed behavior rather than a natural positive response to falling temperatures. He says, "deliberately inducing clustering by practice or poor hive design may be considered poor welfare or even cruelty in light of these findings." How does this argument strike you?

We know the original Langstroth hive was, in fact, insulated by construction design. It was double-walled with interior glass walls, insulating comb honey supers. What we call our current Langstroth hive is a simplified version developed after Langstroth's death. The simpler design was easier to construct and could be shipped in pieces for assembly by the beekeeper. Recommendations in the coldest climate was to insulate the entire hive during the winter months.

Additional to the element of hive design is the issue of providing an upper opening for the winter hive. Mitchell and others, such as Etienne Tardif, also a mechanical engineer, a data-driven beekeeper who has successfully kept bees for 13 seasons in the far north of the Yukon in Canada, stressed the importance of the size of the heat sink in a wintering beehive. Basically, in the colony, cold air enters the hive at the entrance is then heated by bees. This heated air, which holds more moisture, rises and is trapped at the top of the hive. It forms a heated pool.

If a bee cluster is located when this heat bubble, this heated pool, the bees will need to produce less heat, meaning they will consume less honey to shiver to activate wing muscles. If there is an upper entrance, this heat pool will be smaller as some of the warm air will be whisked out the upper entrance. We can see that bees, in fact, do not welcome an upper entrance, as they will propolize it closed or reduce it to the size of a single bee opening in the fall before the temperature drops below 57 degrees. After that, they may not have a temperature warm enough to process the propolis to close in upper entrances. Upper entrances, for those who like them, located below this heat pool, are okay. In fact, bees in the coming spring may be using that upper entrance, especially when the bottom entrance is a ways away or gets iced over during various storms.

Well, beyond temperature and fuel, the management of moisture is a factor in winter mortality. When bees metabolize honey for heat, they produce water vapor as a byproduct. If the hive lacks adequate ventilation, this warm, moist air condenses on cold surfaces at the top of the hive, chilling the bees and creating an environment conducive to mold and disease. A colony is often more susceptible to death from damp conditions and this moisture condensation falling back on them than from the cold air itself. This can simply be avoided by adding supplemental top insulation to hives. How do you feel? Is this intricate, deliberate behavior of clustering not an adopted behavior evolved as bees move from one more tropical to more temperate climates, from out of Africa into Europe, for example, or is it adding more stress during an already stressful time for the colony?

In summary, it's really apparent that bees benefit from extremely close contact with related family members, especially exemplified by clustering. It is bee teamwork. It enables communication. Our bee colonies may look congested, but in fact, that is the normal condition. Through teamwork and careful energy management, the honeybee cluster allows bees to live through cold temperatures that no one bee could survive alone. Each bee plays a role in maintaining the cluster, whether it's generating heat at the cluster core or rotating in and out of the shell. Beekeepers who understand the dynamics of this behavior are best equipped for beekeeping success.

This is the first of what I hope will be a series on Bee Science. I welcome your suggestions. Until next time, bee well.

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