After writing a post about upper entrances in winter, I received a lot of mail from beekeepers who insisted that an upper entrance in winter would place the colony at risk of freezing. My own experience with upper ventilation has been the exact opposite, and my colonies have thrived since I began using upper ventilation combined with a moisture quilt. Nevertheless, I decided I should consult other sources.
In Honey Bee Biology and Beekeeping (2013) Caron and Connor explain:
Colonies can survive very well without elaborate wintering preparations by the beekeeper as long as the bees are protected from winter winds and they are able to vent excess moisture. . . . Beekeepers should provide upward ventilation in every hive during the winter . . . An alternative is to place a wooden shim, carpet tack or stick in one of the corners at the top of the hive. Some beekeepers prefer to drill holes in hive bodies or use spacers or inner covers designed to allow air ventilation.
Beekeeping in Western Canada, published by Alberta Agriculture and Rural Development (1998), puts it this way:
An upper entrance is an important requirement for successful outdoor wintering. The colony cluster gives off water vapour as it respires, which rises to the top of the hive and must be allowed to escape.
A passage in The Hive and the Honey Bee (2015) is equally emphatic:
Thus, beekeepers in cold climates must provide an upper entrance to allow water to evaporate out of the hive by drilling a hole in the box or providing an inner cover with an opening in the rim.
In The Beekeeper’s Handbook (1998), Sammataro and Avitabile insist the winter survival depends on a number of items, including
“An upper entrance for winter/spring cleansing flight” and “Top ventilation to release moist air.”
Bees rarely die of cold
Although it is written in many texts and papers, beekeepers tend to forget that bees are extremely well adapted to staying warm in winter. Contrary to popular thought, they do not keep their entire hive warm in the way a human keeps a house warm, instead they only keep the cluster warm. This is aided to some extent by the structure of the hive, but even open-air colonies can survive a moderately cold winter if they can stay dry and out of the wind.
In chapter 21 of The Hive and the Honey Bee, Currie, Spivak, and Reuter report that a cluster of 16,000 bees can survive -112°F (-80°C) for 12 hours. That is an amazing feat, but to succeed at those temperatures, the cluster must be dry and free of drafts.
This phenomenon is easy to reproduce. Just go outside wearing your regular clothes on a 40°F day (4°C) for 15 minutes. Then try it again wearing the same clothes doused with water. Evaporation is a cooling process. When the water on your skin evaporates, it makes you cold. It will evaporate even faster in the wind, which makes you even colder.
Finding a compromise
Dry bees can withstand extremely cold temperatures, but as they say, a wet bee is a dead bee. So your priority for wintering bees should be to keep them dry and out of the wind. Yes, ventilation provides some amount of air movement through the hive, but there is a give and take between too much and not enough. The trick is to find the sweet spot in the middle.
My enthusiasm for moisture quilts is partly due to this give and take. The moist air moves through the wood chips, condenses on the inside of the lid, and rains down on the chips. The chips collect the moisture and then dry slowly. They dry because the space above the chips is vented to the outside. Once dry, they are ready to collect again. And since the air going through the chips does not have a straight-line path—it must wend its way between chips—there is very little draft inside the hive.
Bumble bees can do it too
The amazing ability of honey bees to keep their nest warm is shared by other bee species as well. In A Sting in the Tale (2013), Dave Goulson describes how he had to destroy a colony of bumble bees because they were brought into his country for research only and could not be released. He decided that freezing them would be the most humane way to kill them, so he put the entire colony in a freezer at -30°C (-22°F). He writes:
The next day I came back to find the colony very much alive and buzzing loudly; the workers had gathered into a tight clump over the brood and were presumably shivering at maximum capacity. The queen was hidden in their centre, and seemed quite unperturbed.
A convenient opening
As I mentioned in my previous post, An upper entrance in winter, before this year I never used an upper entrance, only a number of ventilation ports above the wood chips. This worked fine, but this year I added an Imirie shim with an entrance just below the candy board.
All of my colonies have taken a shine to this opening, peeking out of it on cold days and flying out on warm ones. In fact, the bees in all my hives seem to have abandoned the lower entrances altogether. I wondered about this, but realized that part of the attraction may simply be that it is close. As winter progresses, the cluster moves up. How much easier it is to exit through the top, than to go down three stories and exit through the bottom?
Of course, there may be more to it than that, and it will be interesting to see what they do as the winter warms into spring.
Honey Bee Suite