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Physics for beekeepers: mold in a beehive

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.

Tip: If your moisture quilt is dry on the bottom and wet on top, it is working. A dry bottom means the air rising up through the quilt still has capacity to hold moisture. Only when the warm air contacts the cool under surface of the cover does it lose that capacity. The moisture it previously held condenses on the lid, drips down, and is captured by the woodchips.

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.

Rusty
Honey Bee Suite

Mold in a beehive, starting on a brood frame.
Mold getting started in a dead out. ©Rusty Burlew.

Find the rest of the Physics for Beekeepers series here.

Comments

Bernie
Reply

That happened to us this year, lost all 6 hives. We were thinking it was the moisture. Now we need to figure out how to clean up the mess so we can get new bees in place.

Dieter
Reply

“… Complete your regular colony postmortem and try to learn what really crashed your colony…”
Would this be worth to elaborate on in a blog?

Peter
Reply

Hi.

I’m located in London, UK. London is famous of humid winters so it was my main concern to keep hive dry especially, that my only one colony was very small going in to winter. As a self-claimed inventor I created insulation cover out of polystyrene to wrap up entire hive body. On top of crown board I put piece of polystyrene as well with little hole over the feed opening (my bees made it larger in the autumn). In the cold months I added plastic transparent box over the opening with vents to the bottom of it. It condense all moist and water drips to the polystyrene and drips away from the interiour. On top of that my bees seem to be recycling some condensed water to dissolve their stores.

Despite me thinking my hive gone queenless my bees are foraging on sunny days with high traffic by the entrance and seem to be healthy.

I am still hoping my queen is a shy one with laying eggs in cold UK spring.

Peter

frances I Moore
Reply

Bernie did u treat for mites.

David
Reply

Good write up. I was really hoping for a conclusion that included what to do with those moldy frames!

Donald
Reply

Great topic and well written as always Rusty. So . . . got a question for you . . . when you find a few combs with mold/mildew on them, do you feel ok with placing them into strong colony to clean and reuse, or woud you rather scrape dissenfect, and start all over.

Katie
Reply

Hi Rusty, thanks for the info – always so helpful! I have a sidebar question about mold: I had some comb/wax that was cut off the frame and stored in a glass jar with a lid. As a beginning beekeeper, I learned the hard way when I took it back out months later to see it covered in mold in the jar. Would it have been salvageable at all at that point? I was planning to use the wax for soap, etc. but pitched it after I saw all the mold.

Rusty
Reply

Katie,

In a case like that I would probably not use it. We are not as adept at cleaning it as the bees are.

Chris
Reply

I’ve found, in my ten years removing bees from their natural and semi natural enclosures. That they prefer a vertical chamber, like a tree, but will take anything as long as it has enough space. I’ve also found they don’t like air flowing through the hive like many bee keepers think. They’ve told me this by plugging up ventilation screened inner covers that I used to install on my hives after extracting them from the wall of a building or a barn. The center oval inner cover is perfectly designed to remove the moisture laden air, but not remove all the heat from the top floor of the hive where the nursery is located especially in early Spring. I think, many bee keepers are “chilling their bees” inadvertently. Just my observations.

Rusty
Reply

Chris,

I don’t know where you live but I would guess it is a place that doesn’t have a nine-month rainy season. The primary thing to remember is that all beekeeping is local. Where I live, moisture is always the main issue. Cold is barely a consideration because it seldom goes lower than mid-20°F, which is not cold for honey bees.

If I use one of those wooden inner covers with an oval hole, the water condenses and rains down on my bees as if they were in a shower stall.

Myrna Warren
Reply

Previously you posted a recipe for mite control strips. The amounts were: 25 ml glycerin and 25 grams of oxalic acid dehydrate (wood bleach). Can you give these amounts in measurements for a measuring cup or measuring spoons. I would like to try this.

Rusty
Reply

Myrna,

A teaspoon is approximately 5 ml, so 25 ml is about 5 teaspoons. It’s hard to measure 25 grams without a proper scale, but it’s roughly equal to 1.94 ounces by weight, not volume. You can buy a scale for not much money, it’s worth it. This is the one I use.

Al
Reply

Hi, I’m VERY new to this. I am very interested in starting beekeeping. If I just start with one hive, how long will it last? Does the colony stay in the hive? Do they leave after a year? Any answers or advice would be greatly appreciated. Thank you!!!

Rusty
Reply

Al,

You really need to read a beginner book. The hive is the house the colony lives in, and it can last many years. The colony will last as long as you can keep it alive using good management practices. Yes, the colony stays in the hive. Part of the colony may leave every year in the form of a swarm, or not, but the entire colony doesn’t leave unless something goes wrong.

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