varroa mites

Oxalic acid best practices: safely treat for varroa

Help your honey bees thrive in spite of the mite. Follow these simple steps to assure their health and yours.

We should not ignore oxalic acid best practices, no matter how safe we feel. Each time I read an article on oxalic acid treatment, the author insists on listing foods that contain oxalic acid. Apparently, this should put us at ease. After all, if rhubarb, pecans, spinach, sweet potatoes, swiss chard, and honey contain oxalic acid, how toxic can it be?

Actually, it can be plenty toxic. The Environmental Protection Agency (EPA) doesn’t affix a skull and crossbones to the Oxalic Acid Dihydrate page because they think it’s cute. No, they put it there to give you a heads-up.

The dose makes the poison

The old saying, “the dose makes the poison” is perfectly apt in this case. It simply means the size of the dose determines whether it is poisonous to an organism. For example, the opioid painkiller Fentanyl is used in medicine to control severe pain. But at the street level, it can easily be deadly.

The powder you use to control varroa mites is 97.0 percent oxalic acid dihydrate. Your average rhubarb leaf is 0.5 percent oxalic acid. In other words, the powder is roughly 194 times as potent. It’s sort of stupid to pretend your OA powder is like a food additive.

Just because a compound exists in food, doesn’t mean it’s safe at extreme levels. The sweet almonds you eat by the handful contain cyanide, but that doesn’t mean cyanide should be treated like a condiment.

The government is here to help

I am not trying to scare you, nor is the EPA. The warnings, the directions, and the safety requirements are published to remind you of potential dangers to yourself, your bees, and innocent bystanders like children and pets. As beekeepers, we are lucky to have oxalic acid in our arsenal against varroa mites, but we should use it with care.

We taxpayers pay for agencies like the EPA and the FDA to monitor chemicals that affect our environment and the health of humans and animals. You’ve paid for their advice so you may as well heed their warnings.

You can wade through blog posts and YouTubes until you’re dizzy, but everything you need to know about oxalic acid is in one concise document, called a label. That document is online and easily available at any time of the day or night. If you feel confused or uncertain, just look it up. When it comes to chemicals and drugs in the states, “the label is the law.”

And yet, nobody reads the label

I know that not many people read the label, especially bloggers. I read two blog posts published in August 2022 that say specifically that the EPA does not allow honey supers to be in place while using oxalic acid treatments.

Yet, if you read the label right now, it says, “Oxalic acid can be used when honey supers are on the hive.” What does that mean? It means bloggers are not doing their homework, at least since April 30, 2021, the date of the supplemental label.

General directions for treating varroa mites

Read the general directions carefully before starting to use oxalic acid. I will highlight some instructions here, but the EPA lists more.

Oxalic acid must be used in the hive only, but not in enclosed overwintering facilities.
Use only when mite treatment thresholds are met. The thresholds will vary by state.
Use when little or no brood is present. OA will not kill mites beneath wax cappings, but may damage bee brood.
At the time of application, the user must be in possession of both the Oxalic Acid Dihydrate container label and the EPA supplemental label.

Methods of applying oxalic acid

The EPA currently recognizes two methods for applying oxalic acid. The first is the Solution Method and the second is the Vaporizer Method. The Solution Method is further broken down into the trickle (or dribble) method and the spray method.

Please note that the so-called shop towel method is not approved at this time, and asking me in an email won’t change that. Sorry, but it’s not up to me. For now, let’s look at the approved methods separately. Each has its uses.

Oxalic acid in solution

According to the label, oxalic acid can only be applied as a solution when it is mixed with sugar water. Furthermore, the label recommends that the syrup be warm to assure the powder dissolves completely.

Anyone using the Solution Method must wear:

A long-sleeved shirt and long pants
Socks and shoes
Chemical-resistant gloves (see the label for details)
Goggles or other protective eyewear
A NIOSH-approved particulate-filtering facepiece respirator with any N, R, or P filter (the letters refer to the amount of oil resistance, which doesn’t apply here)

The trickle method (aka dribble method)

Dissolve 35 grams of OA dihydrate in 1 liter of 1:1 sugar water
Smoke the bees down into the brood box
Trickle 5 ml of the solution into each seam of bees (a seam is the space between two adjacent frames)
Use a maximum of 50 ml of solution per colony, regardless of the number or size of brood chambers (or 5 ml in each of 10 seams)

The spray method for packaged bees

Spray a broodless package of bees with 1:1 sugar solution two hours before treatment (to fill honey stomachs)
Dissolve 35 grams of OA dihydrate in 1 liter of 1:1 sugar water
Spray 3 ml of OA solution for every 1000 bees (a two-pound package contains about 7000 bees)
Store sprayed package in a cool, dark room for 72 hours before installing the package into a hive

Vaporizer application of oxalic acid

Anyone using the Vaporizer Method must wear:

A long-sleeved shirt and long pants
Socks and shoes
Chemical-resistant gloves (see the label for details)
Goggles or a face shield
A NIOSH-approved elastomeric half-mask respirator with acid gas cartridges and combination N, R, or P filters

Once your safety equipment and vaporizer are ready, just follow the instructions on the EPA label:

Use only on colonies that have a restricted lower hive entrance
Seal all upper entrances and cracks with tape (in order to keep the vapor from seeping out)
Smoke the bees using the lower hive entrance (so bees go up into the hive)
Use 1.0 grams of oxalic acid dihydrate power for each brood box
Add the measured powder into your vaporizer and follow the manufacturer’s directions (all vaporizers are slightly different, but they all come with directions)

Application isn’t difficult once your equipment is ready

Most of the horror stories I’ve heard about using oxalic acid were the result of not following directions. Stories of choking on fumes, splashing hot acid, or burning down hives are not hard to find, and some are quite scary. You can avoid all that by just reading the label and following these simple instructions.

As far as equipment goes, everyone has a favorite vaporizer they prefer to use. Some of the popular ones are fairly inexpensive on Amazon.

Applying oxalic when lots of brood is present takes some planning. Since OA doesn’t kill mites under wax cappings, repeated applications are needed to kill the mites as the new bees emerge. For more information, see “Using oxalic acid vaporization when brood is present.”

Additional Restrictions

According to the EPA label, “This product must be used strictly in accordance with this label’s precautionary statements and use directions, as well as with all applicable State and Federal laws and regulations.” [emphasis added]


Varroa mite resistance management

The EPA label points out that resistance can arise at any time to any treatment. Because of that, oxalic acid best practices include a recommendation related to resistance management.

The EPA recommends rotating the use of mite treatments rather than using the same one time after time. Specifically, the label reads, “When possible, rotate the use of miticides to reduce selection pressure as compared to repeatedly using the same product, mode of action, or chemical class. If multiple applications are required, use a different mode of action each time before returning to a previously used one.”

Lots of folks insist that mites cannot possibly become resistant to oxalic acid. Well, maybe. But just because it hasn’t happened yet, doesn’t mean it won’t. I put odds on the mites.

Rusty Burlew
Honey Bee Suite

Note: the post contains an Affiliate Link.

24 Comments

  • “ Spray a bloodless package of bees”… while I think the typo is amusing, pretty sure you mean a “broodless package”. (It took me three tries to get that past autocorrect!)

    • Bill,

      Thanks for that suggestion. I added a short section on the necessity of heeding all applicable state and federal laws and regulations.

  • I read too fast and sloppy to catch typos, and I’m just here to sign up for comments, so pay me no mind.

    Although maybe it’s worth noting that I bought the equipment to do oxalic acid vaporization years ago and I’ve never used it because [pick one] it’s too scary / it sounded too much like work.

  • Yes, read the label closely: “This label expires on November 3, 2022 and must not be distributed or used after that date.”

    • Matthew,

      Correct. That date pertains to the Supplemental Labelling Update, EPA Reg. No. 91266-1 dated 04/30/2021. We are still subject to it as written through November 3, 2022. The update was in regard to Personal Protective Equipment and Directions for Use, and will likely be incorporated into the standard label. It’s on my calendar to check and update as needed.

      • Are you remiss in not stating that ‘technically’ Api Bioxal is the only approved Oxalic Acid? We are always pushing ‘The label is the law’ but it should also be stated that wood bleach from your local hardware store isn’t what you should be using. Yes, I know. very few folks are really using Api Bioxal but, to my knowledge, they are the only approved product in the US.

        • Matthew,

          I don’t see any reference to specific brands of Oxalic Acid Dihydrate in the EPA regulations, only that it must be 97.0 percent pure. Could you point out where I can find that information?

          • This is where things get REALLY confusing and even I don’t follow it. However, the USDA is the registered owner of OA as a varroa treatment. The USDA then states that Api Bioxal is the only approved product. See quotes below and link to a great USDA site that talks about it.

            “The in-hive use of oxalic acid to control Varroa mites (Varroa destructor) is currently a USDA-held registration.” And then this: “With this it is important for beekeepers to understand that the registration and legal use of oxalic acid in the U.S. for in-hive use is currently only through the registered product API-Bioxal™ (of which oxalic acid is the active ingredient).”

            https://www.ars.usda.gov/northeast-area/beltsville-md-barc/beltsville-agricultural-research-center/bee-research-laboratory/docs/oxalic-acid-faqs/

          • Matthew,

            Here’s another interesting tidbit: In this YouTube recorded August 4, Randy Oliver states, “Oxalic acid is not registered for use at all anywhere in California. I am the only beekeeper in California who can legally put oxalic acid in a bee hive in order to kill mites.” (Start at minute 25:59.)

            The reason, apparently, is that oxalic acid is not a registered pesticide in any California county. I could not find anything about oxalic acid on the California Department of Pesticide Regulation website because it is not listed there.

          • No OA in CA for VD… that I did not know! I bet all top bars of frames in CA are bleached quite nicely! 🙂

  • Where is the proof that oxalic acid is illegal in California? Randy Oliver has his hives in CA. I find it hard to believe that Randy Oliver would be himself using and promoting something that is illegal in his home state.

    • Matt,

      Randy has a pesticide Research Authorization and is working directly with the EPA, Agricultural Research Service, and California Department of Pesticide Regulation to get his method approved.

      • Rusty,

        I think Matt was referring to Bill Hesbach comment about OA being illegal in California. I think your reply was in reference to OA sponges.

      • Right, but his permit is for testing “extended relaease” oxalic acid in glycerin and associated delivery methods. Not oxalic usage as a whole. Only extended release in glycerin. He says he’s been doing dribble method for years. Hard to believe oxalic acid dribble with no supers is illegal in CA. Respect to you and the blog.

        • Hi Matt,

          I intended to do a state-by-state chart of regulations, but it’s overwhelming, so I’m sticking with a reminder to check your own state and local regulations before using oxalic acid. Some states are more restrictive than the EPA and at least one state has a special exemption for using extended-release OA. One of the California bee clubs said California is county-by-county for methods of oxalic use, but I can’t find any verification for that either. After your comment yesterday, I spent hours digging through California codes and regulations but found no satisfactory answers. If you know where to look, that would be helpful.

          The main point of my post was that users, regardless of the application method, should be careful with OA. I hear of many accidents which should not have happened. If regulators see too many of these “accidents” they have the option of retracting a registration. The loss of OA would be devastating for both bees and beekeepers, so it pays to play it safe.

  • I understand the dangers, skull and crossbones, not approved yet Oxalic Acid Dihydrate very dangerous warnings. Please don’t use it! Only those with the proper paperwork, training, PPE, and strict experimental conditions should qualify for its use. But if I had to choose honey and comb honey from a grocery store shelf with EPA/FDA approved fluvalinate, coumaphos, and amitraz in it or my own that I only use oxalic acid on (which I would never do) for a classroom of children to sample I know which one I would choose. I guess let the EPA/FDA or your conscience be your guide.

    • Mike,

      I agree. But fluvalinate, coumaphos, and amitraz are not authorized for use when honey supers are present. Do beekeepers follow that advice? Who knows? This same question resonates throughout the entire human food supply.

  • Oxalic acid vapor is seriously dangerous and can cause permanent injury to the respiratory system or death if breathed in high concentrations. A protective face mask covering the nose and mouth so user breaths filtered air is important. According to the 3-M tech rep, a correct filter for use with the protective mask is (OV), for organic vapor. Acid vapor filters are harder to find but OV filter mask kits are readily available in home improvement stores and on-line. The N, R, and P ratings alone do not provide needed protection.

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