physics for beekeepers

Physics for beekeepers: mold in a beehive

Moldy combs: Mold on a brood comb.

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.

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.


  • 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.

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

  • 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.


  • 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.

  • 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.

  • 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.

    • 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.

  • 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.

    • 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.

  • 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!!!

    • 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.

  • My bees died this year, in Ohio. I did find mites on the dead bees.They left lots of honey. There is a light coat of mold on the outside of the honey. Can I use that honey for human consumption? Thanks

    • Dani,

      First make sure it is actually mold and not bloom. Id it turns out to bee mold, wash or spray the combs with a mild bleach solution before you extract. The mold is harmless but it can impart a mildewy taste to the honey if you don’t remove it first.

  • Hi Rusty,

    First off I want to say that I have found much wisdom on your website; and I would like to Thank you for it!!

    Secondly; a question: a second year hive was of “medium-strength” in March. Treated for varroa with the ‘lighter option; Mite-Away Quick strips”, and could not remove until more than 3 weeks later (due to weather).

    I did the same with my other hives too. We had a dreadful month, with freezing rain, ice pellets, snow and plus brutal winds.

    Two days ago I found the hive I had labeled ‘Medium’ was a dead-out. ): Other two are “strong” hives nearby, no evidence of disease or Varroa

  • Hi Rusty!

    I so appreciate your posts as I am a newbie beekeeper.

    We are in our second season and while our bees survived the winter, I mistakenly neglected our colony as we had a new baby and life happened. Only now did I opene up our hive (2 deeps) and found our bottom box empty with what I think is mold and MAYBE wax moths? The top box is full of honey. I think somewhere along the way the queen died or left as I see no eggs in any of the cells.

    I quickly picked up a new queen.

    I took photos and would love your opinion.

    I’m in Seattle.

    Thank you!!!

    • Erika,

      Mold is the result of too much moisture in the hive. If the population was low, there may have not been enough bees to fan out the excess moisture from the honey.

  • In our County, because of one very highly respected beekeeper, who does have lots of good information to teach, “ventilation” is prescribed. Inner covers with 4” notches and screens are recommended so as to “prevent moisture from raining down on bees.”
    I took his classes 11 years ago when I started, and did what he said. But since I remove bees for a living and have done over 500 extractions, I now have seen how bees “live”. They don’t like drafts, or, put another way-“ ventilation”. Even when we think we are doing them a favor by “cracking open the top cover” on a super hot day-we are not helping them. They have their own air conditioning system and we just override that by opening hive to hotter, outer temps.
    There will be moisture in hives just like the one described, weak colony, dead of winter, no big deal. Hive will either recover or not but moisture won’t hurt them. It’s a lack of bodies to keep warm that will kill them perhaps, but not mold. People are “chilling their brood” with the best of intentions here in Sonoma but bees like stuffy, dark, claustrophobic cavities, preferably vertical-think tree, not top bar, so they can control heat dynamics in hive best.
    My two cents…

    • Chris,

      I disagree completely. Left to themselves, honey bees choose homes in hollow trees or human structures that have a lot of moisture absorbing capacity, so they don’t need ventilation in those places. Man-made hives, on the other hand, have almost no moisture absorbing capacity, so ventilation is necessary. Comparing a man-made hive to a bee-selected home is like comparing apples and oranges…not the same at all.

  • I could not disagree more emphatically based. On my 1- years of “box” beekeeping an over 550 wall, tree, barn, cement container, bee extractions. I have a rare perspective, I’ve seen way more than, lots of bees living on all types of structures including boxes.”
    So here’s the deal, differing from your description of things, in nature, in a “bee tree” a tree that’s had bees for a long time, the insides are totally coated with the resin propolis-the most water resistant material available to a bee. In nature, in a tee, if there is condensation-not an enemy, the bees drink from that and air condition hive. It never “drips in bees” if anything, it slides down resin laden walls of tree.
    In a Pine box, one of THE most absorbent woods available, there is rarely a “moisture” problem, provided, beekeepers have given bees more of a vertical space. I’ve seen 3’ wide combs, 5’ tall, 3 layers thick, with only a tiny amount of bees, and the opposite, 1 deep, super crowded, either way, no moisture. It’s a big myth that is chilling bees… not what bees want-in my opinion based in my observations of wild and domestic hives..I could be wrong.

  • Thank you for another great article! Question: since a small colony can’t generate as much heat to remove moisture as a large colony, should the small one have MORE ventilation? Seems counterintuitive, but your article sure makes sense!

  • As a new beekeeper, I have enjoyed reading the Q & As. Thank you so much! I think I may have Argentine ants in two newly spilt hives located on a new pallet. I will trim the grass and dusted with cinnamon and hopefully the bees will self-manage the ants. I may have to move the hives to different location. Do you have any other suggestions. Thank you.

  • I had the mold issue in the hives that died this winter. Can I put new swarms in with the moldy frames? Will the bees clean them up or do I clean them? If so, How?

  • Hey,

    Really enjoyed the article! I’m relatively new to beekeeping (2 years, 3 hives) and have had issues with ventilation. I have only now come across having a top entrance and an open bottom to allow air to flow through.

    My bees did pretty well during winter but my super smells really gross! There’s a little mould on the top of the frames and the honey smells fermented and tastes a bit like alcohol.

    Should I remove all of the frames in the super and discard the honey, or would it be okay for the bees to use it?


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