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Winter is Coming
Warm and Snug Beats Cold and Drafty
Theresa J. Martin
Bees evolved to live in a thick-walled, well-insulated, stuffy, humid tree cavity with a single, small 2-square-inch entrance at the bottom of the cavity. The walls of a tree cavity are often between 5 and 14 inches thick, with infinite insulation above and below the cavity (Figure 1). The sides have an R-value of 6 to 30. R-value measures the insulation effectiveness of a material, where a higher R-value indicates a higher resistance to temperature change. A tree cavity with high R-value displays temperature stability and retains heat well. In fact, a tree cavity is so well insulated that research indicates colonies living in a natural cavity rarely cluster in Winter and instead maintain a dispersed configuration.
Bees also capitalize on moisture inside a natural cavity in Winter. Cold air comes in through the single small entrance at the bottom of the cavity, rises, and as it passes over the nest, picks up heat and moisture expelled from the colony of bees. The air continues to rise until it reaches the top of the cavity. Because the top of the cavity is usually a tree trunk containing thermal mass, the warm moist air moves to the relatively cooler sides of the cavity and drips down the waterproofed sides coated in propolis. The bees collect the condensed water from the sides of the cavity and use it for homeostasis and brood rearing.1
Most colonies select a natural tree cavity that contains a single, small entrance at the bottom, with no upper entrance. Multiple studies demonstrate the advantages of having no upper ventilation from a thermoregulation perspective. Because heat rises, the lack of an upper opening reduces heat loss. In addition, a lack of upper ventilation increases carbon dioxide levels. Honey bees exist in environments with carbon dioxide as much as 30 times the normal amount. Multiple studies found higher levels of carbon dioxide significantly increase Varroa mortality.²
Clustering Rare
While colonies occasionally cluster in a tree cavity, up-to-date research indicates it is likely a rare behavior. Conversely, managed hives that are ¾-inch thick have an R-value of only .84, demonstrating poor, almost non-existent insulating qualities. Clustering in a managed hive “may only be a coping mechanism” and a “compulsory, frequent, life-saving behavior” of bees trying to survive Winter in an impoverished thin-wall hive (Mitchell, 2016, p. 1).
In addition, the prevailing guidance is to put an upper entrance on poorly insulated hives to release moisture. Upper entrances let out moisture yet also let out heat the colony worked hard to generate to keep themselves warm. This causes the colony to vibrate their indirect flight muscles continuously to replace the lost heat. Colonies consume more honey because of the increased effort required to replace the lost heat.3
While colonies can and do survive Winter in an uninsulated managed hive, the metabolic cost is higher, reducing the longevity of long-lived Winter bees. Shivering adds to the immunity challenge and energy demands, compounded by other pressures such as pathogens, pesticides, and poor nutrition.
Operation Size Dictates
Winter Approach
Sideliners and commercial beekeepers with hundreds or thousands of colonies often move colonies to warmer locations or overwinter inside cold storage facilities held at ~46°F.4 Beekeepers with a smaller number of colonies can insulate their hives with the goal of creating an environment that is closer to a tree cavity. “The implied improved survival in hives with thermal properties of tree nests may help to solve some of the problems honey bees are currently facing in apiculture” (Mitchell, 2016, p. 1).
As the research on insulation advanced, beekeeping supply companies created insulated options, such as polystyrene hives, which provide over six times the insulation value compared to a ¾-inch wooden hive body. Polystyrene hives also provide year-round insulation, including in the Summer (Figure 2). This may be contrary to conventional thinking, yet research shows colonies also expend significant energy cooling colonies in Summer.
In 2020, Bill Hesbach put forth the “condensing colony” concept that recommends a heavily insulated hive top with no upper ventilation.5 Because the top of the hive is well-insulated, the moist warm hits the well-insulated cover, moves to the sides of the cover where it condenses, and may run down the sides of the hive. This configuration provides moisture away from the Winter cluster and in a location where it can be used by thirsty Winter bees.
Warm and Snug
I live in Kentucky, where we experience several weeks of sub-freezing Winter temperatures. In mid-October, I winterize my 20–25 hives and remove winterizing materials in early April (Figure 3). Starting in October, I stay out of my hives completely and avoid disturbing the colony. My winterizing regime includes:
• Add 2.5-inch foam insulation under the telescoping cover (R-Value 12.5) (Figure 4).
• Under the 2.5-inch foam insulation, add a medium box with two layers of fiberglass insulation (R-Value 38) (Figure 5).
• For insurance against starvation, place a sugar block on top-most frames (Figure 6).
• Include zero upper ventilation or upper entrance of any kind.
• Add 1.5-inch foam insulation on four sides (R-Value 7.5).
• Reduce the entrance to 2-inch x ¾-inch and install a mouse guard (Figure 7).
• Beneath the IPM board, add 1-inch foam insulation under the hive (R-Value 5) (Figure 8).
My colonies are as warm and snug as I can get them for our five months of Winter. Yet I know that no matter how hard I attempt to give them the best overwintering experience possible, I am still not getting close to the superior Winter conditions colonies experience in a tree cavity. This is the nature of beekeeping. We have so drastically modified the environment in which our managed colonies live that even with our best efforts, honey bees can do better without us.
Theresa J. Martin is the author of Dead Bees Don’t Make Honey: 10 Tips for Healthy Productive Bees, which includes a Foreword by Dr. Thomas Seeley. Theresa has achieved 99% colony survival and honey production that is twice the local average in her seven years as a beekeeper, with 20–25 colonies in Kentucky. She can be reached at theresa@littlewolf.farm
REFERENCES
1Seeley (2019) provides our best understanding of colonies living in natural cavities. Radcliffe and Seeley (2022) contrast the temperature dynamics in a tree cavity and wooden box with and without insulation. Mitchell (2016, 2017, 2023) provides engineering studies that detail thermal properties of cavities versus managed hives, which casts doubt on the long-held assumption that colonies cluster as a normal method of keeping warm in Winter.
Seeley, T. D. (2019). The Lives of Bees: The Untold Story of the Honey Bee in the Wild. Princeton University Press.
Radcliffe, R. W. & Seeley, T. D. (2022). Thinking outside the box: Temperature dynamics in a tree cavity, wooden box, and langstroth hives with or without insulation. American Bee Journal, 162(8), 893-898.
Mitchell, D. (2016). Ratios of colony mass to thermal conductance of tree and man-made nest enclosures of Apis mellifera: implications for survival, clustering, humidity regulation and Varroa destructor. International Journal of Biometeorology, 60(5), 629-638. https://doi.org/10.1007/s00484-015-1057-z
Mitchell, D. (2017). Honey bee engineering: top ventilation and top entrances. University of Leeds. American Bee Journal, 157(8), 887-889. ISSN 0002-7626. eprints.whiterose.ac.uk/141140/5/honeybee_engineering.pdf
Mitchell, D. (2023). Honeybee cluster-not insulation but stressful heat sink. Journal of the Royal Society Interface, 20:20230488. https://doi.org/10.1098/rsif.2023.0488
²Studies show colonies living in natural cavities without upper ventilation experience higher levels of CO2, which has the advantage of increasing Varroa mortality
Seeley, T. D. (2024). Piping Hot Bees and Boisterous Buzz-Runners, p. 1-13. Princeton University Press.
Onayemi, S. O., Hopkins, B. K., Sheppard, W. S. (2022). Elevated CO2 increases overwintering mortality of Varroa destructor (Mesostigmata: Varroidae) in honey bee (Hymenoptera: Apidae) colonies. Journal of Economic Entomology, 115(4), 1054–1058. https://doi.org/10.1093/jee/toac065
Kozak, P. R., Currie, R. W. (2022) Laboratory study on the effects of temperature and three ventilation rates on infestations of Varroa destructor in clusters of honey bees (Hymenoptera: Apidae). Journal of Economic Entomology, 104(6), 1774–1782. https://doi.org/10.1603/EC08278
3Weinberg (2023) conducted controlled studies that show colonies consume more honey to fuel both heating and cooling activities.
Weinberg, I. P. (2023a). Honey bee ecology in a changing landscape: Comb phenotype, nutrition, and self-organization in response to exogenous pressures (Doctoral Dissertation, Tufts University).
4Indoor Winter storage has become more common amongst commercial and sideline beekeepers.
Hopkins, B. K., Long, J., Naeger, N. L., Sheppard, W. S. (2023). Comparison of indoor (refrigerated) versus outdoor winter storage of commercial honey bee, Apis mellifera (Hymenoptera: Apidae), colonies in the Western United States, Journal of Economic Entomology, 116(4), 1063–1068, https://doi.org/10.1093/jee/toad109
5Bill Hesbach explains the condensing colony in several papers and in Episode 50 of Two Bees in a Podcast.
Hesbach, W. (2020). The condensing colony. American Bee Journal, 160(2), 170-180.
https://bluetoad.com/publication/?m=
5417&i=646365&p=56ver=html5ation/?m=5417&i=646365&p=56&ve
r=html5
Ellis, J., Vue, A. (2021, March 18). Two Bees in a Podcast. University of Florida. Episode 50: Insulating Honey Bee Colonies & Good Neighbor Guidelines. https://entnemdept.ufl.edu/honey-bee/podcast/
