Oxalic Acid for Bees
Oxalic acid is one of the most common treatments for controlling Varroa mites in honey bee colonies. It is inexpensive, effective, and approved for use in many situations. However, because oxalic acid only kills mites on adult bees—not mites protected inside capped brood—timing the treatment is one of the biggest factors in its success. In this guide, I share the two most common ways oxalic acid is used in beekeeping.
When I first became a beekeeper, I struggled to avoid the harsher varroa mite treatments. I wanted a natural treatment that would work effectively in hot weather. Oxalic Acid vaporization – while not perfect – has helped make a difference in my apiary.
Why Many Beekeepers Choose This Treatment
Varroa mites (Varroa destructor) are tiny, external parasites that target honey bees. (I think of them in the same terms as fleas feeding on a dog – however it is a bit different).
Mites feed on the fat bodies of both adult bees and developing young. This weakens bee brood and causes overall bad colony health and decline.
The mites you can see on the bees are called “phoretic” mites. But, it is the mated females hidden inside brood cells that are the major threat. They are reproducing.
What is Oxalic Acid?
One reason oxalic acid has become so popular is that it is both effective against Varroa mites and relatively easy to apply using approved treatment methods.
Oxalic Acid is an organic compound that occurs naturally in nature. The white crystalline solid is colorless in water. You will find it in such things as: peanuts, sweet potatoes, wheat bran and pecans.
How Oxalic Acid Affects Varroa Mites
Used correctly, oxalic acid targets the phoretic mites that are attached to adult bees. When oxalic acid is applied to a hive, it comes into contact with the varroa mites. Sharp acid crystals penetrate the mite’s exoskeleton causing damage – dehydration and death.
There is still some debate among researchers over the exact way mites are killed. Whether it is partially through inhalation of the vapor by mites or solely direct contact with spiky dried crystals. But, the benefits are clear:
Benefits of Oxalic Acid
- Kills phoretic Varroa mites while doing little harm to bees when used properly.
- Varroa mites have not developed resistance to oxalic acid.
- Leaves no toxic residue in hive products such as honey and beeswax.
Because oxalic acid only affects mites riding on adult bees, understanding the colony’s brood cycle is the key to getting the best results.
When It Works Best
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.
I’m not sure it really matters a great deal what time of day you do your oxalic acid treatment. As for best time of year to treat, doing your varroa mite tests or counts is the only way to know if you have a problem.
It is important to note that – oxalic acid vaporization is most effective when there is little or no brood in the hive.
Why Brood Matters
Oxalic acid treatments only kill phoretic mites – the foundress mites found on the adult bees. 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 Southern beekeepers-our colonies may have brood year round. 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)
For many Southern beekeepers, finding a natural treatment to use for mite control is very difficult. At the time of year when I really need to treat my colonies, it’s too hot to use the temperature sensitive natural methods.
By the time the daily temps cool down, my colony may be so infested that I find a dead beehive or one with one foot in the grave.
My Three-Treatment Summer Strategy
To overcome this challenge, I 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.
A recent study suggested that this 3 treatment regime is just not enough to kick the mite load down far enough. I respect that but have not found a better alternative.
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 may be an option.
Understanding the Different OA Application Methods
Oxalic Acid (OA) was approved for beehive use in the US in 2015 (Api-Bioxal). Each method has advantages depending on your climate, management style and the time of year you are treating.
- dribble method
- vaporization
Dribble Method
I do not use this method only because it does not fit in my apiary management plan. But, it is effective for beekeeping in cooler climates.
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 clustered there.
Care must be taken to not use more than a total dose of 50 ml per colony (of course you would adjust for smaller colonies).
Because the dribble method is hard (harsh) on the cuticle (exoskeleton) part of honey bees, it should only be done 2 times a year.
Also, the bees need to be somewhat clustered together between the frames for application. This means that dribbling is best in cooler weather.
In a similar application, spraying bee packages is sometimes employed by those in the bee farm or beekeeping business that sell bees.
Vaporization
The most popular method of mite control with OA is – “oxalic acid vaporization“. A small amount of OA crystals are placed on a special wand – that will heat up.
The wand is inserted into the hive and heated (external power source) to vaporize the crystals. Once the temperature reaches about 314° F, sublimation occurs and the solid crystals become gas.
This vapor disperses through the hive. As the gas vapor cools, small spiky crystals are left behind on the bees, comb and hive walls. Varroa mites exposed to the crystals will die.
With vaporization we are not trying to coat the bees, rather we are injecting the heated vapor into the hive environment.
How to Use Oxalic Acid Vaporization
This is an overview of one way to use an oxalic acid vaporizer. This is my process – always follow directions that come with your equipment and materials. New products are constantly being developed.
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 and power source
- 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
Choosing a Vaporizer Wand
There are many types of wands available. Some work great but are expensive, some are more affordable and work good and others are pieces of “kaka”.
The wand I currently use is called the Varrox from Oxavap. It has held up for many years and is still working great.
I chose it because I could dip the heated bowl into water to quickly cool it down and move on to the next hive – you can’t always do that with the cheaper products.
The cheaper options can work well too – shop around before deciding. Most will require a power source – electric, battery etc.
The Process
Once everything is assembled, the treatment itself only takes a few minutes per hive. This is the process I follow.
1. Insert the IPM grid board (flexible white board) under your hive if you are using screened bottom boards.
We do not need an air-tight seal, just something to reduce drafts. Solid bottoms are ready to go.
Gently remove the hive top and set aside. Then, slowly use something to plug the hole in the inner cover.
I use whatever is handy, a piece of wood, packing foam etc. (I am simply trying to slow down the rate at which the vapor leaves the hive interior).
2. Measure the required amount of crystals (for your hive size and bee population) and place on a cold vaporizer wand. (Do not heat the wand yet – the fumes are dangerous.)
Slowly insert the vaporizer wand into the hive entrance.
4. After inserting the wand bowl (- with crystals) into the hive entrance. I 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 or anything that keeps vapor in for a few minutes but a cloth or canvas is quick and easy for me.
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!
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.
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 while longer – or maybe your vaporizer is not working properly.
7. After removing the wand, I open up a tiny spot on the toweled front to let some unhappy bees locked out – get back in. Wait an additional 8-10 min before opening all the entrances and beehive ventilation ports.
Follow Up With a Mite Count
I too am guilty of getting lazy – but every mite 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.
Safety Precautions
Oxalic Acid is safe to use for the beekeeper and the bees as long as proper safety protocols are used. Protect Yourself:
- Wear the respirator specified on the label.
- Wear gloves.
- Stay upwind during treatment.
- Use your smoker to monitor wind direction.
An Exciting Breakthrough
2024 saw the introduction of a new method for using oxalic acid for mite control. VarroxSan is an organic acaracide that kills mites on contact.
The special strips are placed in the brood box of the hive. With the longer treatment period and ease of use – this may become the major way of using oxalic acid in beekeeping.
FAQs
As long as the outside temperature is at least 37° F for a couple of hours following treatment – you are good to go.
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 for a few minutes.
With a good wand, it should only take 2.5 – 3 minutes to vaporize your OA crystals.
No, if you follow the directions and use the proper vaporization process no damage should occur to your comb or bees.
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.
This depends on several factors including the genetics of your bees and where you live. Whenever you treat your colony, be sure to do another mite check in a couple of weeks to make sure it worked.
Final Thoughts
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, oxalic acid treatments for bees is an effective way to control varroa mites in your hives.
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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..!
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.
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.
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.
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.
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.
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.
Thank you and best of luck with your bees.