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Experimenting with Formic Acid


Formic Vapors and their distribution. 1

Experiment #1: Applying Formic Pro on the bottom board, with a temporary top entrance. 2

Results. 5

Experiments on queen loss due to formic. 6

Experiment #2: Can you remove, and then reintroduce the queen?. 7

Experiment #3: Is it the formic or the bees that kill the queen?. 13

Discussion. 15

Citations and notes 15


Experimenting with Formic Acid

Randy Oliver


First Published in ABJ March 2024


Formic acid is the trickiest miticide to use in hot weather, so I continue to experiment with various application methods to improve its efficacy, and to better understand why it sometimes causes queen losses.

Last summer I ran several small “quick and dirty” preliminary experiments with formic acid; in this article I’ll share three of them.

Formic Vapors and their Distribution

Formic pad fumes, depending on the concentration of the acid, are denser than air (Table 1) and will thus tend to settle in a hive unless the bees actively fan them away.

Table 1 Notice that the more formic acid is diluted with water, the less dense is the mixture of their combination of vapors as they evaporate (they evaporate together at about the same rate).

Today, formic is generally applied above the brood nest, under the assumption that its fumes will tend to sink down across the brood. But back in the day, Canadian beekeeper Jean-Pierre Chapleau promoted applying formic on the bottom board. This got me wondering whether I could slip a single Formic Pro strip into the lower entrance, then seal that entrance and force the bees to use a temporary top entrance. My reasoning was that the formic fumes would tend to pool near the bottom of the hive, and slowly diffuse upwards into the brood area, where the bees would be forced to fan in fresh air from above. That might result in better distribution of the formic fumes throughout the hive.

Experiment #1: Applying Formic Pro on the bottom board, with a temporary top entrance.

So on a hot day in July I tried it on a double-deep hive having a starting mite wash count of 34. The colony took the treatment well, with few dead bees in evidence the next day, and a mite count after 24 hours of zero! Excitedly, I set up several more hives the same way, but found that the great reduction in mites in the first test was an anomaly — I’d need a stronger dose.

So I made up temporary hive covers with ¾” x 3” openings, plus wedges to seal the lower entrances. I took starting mite wash counts from 13 hives in various conditions (mostly poor — since I didn’t want to sacrifice any “good” colonies), and replaced each hive cover with a ventilated lid. I then shoved a single Formic Pro strip into the entrance, and sealed the entrance with a wedge.

The formic vapors were thus “trapped” in the hive, and the bees needed to learn to use the top entrance for access and ventilation. The results were surprising (Figures 1-3).

Fig. 1 There were huge colony-to-colony differences in response across the board — some bearded up, others ignored the fumes! I took this photo a couple of hours after applying two strips of Formic Pro on the bottom boards.

Fig. 2 The next day, in two hives there were kills of 800 and 1000 bees. In others, there were no dead bees at all! Go figure …

Fig. 3 In some there was kill of open and sealed brood; in others no apparent effect on the older larvae or pupae.


I’ll let you try to make sense of the results yourself (Table 2):

Table 2 (QR=queenright; QL=queen lost)

  • Large colony-to-colony variability in response — some colonies tolerated the treatment, some didn’t!
  • Clearly unacceptable adult bee kills in two hives.
  • Good average mite reduction, but wildly inconsistent (compare hives 8 and 9).
  • Surprisingly little queen loss.

We didn’t follow up on these colonies, but happened to perform mite washes on some of them about a month later (when prepping for another experiment). We were surprised by the number showing mite counts of zero at that time.

I followed up with another small experiment — allowing the bees a couple of days to get used to the top entrance before treatment — and plan to experiment more with the method.

Experiments on Queen Loss Due to Formic

Beekeepers often complain that application of formic acid in hot weather may result in the loss of some queens — especially if they are old or failing (the queens, not the beekeepers). But in our operation, we use that observation to our advantage. When we nuc up our colonies after almond pollination, we put the best-performing queens back into nucs for a second season. By late August, they apparently begin to run out of sperm, and their colonies naturally start to rear supersedure replacements. But there unfortunately aren’t enough drones around at that time for proper mating.

Luckily, by that time our last rounds of nucs have built up into strong singles — and need a second box of honey and brood. So we harvest most of the honey from the hives with second-year queens, and shake all the bees down into a single, along with their brood. We then hit that single with a strong formic treatment — typically in 95°F weather — in the hope of eliminating not only its mites, but its aged queen as well, so that we can then put that box of “clean” bees, brood, and honey on top of a single in need of stores for the winter.

OBSERVATION: When we actually try to kill aging queens with formic in hot weather, darned if half of them don’t just laugh at us.

This brings me back to a subject that I’ve written about previously [[1]] — is it the formic that actually kills queens, or do the formic fumes induce the workers to kill their mother? This question has bugged me since Dr. Amrine posed it years ago [[2]]. So when some opportunities arose in September, I performed a couple of small experiments.

Experiment #2: Can you remove, and then reintroduce the queen?

Based upon my finding that reducing the first-day flash off of fumes from Formic Pro pads pretty much eliminated queen loss [[3]], I wanted to look more deeply into this phenomenon. I’ve noticed that after a day or so, the bees become acclimated to the odor (and irritation) of formic fumes and often walk right over the pads. So I wondered whether temporarily removing the queen during the initial flash-off and acclimation process, and then reintroducing her after a couple of days, might reduce the “queen loss problem.”

So I ran a small experiment, which I’ll go through step by step (Figures 4-10).

Fig. 4 At 5:00 in the afternoon on September 22, with temperatures in the mid-70s F, I removed the queens from ten hives and caged them with seven attendants each.

Fig. 5 The test hives were all single deeps with 9-10 frames of bees. After I pulled the queens, I applied recycled Miteaway II pads charged with 50% formic acid (which applies an acceptably strong dose to a single), in a 1½” rim [[4]]. This photo was taken at noon a day and a half later, during which the average evaporation rate was 35 grams per day. Note that by this time the bees were well acclimated to the fumes.

Fig. 6 In the meantime I maintained the queens in an incubator at 86°F (30°C) and 55% RH, for 43 hrs. They each got a small plug of stiff candy for a food source, and I gave them a couple of drops of water every day.

At noon two days later I returned with the queens. To my surprise and dismay, two had died, with their heads stuck to the candy (I have no idea why — their attendants were fine). The others all appeared to be healthy.

The ambient temperature was 75°F, with some guards fighting off yellowjackets and potential robbers at the entrances. I gave each hive two puffs of smoke, and allowed their queen to walk back into the entrance of her hive. (Some were reluctant to walk in, apparently in response to the odor of formic fumes emanating from the entrances.)

Once I’d reintroduced all the queens, I lifted the hive covers to measure the evaporation rate of the pads during reintroduction (~39 g/day). To my surprise, one queen was already being balled on the top bars (Figure 7).

Fig. 7 This queen had made her way up to the top bars, where she then got balled! I brushed off the balling bees and reintroduced her at the entrance.

A half hour later, I tipped up the boxes to inspect the bottom boards, and again looked under the lids. Three queens were being balled on the bottom boards (including the earlier one), so I recaged them each with one attendant and pushed their cages between two combs away from the fumes. Two other queens were being joyfully mobbed by their workers (one in a ball hanging from a bottom bar), but with no signs of aggression.

Fig. 8 These bees on the bottom board were happy for Mom’s return. They mobbed her, but allowed her to walk freely.

Fig. 9 Three colonies aggressively balled their queens, which I rescued, recaged, and placed between two combs of bees.

A half hour later I repeated the inspections. One queen was still being happily mobbed on the bottom board, and in another hive the bees were balling one dead worker and one yellowjacket.

Fig. 10 Five out of eight queens were reaccepted without issue.

The next day, 24 hours after reintroduction, I repeated the inspections (including looking for dead queens in front of the hives). There was one worker being balled, but no signs of any queens dead or being balled. I re-inspected two days later. The three queens that I’d recaged due to balling were dead, but the rest were happily on the combs.

Results: Out of 10 queens, two died in the incubator, three were balled and eventually died in cages, and five were reaccepted during strong formic application. How’s that for another set of ambiguous results?

So I ran another experiment a few days later in another yard.

Experiment #3: Is it the formic or the bees that kill the queen?

The queen is the largest, best fed, and longest-lived bee in the hive. So why would she be more susceptible to formic fumes than would expendable workers? To see whether that was indeed the case, I exposed some queens and workers, side by side, to exactly the same formic fumes.

By now it was late September. I set up ten hives with second-year queens in single deeps containing 8-9 frames covered with bees. I caught and marked the queens, then placed them in push-in cages midway down a comb, off center, but below a fresh formic pad. The cages were placed over comb with no brood inside, but with open nectar (so that the queen would not be dependent upon the workers to feed her), and with 3 workers as attendants. All cages were placed facing a comb of brood, so that the caged bees would remain within the cluster. I applied MAII formic pads in early afternoon, made with 50% formic, in a 1.5″ rim. Temperature was ~80°F.

Fig. 11 It’s easy to make a handy push-in cage out of a wide-mouth Mason jar rim, with 1/8” hardware cloth soldered or glued in place. (I haven’t yet tried it, but 5-mesh hardware cloth might be used to function as a queen excluder.) I included three workers from the brood area with each queen, and placed the cages over open nectar, halfway down the comb, beneath the formic pad.

After several days, the formic had evaporated from the pads, and I inspected the cages to see how the caged queens and workers made out.

Results: Eight out of ten queens survived and looked fine. But in the cages of the two that died, the attendants were still alive. So some queens do appear to be more susceptible to formic fumes than are workers. But 8 out of 10 presumably old queens tolerated being trapped beneath a strong formic treatment just fine.


Formic acid has some very desirable qualities as a mite treatment:

  • It is fast acting (you can eliminate most all the mites in a hive overnight with a strong flash treatment).
  • A strong dose can penetrate the cappings.
  • It doesn’t contaminate the combs or honey.

But it also has its downsides:

  • In hot weather it may induce queen loss — although I’m still not clear whether the majority of those losses are directly due to the fumes, or instead from being balled by their daughters. (Covering the upper side of Formic Pro strips with their wrapper will largely eliminate queen loss in hot weather.)
  • Formic also exhibits great hive-to-hive variability in efficacy when it’s hot.

Formic is a great choice in early spring, since you can use it to create a short brood break to reduce swarming, and if a queen should be lost, the colony can easily rear a replacement at that time of season. But later in the season, due to lack of drones in my area, I’m concerned that any replacement emergency queens might not get properly mated.

Anyway, formic’s been used for many years, especially in Europe and Canada (where there is a greater choice of approved application methods). I can only hope that now that the EPA has spoken, that our state regulators will allow us to experiment with the generic liquid more freely.


Citations and Notes

[1] Oliver, R (2022) Formic Pro and queens In hot weather. American Bee Journal September 2022 https://scientificbeekeeping.com/formic-pro-and-queens-in-hot-weather/

[2] Amrine Jr, J, & R Noel (2006) Formic acid fumigator for controlling varroa mites in honey bee hives. International Journal of Acarology 32(2): 115-124.

[3] Oliver, R (2022) op cit

[4] This is not an approved method for varroa control, but since I was not applying the formic for pesticidal purposes, it was not against the law.