Preparation for winter (“overwintering“) and winter management is critical to the survival of honey bee colonies. As a beekeeper, your apiary management strategy should be to help bees stay healthy, lessen the burden of winter feeding and reduce winter losses. The best-managed hives are ready for winter well ahead of the first frost in fall. “A beekeeper should be ready for December in November (or sooner).”
What are these preparations? Fall varroa mite treatments should long be finished. (If you just read this sentence and had an “OMG!” moment, mite treatments should be prioritized!) Ventilation and insulation materials should be in place or close at hand. Winter feeding decisions should be determined. Higher colony weight – winter bees with sufficient nutrition – is a strong predictor of overwintering survival. To reach this goal, the colony must have adequate honey stores and bee bread reserves to sustain the colony during the winter.
As temperatures drop, adult bees form a “thermoregulatory cluster” around their queen and remaining brood. Honey stores provide energy for the bees which contract (flex) their flight muscles to produce heat for the cluster which, at the center, remains a comfortable 75 to 93 degrees Fahrenheit. Once the beehive’s internal temperature reaches 44 degrees (7 degrees Celsius), winter bees become too stiff to move around and risk freezing and/or starvation. [1]
Ventilation
Several factors affect the beekeeper’s decision to add (or not) an upper entrance – aka ventilation – to a beehive as part of the winterization process. (Upper entrances and ventilation for use in the spring, summer and fall could be considered, but this overview evaluates the upper entrance for winter bees.) These factors include the climate and local environment, use of a single or double brood box, and existing (or planned) beehive insulation.
Living in a wet, cold environment may necessitate ventilation: mold and mildew can buildup on the top board and excess condensation inside the hive can rain down and quickly kill the colony. (When warm, moist air comes in contact with the cold air surrounding a cold surface – which is at or below the current dew point – condensation on the surface occurs.) But humidity inside the hive is necessary for normal bee activities such as brood development. Another factor to consider is the likelihood that a bottom entrance could be blocked by snow or dead bees. So, the beekeeper must determine this balance for their apiary.
Winter months can be cold, and in some places very cold, and less humid. For areas with a cold winter, the bees are more dormant, less active. Inside the beehive, a lack of airflow can increase the carbon dioxide level and a high dew point can result in condensation; too much ventilation could allow the temperature and humidity level just outside the winter cluster to be similar to the exterior temperature and humidity, a situation which is detrimental to the colonies health. Again, the ventilation decision requires an understanding of the apiaries environment and a delicate balance of conditions affecting the bees health.
Can research guide our decision? Since Langstroth created a modular design for beekeeping, ventilation has been part of the winter preparation conversation. This ritual may stem, in part, from an experiment conducted in 1943 [2] that concluded top entrances in winter did not cause heat loss. For decades this experiment has been used as a reference to support winter ventilation. Refinements in the study of fluid mechanics and heat transfer, and recent research, help explain why this experiment may no longer be relevant: ventilation reduces (or eliminates) condensation on the top board but at what cost to the colony?
In 2017 an American Bee Journal article by engineer Derek Mitchell [3] described how the original experiment’s conclusions, based on a single thermometer and the application of a 15-watt heating bulb, were not inclusive enough to deal with a complex system of heat transfer. Mitchell’s research describes a “heat pool” created from heat generated by the cluster. Heated air rises and carries moisture but Mitchell’s modeling shows that maintaining temperatures above dew point is not possible even at moderately low outside temperatures. Adding a ventilation hole that resides in this pool invariably results in additional and unnecessary stress on the colony as bees must expend energy to replace lost heat due to ventilation. Placement of the ventilation hole should be carefully considered so temperature fluctuations and convective flows are directed away from the cluster’s center. More research that include the use of in-hive sensors will help guide and refine our winter ventilation decisions.
When ventilation is used and given sufficient time before winter sets, bees will propolyze upper entrances to reduce hole size which affects both heat loss and humidity within the hive.
Without ventilation holes, how do we eliminate (or dissipate) this killing condensation? Condensation can be minimized or eliminated in a couple of ways. A moisture board, wood shavings, or other absorbent material can be placed above the cluster. Depending on the recipe, a candy board could absorb condensation too. Consider how the absorbent material could also provide insulating value. Another way to reduce condensation is to insulate the hive either through exterior insulation or with a polystyrene hive.
Exterior insulation
In the winter, a cold beehive requires the bees to consume food stores for energy as they generate heat to maintain a cluster’s core temperature. Conversely, and perhaps counter intuitive to this thinking, bees in a “warm” hive consume fewer resources. If we consider insulating a beehive, we should understand an insulation’s “R-value” which measures its resistance to heat flow. But, insulation does not add heat, it only prevents heat from from being lost. Ideally, we want to have less heat flow out of the hive which equates to less heat loss. For this reason – to reduce the stress on bees and reduce the amount of consumed stores – it is important to increase R values for the hive body. This can be accomplished by adding insulation to the exterior of the hive.
Covered beehives have a higher percentage of surviving the winter than uncovered beehives. Even using tar paper as a minimal covering and wind breaks can help.
Highly insulated hives
Polyurethane hives and polystyrene hives provide better insulation than wooden hives. They maintain a higher internal temperature and more optimal relative humidity. They weigh less, are durable and long lasting. The cost of a polyurethane or polystyrene hive will be approximately 2+ times that of a similar wooden hive. But, when calculating costs, the beekeeper should include any costs of external insulation applied to wooden hives to get a better cost estimate.
Quilt boards and upper insulation
When air is heated, its density decreases, making it lighter and able to carry more water vapor. As hot air meets cold air, condensation forms. It is critical to the survival of the colony that this condensation not fall on the cluster. Winter clusters will potentially lose a majority of their heat at the top of the hive rather than at the hive sides. The advantages of top insulation are less condensation under the inner or upper board (crown board) and a warmer upper part of the hive where the cluster forms because heat is not lost from above and the cluster can be looser making movement to collect stores much easier. Insulating the top of the hive should be a standard operation. [1]
Candy boards
With sufficient honey stores, a colony should not require additional food resources. However, when honey stores are light or insufficient, or the beekeeper wants to provide an emergency food supply, a candy board, also known as fondant or sugar boards, provides a secondary (or primary) source of winter feeding. There are several excellent sources for candy board recipes and candy boards for DIY woodworkers.
Wind breaks
While it may not be considered “insulation”, an effective windbreak reduces the speed of air, its velocity, flowing at the beehive. This break reduces heat loss and acts as a buffer that enables insulation to be more effective. Types of windbreaks include wrapping the hive with tar paper and/or foam insulation boards, pallets and haystacks.
[1] Caron, Dewey. “Wintering Success”, ABJ, November, 2021
[2] Bill Hesbach, B (2020). The Condensing Colony. American Bee Journal, February, 2020
[3] Mitchell, D. (2017). Honey bee engineering: Top ventilation and top entrances.
American Bee Journal, 157 (8). p. 887-889.