Last time, I wrote about my over-wintering misadventures with wet hives. In many parts of the world (I’m looking at you, England.), the biggest winterkill comes from moisture, not disease or starvation. During the winter, water may collect under the hive’s lid and drip down on the cluster, soaking the bees and ultimately turning them into moldy compost.
it doesn’t take much to understand the problem – wet bees chill easily in winter. Chilled bees die. You can talk to local beekeepers to find out how they vent their hives – upper entrances, straw, or burlap bags draped like wicks hanging out of the upper cover. I’ll leave it to you to research what you need in your area. Instead, I’d like to look at how all that deadly water gets into the hive.
The water comes from the honey which the bees eat. Normally, there are about 9 pounds of water in every 50 pounds of honey. As the bees eat, the water is released. In summer, bees need water – for cooling the hive, diluting the honey they eat, mixing with bee bread, and as a mild laxative (“cleansing their metabolic wastes”). I’ve seen estimates that bees produce 68 pounds of water for every 100 pounds of honey they make. (I’m not sure how that figure was derived, but it’s in this paper.) This is all great in the summer when bees are cooling hives, feeding brood, and flushing toilets, but in the winter, little water is needed and usually none is actively collected.
In warm weather, vapour exhausted by the bees drifts out of the hive. Wrapped in winter long johns, the hives have trouble getting rid of water vapour. Significantly, and compounding the problem, cold air is not able to contain much water. That’s why you’re more likely to see dew in the cool morning, not in the warm afternoon. As water escapes the winter brood nest, it collects like dew above the cluster. There, it may freeze, then thaw and drip down onto the bees.
The difference in the amount of water in cold air, compared to warm air, is dramatic. This has nothing to do with warm air “holding” water more easily – the oxygen and nitrogen don’t slide apart, making way for water molecules, nor does CO2 grab water and ‘hold’ it in the air. Instead, with warmer temperatures, water molecules are more energized and bounce around. When chilled, the low-energy water is likely to condense. This condensation is the cause of morning dews and beekeeper woes.
In a bulletin written by Andony Melathopoulos, now a professor at Oregon State University, Andony shows us a breakdown of the water vapour concentration and the quantity of air which needs to vent out of a hive to get rid of a cluster’s exhaust. He shows that a colony gives off 3 to 10 grams of water each hour. (This assumes that the bees are eating about half a kilo of honey each day, roughly one pound per day and 18% of the honey is water.) Extremely cold winter air at 100% humidity has a water capacity of only 0.4 grams per liter. Warmed by the bees’ heat, the air above a cluster may reach 5 ºC (40 ºF) and then (temporarily) it can ‘hold’ 3.4 grams of water, hence taking up the 3 grams of water exhausted by the bees. But if that slightly-warmer, moisture-saturated air does not move out of the hive, the air rises and cools and then releases its water inside the hive. To prevent this, the water-laden air must move out of the hive before it cools and before it rains down on the winter cluster. To stay dry, a hive must remove a liter of air every hour. A sealed hive can’t do that.
So, your winter hive needs to breathe. Place the apiary on a sloped hillside where you have some air drainage. Use a hive vent system appropriate for your geography. If you make the mistake which I made a few years ago (see my last blog post), you may end up with a box of dead moldy bees. Don’t be me!