Using Oxalic Acid to Reduce Varroa.

Winter Treatment.

Last week, I treated all my colonies with oxalic acid before the winter solstice on 22nd December. After that, the queens will start laying again increasing production as the days lengthen. So, there is unlikely to be much brood just now and it is a good time to kill off the phoretic mites (those that are being carried around on worker bee bodies). In the past, I’ve trickled a solution of oxalic acid over the brood frames but I now prefer the less invasive method of heating the oxalic acid crystals and sublimating the bees. Trickled oxalic acid can kill any open brood present. When the crystals are heated they are converted to vapour which penetrates the hive and winter cluster of bees. The vapour kills the varroa but doesn’t harm the bees. However, I don’t sublimate when the ambient outside temperature is lower than 5 degrees Celsius because the bees will be a tighter cluster at low temperatures and the vapour may not penetrate the cluster suffiently.


Oxalic acid is dangerous for humans, and if vapour is inhaled it can reform as crystals and severely damage lungs. I never take chances and I wear an appropriate face mask that conforms to EN14387 standards. It has ABEK1 filters. Protective gloves are necessary.

Monitoring Varroa Infestions.

During the late spring and summer I use the sugar roll test as explained in a previous blog. I treat the colony if test results show more than 2% varroa in my sample of 300 bees. If varroa levels are high then I use formic acid. I remove the honey supers for the period of treatment. After removing the honey supers at the end of the season, I sublimate using oxalic acid if varroa levels have increased. I used to monitor the natural varroa drop regularily using the BeeBase varroa calculator to work out when treatment was needed. However, I now don’t believe that measuring the natural drop is as reliable as performing a sugar roll or alcohol wash test. So, in winter I routinely treat all colonies with oxalic acid sublimation since it it well tolerated by honey bees. I count dead varroa post-treatment but don’t base winter treatment on natural varroa drop.


John Thorburn, inventor of Gas-Vap, and prize winner at the 2019 National Honey Show.
Using Api Bioxal; the only form of oxalic acid licenced and permitted for honey bee treatment in the UK. Demonstration of vapour which is directed into the hive and takes only a few seconds to administer.

More About Gas-Vap at Bridge Cottage.

Until Fred Mollison told me about his Gas-Vap, I’d been using a Varrox sublimator that hooked up to car/wheelchair battery and heated the oxalic acid crystals. It worked fine on wooden hives but it was impossible to insert into a poly hive so I worked from underneath the open mesh floor rather than through the front entrance. So, it may not have been so efficient. I’m really pleased with the Gas-Vap because it has an extension that fits well into the poly hive entrance without heating and melting the hive. Also, it is very quick in comparison with the Varrox method. The detachable caps get hot and I used a cloth to remove them to avoid melting my gloves. The cloth scorched so I shall find some heat resistant material to carry in my kit. The metal that fits onto the torch gets hot too so I will rest the Gas-Vap on a piece of wood on the landing boards of my poly hives in future.

Gas-Vap Review: A Year On, 18/01/21.

Where this gadget is excellent in terms of getting into hives with wasp deterrent floors, I’ve found that the attachments have fallen off the blowtorch once it heats up now. A year on and I am wondering if it is such good value. It is cheap in comparison with most other products but I need to rely on something that will be easy to manipulate and safe. The caps are difficult to remove sometimes without using pliers and the lighter fuel leaks out frequently when I am filling the torch. I’ve asked another beekeeper who tells me that the same thing happens for him, apart from the attachments falling off. He tells me that he there is a screw on the attachment of his kit which is not present on mine. I haven’t noticed the long nozzle blocking but my beekeeper friend has had to unblock his. A long wire should solve this.

If you are thinking of buying this product, it might be worth asking around and getting more feedback and opinions first. I know someone who has no complaints at all about it. Although I wear the correct PPE and mask, I don’t want to be exposed to any vapours at all. This can happen if the kit falls apart after the oxalic acid crystals have been heated and sublimated. I’ve contacted the manufacturer following my recent problems and I have been told that the latest models have an adjustable screw so this should not happen now.

19 thoughts on “Using Oxalic Acid to Reduce Varroa.”

  1. Excellent. Thanks for this post. I’m intrigued by your comment that the queen will start laying again after the winter solstice on 22 Dec. I’m sure she’s not sitting there with a calendar, but is the solstice thing one of those beekeeper’s old saws, or is it a real approximation of when thing begin to get going again? I had been thinking about some trigger that was temperature based.

    More prosaically, did you find any issue with inserting the Varrox sublimator through the entrance and the heat being so close to the bottom of the brood frames – melting the comb?

    1. Thanks, Avery. I believe that the lengthening days, in terms of more light, influence egg laying rather than temperature. I don’t understand how this works because the queen works in the dark. They can thermoregulate the brood nest to optimum temperatures if they have sufficient stores to produce energy. The Varrox sublimator did melt wax and fry some bees when it went through the front entrance.

      1. Hi Ann, just been reading your extremely fascinating articles and so glad I have found your blogs, wonderful Ann thankyou so much, bless you x

      2. Thanks Ann. I went for a solid block at the entrance and putting the sublimator on an inspection tray under the mesh floor. Won’t be opening them to check the result yet but I’m hoping the mesh will have dissipated the heat without condensing the vapour. We’ll see.

    2. Interestingly, Avery my neighbours with hens tell me that they started laying again directly after the solstice. Yes, I have melted wax at the bottom of the brood frames in the past with the Varrox sublimator.

  2. Hi, This looks like a very interesting vapourizer and a lot easier than carting around large and heavy batteries. Most of the other commercial units have a temperature control to prevent the temperature over cooking the oxalis acid.
    The stuff one buys is usually oxalic acid dihydrate, which is a crystal which has two water molecules attached to each oxalic acid molecule. The CRC Handbook of Chemistry and Physics gives the following properties for oxalic acid dihydrate:

    On heating:

    1) The water of hydration leaves at 101.5° C (214.7° F) The water boils off leaving anhydrous oxalic acid crystals.
    2) At 157° C (314.6° F) the oxalic acid starts to sublime (goes directly from solid to gas)
    3) At 189° C (372.2° F) the oxalic acid which has not yet sublimed decomposes to formic acid and carbon monoxide.

    What is the temperature of your unit? or are you just overheating the oxalic acid and turning it into CO1.?

    1. Thank you for your interesting comments and information on sublimating oxalic acid. You are correct, like the varrox sublimator this gas-vap applicator does not have a temperature control. It is heated for a short time then the gas is given off over 30 seconds so I’m hoping that that it is achieving number 2. Did you check out the link to the company who made this equipment? You can see the precedure demonstrated. I doubt that many hobby beekeepers could afford to buy a commercial unit but your question raises another, are other methods involving uncontrolled heating effective?

      1. Thanks for your reply. It might be of interest to see if a lower intensity of blow tourch flame would increase the time it takes to vapourize in the oxalic acid and allow it to be in the range of 157 deg C for a longer period of time before it breakes it down to Formic Acid and CO1.
        Always learning and experimenting!

        1. That’s mentioned in the instructions, Martin, to keep the flame as low as possible. I will certainly be mindful of keeping a low flame and checking on the gas by lifting the roof and crownboard.It is all very hit and miss though without a temperature control isn’t it? I’ve found that it works though regarding varroa drop.

  3. Having read this, I’ve been having a look at research papers which have used the Varrox Sublimator. The papers are mostly concerned with evaluating the relative effectiveness of different methods if applying Oxalic for Varroa control. They give all sorts of parameters for their tests – excluding the actual operating temperature of the Varrox. On the other hand the instructions for the Varrox say that: The VARROX®-Vaporiser reaches temperatures of up to 400°C. – which would be troubling.

    However, we do know that, in practice, the vaporiser is at least partially effective. Obviously, as the temperature of the pan increases, it would pass through all the temperatures and would only overheat the oxalic when (or if) the temperature of the pan did exceed 189°C. So it would not be ineffective, just less effective than it could be. A related question would be how long the pan would take to reach 400°C — 400°C is approaching red-hot. I suspect it’s considerably longer than the 2½ minutes application recommended by the manufacturer, and, operating in outside temperatures, with an airflow, as opposed to in a testing laboratory, perhaps 400°C is not achievable.

    The critical time is how long the bees are exposed to the Oxalic, not how long it takes for the crystals to sublimate – once they have sublimated, any extra heat applied to the pan makes no difference. Perhaps it would be worth experimenting with the Varrox to see how long it takes to sublimate the dose.

    It is a very relevant point though. I couldn’t find a proper ‘spec’ for the Varrox on the manufacturer’s website.

    1. Avery, thank you for coming back in here in with the info that you’ve been researching. It would be super if you had time to do some experimenting and report back on it?

      1. I was thinking about that but it would mean vaporising in the open air whilst being close enough to see what was happening. With a basic dust mask, my main precaution is ‘light the blue touch paper’ and get as far upwind as possible. Expensive too, with Api-bioxal coming in at £13 a sachet – though I expect that plain old ‘Oxalic Acid Crystals’ should give the same results. It’s amusing that Thorne’s sell them to “Return your beehive to its natural colour”. I’ll give it some thought.

        1. If you do give it a go, Avery, you will be safer with a mask designed for gases etc. See the recommendations on the gas-vap site in previous blog. You really need one of those anyway as the dust mask doesn’t protect you.The problem is the crystals reforming in your lungs if you inhale the vapour I believe.

          1. You’re quite right of course. The ‘blue touch-paper’ method is far from ideal. Will be ordering mask for next use.

          2. Good to know, Avery. I always work on the assumption that I might fall over on the job and not get away from the fumes. I tripped over in the apiary a few weeks ago.

    1. I haven’t forgotten formic acid, Thomas. If you look at a previous post you will see that I used Mite Away Quick Strips (which is formic acid) in the summer). It can be hard on the bees and sometimes the queen dies/is superseded so I don’t use it all the time.

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