Oxalic Acid Vaporization

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Oxalic Acid Vaporization is one of the best tools available for beekeepers to use in the battle against varroa mites. A naturally occurring substance, oxalic acid provides an opportunity to step away from the use of synthetic chemicals in beehives. It has proven to be very effective at killing mites and when used correctly with no damage to the colony.

Varroa mites on bee brood and oxalic acid powder in vaporizer image.

When varroa mites (Varroa destructor) arrived in our country, the face of beekeeping changed forever. These pests of honey bees weaken and kill thousands of colonies-beekeepers must monitor mite levels and perform mite treatments if needed.

Using Oxalic Acid Mite Treatments

With the arrival of varroa mites, anyone involved in beekeeping as a business had reason to worry. Many large companies went out of business and almost all of the feral or wild bee colonies died. 

Since the mid 1980’s, the fight between varroa mites and bees has raged on and on.

One of the biggest challenges of varroa mite control is this: How can we kill the mite and not harm the bee? They are both somewhat related on the scientific classification chart.

Another important issue that must be considered is the actual products we use for mite control. Do we need to worry about chemical residues left behind in the honey and beeswax?

Is it enough to matter? How much is too much and who decides that-do we trust that they know the answer? Remember, humans eat some of these products from honey bees.

Protecting the honey bees and ourselves in part of the reasoning for the interest in oxalic acid.

What is Oxalic Acid?

Oxalic Acid is an organic compound that occurs naturally in nature. It is a white crystalline solid that is colorless in water. You will find it in such things as: peanuts, sweet potatoes, wheat bran and pecans.

Some of this acid occurs naturally in honey. And, because it is not fat soluble-t does not build up in the beeswax honeycomb.

But, just because it is a natural organic compound, that does not mean it is not powerful. Oxalic acid is several thousand times stronger than vinegar.

It is bitter to the taste and irritating to the nose and mouth. Beekeepers should use proper chemical protective wear when using this or any other acid.

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History of Use

Oxalic Acid (OA) has been used by European Beekeepers for many years. With reports of 90-99% efficacy in killing mites, they found it be a useful tool in the fight with varroa.

It was only approved for use in the US in 2015 (Api-Bioxal). Be sure to check with your state officials to ensure that the registration for OA is approved.

Thankfully, it is not very expensive. Bee supply companies have product labeled for use in beehives. This is the legal way to proceed. Some beekeepers go off label and use generics too. That is up to the discretion of the beekeeper.

How Oxalic Acid Kills Varroa Mites

We know that oxalic acid kills mites and does little harm to bees. It is about 70 times more toxic to varroa mites than honey bees.

But, there is still debate over the exact way mites are killed. Whether it is through inhalation of the vapor by mites or direct contact with spiky dried crystals?

One theory is that the vapor enters through the soft pads of varroa mite feet and gets into their blood stream. It may also damage varroa mouth parts.

With vaporization we are not trying to coat the bees, rather we are injecting the heated vapor into the hive.

As the vapor cools, small crystals are left behind on the bees, comb and hive walls. As field worker bees come and go-they will be exposed to the crystals.

Either way, we do know that it kills varroa mites with minimal affects on the colony. There are (at this time) 2 approved methods of using oxalic acid for mite control

  • dribble
  • vaporization
Beekeeper using syringe to apply dribble oxalic acid treatment to hive image.

Dribble Method of Using Oxalic Acid

In the dribble (or trickling) method, the acid is mixed with warm 1:1 sugar water (equal parts of sugar and water). Then, (using a syringe) the mix is applied between the frames in the hive – directly wetting the bees.

Care must be taken to not use more than a total dose of 50 ml per colony and of course you would adjust for smaller colonies.

Because the dribble method is hard on the cuticle (exoskeleton) part of honey bees, it should only be done 2 times a year.

And, the bees need to be somewhat clustered together between the frames as well. This means that dribbling is best in cooler weather.

In a similar application, spraying bee packages is sometimes employed by those in the business of selling bees.

Vaporizing Oxalic Acid With a Heated Wand

The most popular method of application is – “oxalic acid vaporization“. A small amount of crystals are placed on a special wand. Most wands require an external power source – like a battery (lawn mower or car).

The wand is inserted into the hive and heated to vaporize the crystals. First the water portion of the crystals burns off.

Once the temperature reaches about 314° F, sublimation occurs and the solids become gas.

There are many types of wands available to purchase. Some work great but are expensive, some are more affordable and work okay and others are pieces of “kaka”.

Varrox vaporizer with oxalic acid powder in the heating bowl image.

The wand I currently use is called the Varrox from Oxavap. It has held up for many years and is still working great.

A cheaper option can work well too – shop around before deciding. This one from Amazon is one option.

When Should You Treat

It doesn’t really matter what time of day you do your oxalic acid treatment. But, very early morning or later in the day will expose more bees to the product quicker.

As for best time of year to treat, doing your varroa mite tests or counts is the only way to know the approximate level of infestation in your hives.

As for when to perform mite treatments, consider the treatment threshold for bees in your area. This number seems to change a bit as the years go by – it is a good idea to keep in touch with local beekeeping associations.

It is important to note that – oxalic acid vaporization is most effective when there is little or no brood in the hive.

This is not always possible for Southern beekeepers-our colonies may have bee brood throughout the season.

A common treatment plan is:

  • late Fall or early Winter
  • before honey supers go on the hive in early Spring
  • for new hive splits with little capped brood
  • mid-late Summer (3 treatments required)

How to Treat Beehives with OAV

This is an overview of one way to use a vaporizer in your bee yard. Please use appropriate safety gear to protect your skin and lungs. Do NOT inhale the vapor – do not.

Also, follow directions that come with your equipment and stay up to date with beekeeping periodicals. Bee stuff changes from time to time.

Materials Needed

Following all safety precautions as set forth on the label-proceed with the following materials.

  • safety gear – as directed on OA label (gloves and mask)
  • oxalic acid crystals ( 1 gram per brood chamber box = approx 1/4 teaspoon (for my 1 deep and 1 shallow brood configuration I use 1/2 tsp).
  • vaporizer
  • power source for vaporizer
  • measuring spoon
  • timer
  • bucket with cool water (if your model allows cooling)
  • rags, old towel, etc – to use as temporary plugs
  • grid board – if you have screened bottom boards

You should keep most of your tools together in one place. I use an old plastic cat litter container. Anything will work.

OAV Steps

1. Insert the grid board (IPM) under your hive if you are using screened bottom boards. We do not need an air-tight seal, just something to reduce drafts.

Remove the hive top and loosely plug the hole in the inner cover. I use whatever is handy, a piece of wood, packing foam etc.

Middle hole in inner cover of beehive closed with foam.

2. Measure the required amount of crystals (for your hive size and bee population) and place on a cold wand. (Do not heat it first – that is dangerous.) Slowly insert the vaporizer wand into the hive entrance.

Oxalic acid crystals on cold wand.

4. Lay an old towel (or similar) across the front of the hive. This temporarily plugs the entrance – keeping foraging bees out and hopefully some vapors in.

You can seal the entrance and any cracks with painters tape but be sure to remove it all at the end.

5. Connect vaporizer to power source and set a timer for 2.5 to 3 minutes. The time varies depending on the model vaporizer you have. This will require a bit of testing. Stand upwind from the hive during treatment.

Towel shown blocking hive entrance and vapor escaping when wind blows towel away image.

6. When the timer is done, wait a couple of minutes to let the vapor cool before removing the wand. It will still be hot – be careful.

If things went well, all the crystals in the wand will be gone. If any remain, you need to leave the wand in the hive for a short while longer – or maybe your vaporizer is not working properly.

We do want to ensure the hive received the full amount of Oxalic Acid for treatment.

7. After removing the wand, wait an additional 8-10 min before opening all the entrances and beehive ventilation ports.

Take care on windy days. A simple towel is fine as a temporary hive entrance block-but if the wind blows it off – your important vapors escape.

Hot wand being removed from treated hive after mite treatment.

Oxalic Acid Vaporization With Brood

No varroa mite treatment is perfect. Oxalic acid treatments only kill phoretic mites. Those are the foundress mites that are riding around on the adult bees.

Most of the time, the majority of mites in a hive will be inside the capped brood. Oxalic Acid does not kill mites under the cap.

This is why the very best time to use OA is when the colony is broodless. For many Southern beekeepers, finding a natural treatment to use for mite control is very difficult.

Some of the other “softer mite treatments” are temperature sensitive. At the time of year when I really need to treat my colonies, it’s too hot to use some of the approved methods.

By the time the daily temps cool down, my colony may already be dead or so infested with mites that it has one foot in the grave.

Three Time Summer Special

To overcome this challenge, we do 3 vaporization treatments that are 4-6 days apart. In this way, as more mites emerge with new bees -most of the mites will be out of a cell and exposed to one of the treatments.

When treating the hives in cooler weather – I often am able to stick the wand through the largest opening of the entrance reducer. Less disturbance for the bees. But, when you remove the wand – watch out!

Oxalic acid vaporizer wand inserted into the front of a hive image.

Testing Efficacy

Even though I too am guilty of getting lazy – every treatment should be followed up with a mite count a couple of weeks later. No method always works. Don’t just treat and forget it – do your counts.

We know that this treatment is best used when no brood is present. But, most beekeepers use it during times when the colony does have brood.

A recent study suggested that this 3 treatment regime is just not enough to kick the mite load down far enough.

This is another reason to do your after treatment check – especially in late Summer. If mite levels are still up, you have time to use another method when cooler weather arrives.

If you are trying to avoid using the harsher chemicals (Apivar, Apistan etc.) and hot weather prohibits the use of formic acid or thymol – OA is the only thing we have right now.

Safety Precautions

Oxalic Acid is safe to use for the beekeeper and the bees as long as proper safety protocols are used.

Follow the instructions on the label – you will need a properly rated acid gas respirator. An N95 is the standard.

It is also a good idea to keep your bee smoker going in the bee yard. Watch the smoke and this will help you remain upwind of the hive and any leaking vapor.


What does the outside temperature have to be to use OA Vaporization?

As long as the outside temperature is at least 37° F for a couple of hours following treatment – you are good to go.

There is no high temperature threshold – I would say – as long as you don’t pass out in your bee suit!

Must you completely seal the beehive prior to vaporization?

It is not necessary to create a tight seal for the hive. If you have screened bottom boards – insert a grid during treatment.

Have something to temporarily close the hive entrance and any large openings – its just for a few minutes.

Will OA contaminate my hive and comb?

No, if you follow the directions and use the proper vaporization process no damage should occur to your comb or bees. Within a few days, the bees will remove all the remaining crystals from the hive.

Can I use OA vaporization with honey supers on the hive?

It depends, a ruling by the FDA regarding tolerable levels of OA in honey and comb opened the way for a exemption to the old label.

Technically, the label is the law – if your package says you can use it with honey supers on – you can.

Otherwise, the honey supers should not be left on the hive during vaporization. You can place a barrier between the honey supers and treatment boxes or simply remove the honey super for a few minutes.

Within 15 minutes of removing all the seals, it should be safe to return your honey boxes to the hive.

How long does it take to vaporize oxalic acid?

With a good wand, it should only take 2.5 – 3 minutes to vaporize your OA crystals.

How many times a year do you need to treat for varroa with oxalic acid?

This depends on several factors including the genetics of your bees and where you live.

Varroa mites are a bigger problem in some areas. Whenever you treat your colony, be sure to do another mite check in a couple of weeks to make sure it worked.

Final Thoughts

For now, there is no perfect way to kill the mites in your colony. All treatments have some effect (if only temporary) on the bees or the comb and honey inside. But, doing nothing is not an option as varroa will eventually kill the colony.

Oxalic Acid Vaporization is an effective way to manage varroa in your beehives. As with any treatment method, it is one part of an overall plan for healthy productive beehives.

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  1. Charles Garvey says:

    Thanks for your helpful info. for this new Beekeeper. You are a lifeline. I trust your advice in this confusing environment. I am amazed at all the differing info out there.

  2. Charlotte Anderson says:

    Thank you and best of luck with your bees.

  3. I read 2 things I think you need to correct and update.
    First is the dosage. 1gram of OA is 1/4 not 1/2 teaspoon. You use 1/4 teaspoon for a single deep brood chamber and 1/2 for a double deep. If you meant a double deep, you should say that.
    Second, if you read the new labels on the OA sold by Better Bee, which I believe is the only approved fistributer, it now says you can treat with supers on thr hive.

  4. Charlotte Anderson says:

    Thank you Jackson, I always use 1/2 tsp for my two boxes even though they are not double deeps = but I rewrote those instructions and hope I made it clearer as to the correct recommendations.

  5. Hi- how soon can you wrap (winterize) hives after a late fall/early winter oa tx? Last year I used oa siccessfully but I had a couple weeks before finishing tx and wrapping . This year I have time to tx then wrap and want to make sure that’s ok. Thanks.

  6. Charlotte Anderson says:

    I don’t think you would need to wait but I would likely give them a few days – just because. As long as you are just wrapping the outside and leaving ventilation and an entrance opening – I don’t see why it would hurt.

  7. Theodore Koutroumbas says:

    Hi Charlotte..! I’ve read your article, about treating honeybees for varroa mites with vaporization, and we thank you with the advice and recommendations that helps everyone. I want to ask you if using oxalis acid with fogger has the same results as vaporizing. Thank you, Theodore..!

  8. Charlotte Anderson says:

    Right now the only two valid methods of using OA is vaporization and dribble. Does fogging work? Does it harm the colony in some way we don’t yet know? Who knows – the fact is we don’t know. I would only recommend doing the methods that we know are effective and safe. In time that could change but right now that is the recommendation. People are always looking for quick and easy. And, that’s okay – but it is not always the best way.