It is early spring and your beehive seems too quiet. You pop the lid only to find mold everywhere. It cloaks dead bees in furry coats, pillows above the bars, and drifts down between the frames. It covers the surface of combs and binds the masses of dead bees together in a smelly mat. There is no doubt in your mind: mold killed your bees.
But did it? In truth, mold in a beehive is a result of colony death, not the cause of it. Mold spores are everywhere in the environment, waiting for the perfect conditions to germinate into hairy tufts. In a beehive, those perfect conditions don’t normally exist until the colony is too weak to keep itself warm and dry. In most cases, by the time the mold starts to grow, the colony is past saving.
The best conditions for growing mold
Mold is fond of four things: moisture, food, pleasing temperatures, and porous surfaces. Beehives can be a dream come true for many species of mold.
Moisture: Just like seeds, mold spores need water to germinate and grow. Water can get into the hive in many ways, including humidity and leaks. But moisture in a winter hive comes mostly from the respiration of bees and from the decomposing bodies of dead bees.
Food: Like all plants and animals, mold needs a source of nutrients. Plenty of things in a beehive can provide nourishment, including pollen, nectar, dead bees, and other detritus. Even the wood is appetizing to some.
Temperature: Molds can be active over a wide range of temperatures, from just above freezing to around 120°F. But between about 70-90°F, mold is ecstatic, growing and reproducing in a frenzy.
Porous surfaces: Porous surfaces provide a nest of sorts, a place where the mold can latch on and not be swept away . Then too, irregular surfaces are good at capturing moisture and debris that the mold may use as food.
Healthy bees keep molds at bay
During the warm months, the bees have no trouble keeping the hive dry even though colony members are bringing in nectar that is full of water. All the fanning drives out the moisture and ushers in a new supply of outside air that can take up even more moisture. Meanwhile, all the busy bees are polishing and cleaning the inside of the hive and removing the detritus that mold loves. The result? No mold in sight.
However, the winter months are different. The bees are huddled inside. Their respiration contains lots of moisture. Since their breath is warm and the cluster is warm, the air leaving the cluster is warm. Warm air rises because the molecules jiggle around faster and take up more space. In essence, the air molecules are further apart, so warm air is lighter than cold air.
This light, warm air usually finds a way out, perhaps through cracks or maybe through vent holes or an upper entrance. When it leaves, it forms a vacuum that sucks denser, heavier air in through the entrance or screened bottom. This chimney has the effect of allowing the moisture and some of the heat to escape from the hive, while it brings in cold air from the bottom.
A typical winter hive
As an example, I have a hive that comprises a single deep, a medium super, a candy board, and a moisture quilt. On a rainy day last week, I measured the temperature of air going into the bottom entrance and coming out of the top entrance. The air going in was 40 degrees F at 100% relative humidity and the air leaving was 92 degrees F at 72% relative humidity. Remember that warm air can hold a lot more moisture than cold air (which is why we don’t have sticky, humid days in winter). Once the bees warm the air, it can absorb some of the excess moisture in the hive and deliver it to the outside, even though it’s raining. It works like magic.
What this means for the beekeeper
As beekeepers, we shoot for the best way to conserve warm air inside the hive but to allow the moist air to escape. These two things are at odds with each other, so we try to find the sweet spot that will best meet the needs of the bees.
But the sweet spot is a moving target. What is optimum will depend on climate, local weather patterns, the size and design of the hive, and the strength of the colony. In truth, the optimum will vary from colony to colony in the same apiary, which is why there is no one-size-fits-all answer to “What is the best way?”
When the colony weakens, mold takes over
Now, let’s say you have carefully designed your winter hive to keep temperature and humidity in balance. You considered local conditions and perhaps installed insulation, a moisture board, or a quilt box. You designed an upper entrance that was big enough to vent the moist air but small enough to prevent massive heat loss. Everything seems to be working.
But somewhere along the line, something goes wrong with your colony. It could be mites, queen failure, or a shrew infestation. For whatever reason, the size of the colony begins to diminish. As it gets smaller and smaller, fewer and fewer bees are available to maintain the temperature. The amount of circulation inside the hive—warm air out, cool air in—which is driven by colony heat, gradually decreases. When the colony becomes too small, the turnover of air is not great enough to keep the interior dry.
In fact, instead of circulating out, the moisture builds up inside the hive. It condenses on cool surfaces including the frames and comb, where mold seizes the opportunity. As the colony shrinks the mold expands, and by the time you open the hive, it appears that mold swallowed the entire colony.
The mold is not a sign of bad beekeeping, it is merely a sign that things got out of balance and the bees were unable to maintain mold-free conditions. The mold didn’t kill the bees but merely took advantage of the environmental conditions created by the faltering colony.
So the next time you find a hive full of mold, do not conclude that mold caused colony demise. Instead, look further. Complete your regular colony postmortem and try to learn what really crashed your colony.
Honey Bee Suite
Find the rest of the Physics for Beekeepers series here.