Creating a Model for Oxalic Acid Vaporization: Part 1

June 17, 2026

Contents

THE EFFECT OF OAV UPON MITES. 2

EXPECTED MITE DROP AFTER AN OA TREATMENT. 4

ADVERSE EFFECTS UPON THE BEES AND BROOD.. 6

DOSE AND EFFICACY FOR BROODLESS COLONIES. 7

OAV FOR COLONIES ENGAGED IN BROOD REARING.. 8

SO HOW MANY OAV APPLICATIONS ARE NECESSARY?. 9

SO WHAT’S THE OPTIMAL OXALIC DOSAGE FOR AN OAV?. 10

THE BOTTOM LINES. 11

FOR COLONIES WITHOUT BROOD. 11

FOR COLONIES WITH SUBSTANTIAL BROOD REARING IN PROGRESS. 11

SO HOW MANY OAV APPLICATIONS ARE NECESSARY DURING SUMMER, AND AT WHAT INTERVALS?. 11

OAV vs OAE. 11

CITATIONS AND NOTES. 11

Creating a Model for Oxalic Acid Vaporization

Part 1

Randy Oliver

ScientificBeekeeping.com

First published in ABJ February 2026

As beekeepers shift to alternative treatments for varroa, I’m often asked about how best to apply OAVs during the summer.  To answer this question, we could sure use a computer-based model.  I’m developing one!

I began writing this article back in 2022, in response to beekeepers wondering why it took so many summertime OAVs (oxalic acid vaporizations) to attain good mite reduction.  Unfortunately, my popular varroa model [[1]] wasn’t designed for predicting the outcome of a series of OAVs.  Since then, I’ve done a deep dive into oxalic vaporization, done some experiments (such as the one in my other article in this issue), and reviewed a number of data sets (from beekeepers around the world)  of mite drop counts (from stickyboards) following series of vaporizations at different intervals and times of the year.

I’ve now spent considerable time creating a new Excel model to predict the expected effect upon the varroa population following treatment(s) with OAV.  Let me walk you through the process of how I developed the model.

Creating a model:  To create the model, I first needed to determine the inputs and factors involved, starting with…

THE EFFECT OF OAV UPON MITES

The first thing to get a handle on is how the microcrystals of oxalic acid resulting from an OAV actually affect a mite.  We can see how the crystals cling to the bees’ feathery body “hairs” (setae)(Figure 1), but it’s not yet exactly clear in what ways the acid affects the mites.

Fig. 1  This bee had received a moderate dusting of oxalic acid.  Judging by how evenly dispersed the microcrystals are, it’s likely that most “phoretic” mites would also get some crystals on their bodies too (I’ll continue to use the improper term “phoretic” instead of the more accurate “dispersing”).

So exactly how would those crystals of acid actually affect the mites?  Here’s a list of some possible modes of action (understand that there would be no acidic effect until a crystal dissolved into water, via absorption from the air, or from coming in contact with a wet surface):

• Acute toxicity: severe damage or lethality from short-term contact with OA (via the mite’s cuticle or foot pads), or by ingestion of the acid (perhaps via feeding on a bee’s contaminated hemolymph).
• Chronic toxicity: deleterious effects or death due to longer-term exposure to OA, or Cumulative toxicity: when  sublethal doses bioaccumulate,  leading to eventual lethality (as with arsenic or fipronil).  Either of these would require extended residual action, which would be more likely with extended-release OA, rather than by vaporization.
• Tissue damage: Cuticle degradation (“acid burn”), perhaps resulting in damage to the mite’s footpads, cell membrane disruption, or intestinal tissue apoptosis following self-cleaning or ingestion of bee hemolymph.
• Sensory damage or behavioral disruptors: indirect lethality due to disruption of feeding or other critical behaviors (perhaps leading to desiccation), or a long-term decrease in reproductive success due to sensory disorientation or behavioral disruption (resulting in “sterility”).

Practical application:  After an OAV, there is typically (but not always) a major mite drop in the first 24 hours, followed by an extended increased drop for at least 4 days afterwards, suggesting that more than simple acute contact toxicity may be involved.

Milani [[2]] found that mites that walked over OA microcrystals died within 24-48 hours, but that those which came in contact only on their backs did not die — strongly suggesting that the acid enters the mites’ bodies through their sticky footpads (empodia) (Figure 2), as opposed to direct absorption through their cuticle (with vaporization, there is no humectant such as sugar or glycerin involved).

Fig. 2  I took this photo of a mite walking upside down under a glass cover slip.  At each step, it temporarily inflated its sticky empodia (which allow a mite to hang tightly to a bee).  The moisture on an empodium’s surface would not only cause any OA microcrystals to adhere to it, but would also dissolve and activate the acid.  At that point, the acid might either “burn” these critical appendages, or be absorbed into the mite’s hemolymph, and then cause physiological problems or cardiac arrest [[3]].

Papežíková [[4]] also observed that mites dropped onto petri dishes lightly fogged with OA picked up OA crystals on their front legs within minutes, and began dying within hours (even if only exposed to the microcrystals for 5 minutes), and were mostly dead by 24 hours.  However, such rapid kill was not found when mites were placed upon caged bees that had been vaporized  — taking 5 days to reach 50% mortality.

Practical application:  Why the difference?  I’ve observed the same when I cage mite-infested bees that have walked over an OA/gly pad.  Is it critical for the mites to already be on the bees during the actual vaporization?  Yet another unanswered question!

Another surprising finding by Papežíková’s group is that they also found that greater mite mortality  occurred when riding on caged bees that had been dribbled with or fed OA in sugar syrup, with that mortality increasing day by day for 5 days, by which point 98% of the mites on bees fed OA syrup had died (as well as killing half the bees).  Could some of the delayed mite mortality after an OAV be due to mites feeding on bees that swallowed OA in the cleanup after the vaporization?  Still sooo much we don’t know.

EXPECTED MITE DROP AFTER AN OA TREATMENT

For most application methods of OA (dribble, spray, or vaporization) mite mortality occurs quickly, with the greatest mite drop (onto stickyboards) typically (but not always) occurring during the first 24 hours, with elevated drops above the baseline value persisting for several days afterward (Figure 3).

Fig. 3 Planinc [[5]] recorded mite drops after OA dribbles, which have a similar result as a vaporization.  Note the stepdown of mite drops post treatment, as well as the recovery of the “phoretic” mite population within a week, as replacement mites emerged from the brood.

But a different data set sent to me by Greek beekeeper Koursoumis Kostas gave a different picture (Figure 4).

Fig. 4  Kostos vaporized each of 10 single-deep broodless colonies with 3 grams of oxalic acid.  The vaporizer was set at a temperature that vaporized the acid rapidly.  The first two vaporizations were performed during cold weather (48°F/ 9°C); the third one when it was warmer (57°F/ 14°C) and raining (are temperature and humidity important factors?).  Note the delay in mite drop after the first two vaporizations, and also that it took three vaporizations of broodless colonies to attain efficacy.

FWIW, as I was running my setpoint temp experiment (see my other article in this issue), I identified a double-deep, collapsing broodless colony, with only three frames of bees left, and having a mite wash count of 43 mites/half cup of bees. Since I already had a vaporizer out and set at 230°C, I gave it a shot of 4 grams of OA (at an ambient temp ~60°F/ 15°C) .  Four days later, their mite count was zero! I didn’t measure the mite drop, but attained 100% efficacy with a single OAV.  Was the difference due to the ambient temperature, the setpoint temperature, the dose, or something else?  We clearly need more comparative data to answer that question (especially since many beekeepers are applying OAVs in cold weather)!

Creating a model:  A valid model would match field data, but there may be a lot of variables involved in rate of mite drop following an OAV.  This may make it difficult for me to match every data set that I’ve received (and many scientific  papers don’t include details of dates, amount of brood, temperature, hive configuration, etc.).

Anyway, although we don’t yet fully understand all the aspects affecting mite drop following an OAV, we do know that it “can work.”

ADVERSE EFFECTS UPON THE BEES AND BROOD

There’s never any free lunch, and no mite treatment comes without a cost.  We must weigh the “benefit” of reducing the mite infestation rate, against the “cost” of any adverse effects upon the colony. When I review various papers on the effects of OAV upon colony strength, there are some disparities in their conclusions.

Following two OAVs during winter, Al Toufalia measured colony strength in spring, and found (not surprisingly) that the treated colonies had grown more than the controls [[6]].  Similarly, Jack [[7]], after applying three weekly OAVs of up to 4 grams per single deep in October, found “no significant interaction between OA dose and time for any of the measured colony strength parameters.”

On the other hand, Tellarini Prieto [[8]] in Canada applied a wide range of weekly doses by vaporization four times to single deeps, resulting in total doses of 0, 20, 40 and 80 g over the course of a month.  They found only a slight increase in adult bee mortality even at the extremely high dose of 20g per vaporization, but clear trends in reductions in brood area (~13% from the 5- and 10g doses, to ~27% from the 20-g doses), and colony growth (up to ~20%).  Fortunately, they found no issues with “ability to rear emergency queen cells,…queen acceptance, performance, nor quality”

And recently, Bozkus [[9]], applied three weekly 1, 2, or 4-g OAVs by Varrox to double deeps in July and August in Oregon (repeated two years).  Their data also indicated a reduction in colony size and brood area (correlating with dose), apparently due to substantial larval mortality (up to 50%) due to treatment.

Practical application:  At the end of a series of OAVs, you may wind up with a somewhat smaller colony, but if it then has a greatly reduced mite level, the colony will be better off for it, and if still rearing brood, quickly recover (and with a lot fewer mite and virus issues).

So now we should look at the effects of dosage and treatment intervals (for colonies either with or without brood).

Creating a model:  I need to make the model adjustable for season, to account for the amount of brood present.

DOSE AND EFFICACY FOR BROODLESS COLONIES

OAV is clearly more efficacious when applied to colonies not containing brood, since all the mites in a broodless hive are on the adult bees, and thus directly and immediately exposed to the acid.

A relatively low dose of OA applied to a broodless colony during winter can be quite efficacious.  In one of the earliest large-scale studies of OAV, carried out by Radetzki [[10]], in which 95 cooperating apiarists from seven European countries tested the Varrox vaporizer in 1500 hives, applying either 1.4- or 2.8-gram doses.  Most colonies were free of brood, with a resulting efficacy from a single treatment of around 95%, whether either 1.4 or 2.8 g were applied.  Even in colonies containing some winter brood, efficacy was above 90%.  And this was the case for either single or double-deep hives.

Later research by Al Toufailia [[11]], demonstrated that giving a single OAV to a broodless colony in winter can kill over 90% of the mites (Figure 5).

Fig. 5  Surprisingly, there was only a slight dose-response curve, suggesting that there were diminishing returns from applying more than 1 gram of OA to a broodless single (chart after Al Toufailia’s data).

Al Toufailia may have gotten lucky with the 1-gram dose, since not all researchers have gotten as good a result [[12]].  In any case, he [[13]]  followed up the next year with applying two OAV applications of 2.25 grams (per single) during the broodless period, achieving a combined efficacy of 99.6%—resulting in a colony nearly completely free of mites.  (He unfortunately didn’t test a repeated lower dose.)

Surprisingly, Al Toufailia got less consistent efficacy in the second application than in the first. Let’s hope that this isn’t an indication that some mites may be more resistant to OAV than others!

Practical application:  For best efficacy in colonies without brood, it appears that two OAV applications of 4 or 5 grams per double deep colony should do the trick.  Starting each season with nearly mite-free colonies (as we do in our own operation by splitting and applying an OA dribble during the induced brood break), results in colonies that may require only a single additional treatment in midsummer each year to keep varroa under control (some colonies will of course require more attention). 

My aforementioned little test in which I achieved 100% efficacy from a single OAV can be compared to a trial by Bahreini [[14]], who tested the efficacy of three different types of vaporizers in small broodless clusters in singles.  Of interest is that they found that “the Varroa infestation levels dropped below the recommended fall treatment threshold after a single application of OA using [two of the devices]; however, it took three applications of OA using the [third device] to obtain similar results.

Practical application:  There is a wide assortment of oxalic vaporizers offered on the market, and they don’t all work as well!  When shopping for one, I again suggest that you ask for hard data (rather than anecdotal reports) on output and residue titrations, as well as for efficacy in field trials (best performed by independent objective organizations).

Creating a model:  I hope to be able to use the model to back-calculate from field data the actual efficacy resulting from whatever combination of variables were involved in the treatment that the beekeeper used (vaporizer type, dose, setpoint temp, colony type, and weather).

OAV FOR COLONIES ENGAGED IN BROOD REARING

Okay, so much for treating broodless colonies in winter (or those made broodless by a beekeeper-induced brood break during summer) — in which nearly all the mites are exposed to the treatment.  It’s a different case during the rest of the year, since more than half the mites at any time may be protected from an OAV due to being hidden in sealed brood (Figure 6).

Fig. 6  An approximation of the percentages of mites in the dispersal (“phoretic”) phase over the course of a season in an area with a 2-month brood break.  The lower the percentage of mites phoretic, the less efficacious an OAV will be.  Even in late summer and autumn, it will take repeated OAVs to attain good efficacy.  (in the chart I allowed for small patches of brood during the winter, following the finding of Al Toufailia [[15]] that mites load up in those patches, and would be protected from an OAV — thus the benefit of a follow up winter vaporization).

Since OA residues rapidly lose their acidity once on a bee (thus exhibiting little residual toxicity), the mite population of a colony given a single application can bounce back in less than a month.  Thus it requires repeated vaporizations to achieve effective mite reduction.

Practical application:  Note that an OAV would be a waste of time during swarming season, since nearly 80% of the mites can be hidden in the brood at that time.  Even in late summer and fall, half the mites will be protected from an OAV, so it will take repeated applications to “get them all.”

Creating a model:  I need to account for the changes in the number of days that a mite is phoretic over the course of the season.

So how many OAV applications are necessary?

Back in 2022, the allowed OA dose applied by vaporization was 1 gram per brood chamber (2 grams per double deep), and the Api-Bioxal label made clear:  When applying Oxalic Acid Dihydrate when capped brood is present … multiple treatments several days apart will be needed to reduce successive cohorts of adult mites.

Practical application:  Multiple treatments was an understatement —it was taking beekeepers a lot of vaporizations (often 10 or more) to get varroa under control.  Although early research in Europe suggested that 2 grams of OAV per double-deep might be enough [[16]], it was becoming clear that it wasn’t [[17]^{, [18], [19]}].

A recent paper by Bozkus (Ramesh Sagili’s team) [[20]] makes this clear. They applied three OAV doses of 1- to 4-grams to double-deep colonies in July and August at 7-day intervals.  They observed a clear dose response curve, indicating that 3 grams per double-deep would be the minimal effective dose.  But even when they applied 4 grams at three 7-day intervals it only “held” the infestation rate steady, but did not decrease it. 

Creating a model:  For some reason, my modeling to date  does not reflect Bozkus’ findings — it predicts that a week after the third vaporization, the mite wash count would have been only 60% of the starting rate, even at only a 90% kill rate.  To match their findings, at this writing, my modeling indicates that their OAVs attained only 50% efficacy at killing the phoretic mites.  I’ll continue to fine tune the model before sharing it.

On the other hand, Cameron Jack’s trial [[21]] was run as the weather cooled in October (Figure 7).  His findings suggest that when applied in “3 applications at 7-d intervals,” either 3 or 4 grams of OA per brood chamber may be necessary for serious mite reduction.

Fig. 7  I checked the weather history for Jack’s trial in Gainsville.  It was dry and cooling, suggesting that his test colonies would have been winding down brood rearing.

Creating a model:  My computer model reflects Jack’s findings.

SO WHAT’S THE OPTIMAL OXALIC DOSAGE FOR AN OAV?

Although some have interpreted Jack’s findings to indicate that one should apply as much as 8 grams of OA to a double-deep hive via OAV, despite that much being allowed by the label, it seems like an excessive amount to me.  There’s a tradeoff — it currently appears to me that any more than 4 grams per double deep provides diminishing returns in efficacy, at the cost of increased adverse effects upon the bees and their brood (this of course assumes optimal output from the vaporizer).  This adverse effect upon the brood would be multiplied with repeated vaporizations.

Practical application:  The residue data that I presented last month, the efficacy data that I present in my other article in this issue, plus reports from beekeepers, suggest that a 4-gram per double deep dose will likely do the trick — according to my modeling, any benefit from increased efficacy may be outweighed by the cost to the brood and bees.

The Bottom Lines

For colonies without brood

The bottom line is that a single OAV application when a colony is broodless may be “enough,” but two vaporizations will nearly eliminate all the mites from a hive.  Depending upon the output and set temperature of the vaporizer, a 4- or 5-gram dose of OA per double deep would likely be enough (we need more hard data).

For colonies with substantial brood rearing in progress

For a colony in full brood rearing mode (during which a single OAV may kill only a third of the mites), it’s gonna take repeated vaporizations, and the efficacy of each vaporization will again be dependent upon the device and setpoint temp.

During summer, three OAVs (at any interval) will hold or somewhat reduce the colony’s mite population, but don’t dream that it will eliminate most of the mites — that would take several applications.

So how many OAV applications are necessary during summer, and at what intervals?

Ah, that is the question!  The value of a model is that, everything else being equal, you can compare the effects of changing any variable (or combination thereof) — the efficacy the OAV application at killing the phoretic mites, the interval between each OAV application, or the number of repeated applications.

OAV vs OAE

O.K., I can’t end this article without mentioning extended-release oxalic application (OAE) (dissolved in glycerin, applied in hung strips or laid-flat pads).  They won’t give as quick a knockdown as an OAV, but have roughly the same effect over time as repeated OAVs, with the labor advantage of having to make only a single application, rather than repeated treatments.  That said, a combined treatment of an OAE application, immediately followed by an OAV after you close the hive, would be worth a try (we ourselves have been happy with the results of doing so with a dribble).  Please let me know your results!

I’m fine-tuning and testing the model as I send this article to press, and plan to share a working version ready for your use next month! Update:  the model is up and running at https://scientificbeekeeping.com/a-model-for-oxalic-acid-vaporizations/

CITATIONS AND NOTES

[1] https://scientificbeekeeping.com/randys-varroa-model/

[2] Milani, N. (2001) Activity of oxalic and citric acids on the mite Varroa destructor in laboratory assays. Apidologie 32(2): 127-138.

[3] Ilanidis, K (2023) The impact of the varroacide VarroxSan® on the cardiac activity of Varroa destructor.  Presented at Apimondia 2023.

[4] Papežíková, I, et al (2017) Effect of oxalic acid on the mite Varroa destructor and its host the honey bee Apis mellifera. Journal of Apicultural Research 56(4): 400-408.

[5] Planinc, A (2004) Dynamics of falling Varroa mites in honeybee (Apis mellifera) colonies following oxalic acid treatments. Acta Veterinaria Brno 73(3): 385-391.

[6] Al Toufailia, H, et al (2015) Towards integrated control of varroa: 2) comparing application methods and doses of oxalic acid on the mortality of phoretic Varroa destructor mites and their honey bee hosts. Journal of Apicultural Research 54(2): 108–120.

[7] Jack, C, et al (2021) Determining the dose of oxalic acid applied via vaporization needed for the control of the honey bee (Apis mellifera) pest Varroa destructor. Journal of Apicultural Research 60(3): 414-420.

[8] Tellarini Prieto, E, et al (2024) Safety assessment of high doses of vaporized oxalic acid on honey bee worker health and queen quality. Frontiers in Bee Science 2: 1442030.

[9] Bozkus, M, et al (2025)Oxalic acid vaporization: effectiveness against Varroa estructor (Mesostigmata: Varroidae) and safety for Apis mellifera (Hymenoptera: Apidae), Journal of Insect Science 25(6): ieaf091.

[10] Radetzki, T (2004).Vaporization of oxalic acid in field trial with 1,509 colonies. Apiarist 1: 33.

[11] Al Toufailia(2015) op cit.

[12] Marinelli, E, et al (2004) Oxalic acid by Varrox® to Varroa control in Central Italy. Apiacta 39: 39-43.

[13] Al Toufailia, H, et al (2018) Towards integrated control of varroa: 4) varroa mortality from treating broodless winter colonies twice with oxalic acid via sublimation. Journal of Apicultural Research 57(3): 438-443.

[14] Bahreini, R, et al (2018) A pilot study to compare the efficacy of oxalic acid sublimation devices to control Varroa mite in Alberta.  https://www.albertabeekeepers.ca/wp-content/uploads/2018/09/2019-Final-OA-vapourizer-report.2.pdf

[15] Al Toufailia, H & F Ratnieks (2018) Towards integrated control of varroa: 5) monitoring honey bee brood rearing in winter, and the proportion of varroa in small patches of sealed brood cells. Journal of Apicultural Research 57(3): 444-451.

[16] Rademacher, E. & M Harz (2006) Oxalic acid for the control of varroosis in honey bee colonies–a review. Apidologie 37(1): 98-120.

[17] Berry, Jennifer (2021) Multiple applications of vaporized oxalic acid.  Bee Culture Dec. 2021: 70-73.

[18] Jack, C, et al (2020) Evaluating the efficacy of oxalic acid vaporization and brood interruption in controlling the honey bee pest Varroa destructor (Acari: Varroidae), Journal of Economic Entomology 113(2): 582–588.

[19] Jack (2021) op cit.

[20] Bozkus (2025) op cit.

[21] Jack (2021) op cit.