Instructions for extended-release oxalic acid

Extended-release oxalic acid for varroa management

Updated 30 March 2025

Legality of use

Understand that the EPA doesn’t give “approval to” any pesticide — it only grants registrations for applicants to distribute and sell specific formulated products for which the EPA has approved the label.  That said, the EPA’s mandate for regulation comes from FIFRA [1], which is completely unclear about “own use” of a pesticide, as opposed to the production, distribution, or sale of a pesticide: ” The dilution by individuals of formulated pesticides for their own use and according to the directions on registered labels shall not of itself result in such individuals being included in the definition of ‘‘producer.”  Furthermore, FIRA Section2(ee) TO USE ANY REGISTERED PESTICIDE IN A MANNER INCONSISTENT WITH ITS LABELING states: The term ‘‘to use any registered pesticide in a manner inconsistent with its labeling’’ means to use any registered pesticide in a manner not permitted by the labeling, except that the term shall not include (1) applying a pesticide at any dosage, concentration, or frequency less than that specified on the labeling unless the labeling specifically prohibits deviation from the specified dosage, concentration, or frequency, (2) applying a pesticide against any target pest not specified on the labeling if the application is to the crop, animal, or site specified on the labeling, unless the Administrator has required that the labeling specifically state that the pesticide may be used only for the pests specified on the labeling after the Administrator has determined that the use of the pesticide against other pests would cause an unreasonable adverse effect on the environment, (3) employing any method of application not prohibited by the labeling unless the labeling specifically states that the product may be applied only by the methods specified on the labeling.” (boldface mine).

The EPA has already registered two sources of oxalic acid (ApiBioxal and EZ-OX), and their labels do not prohibit mixing them with glycerin as an application method.  I’m no attorney, but the above definitions from FIFRA indicate that a beekeeper could dilute either ApiBioxal or EZ-OX with glycerin when they apply it, so long as they did not exceed the allowed dose per hive. Since the approved label dose for VaroxSan is up to 56 g of oxalic acid dihydrate per 20 combs in the broodnest, that would suggest the EPA’s maximum allowed dose.  However, EZ-OX is registered for only 4 g per double deep. so I’ll leave it at that.

The EPA is justifiably concerned about beekeepers contaminating their honey when they mix oxalic acid with other substances.  Their 2024 Advisory states that “EPA considers any application of an unregistered pesticide for other than personal use to be a distribution of an unregistered pesticide and a violation of FIFRA.”  And further ” An individual raising bees as a hobby and consuming themselves whatever honey is harvested might be considered “own personal use.” But as described above, an individual beekeeper cannot sell or distribute (which includes transportation) any unregistered pesticide and cannot sell or distribute any adulterated honey or other edible beehive products.”

The EPA attorneys danced around the subject of “personal use.”  But according to the above, if you are not selling or distributing oxalic acid for pesticidal purposes to others, your own use in your own hives all boils to whether you are “adulterating” (contaminating) any honey for sale.  So we need to understand how to interpret “adulteration.”

To continue from the Advisory: “Off-the-shelf products that contain active ingredients but are not registered may contain “other ingredients” or may be combined with other substances for which tolerances or exemptions have not been established. Honey that contains residues of a formulation comprised of multiple substances (e.g., an active ingredient and other ingredients) where those residues are not covered by a tolerance or exemption from the requirement of a tolerance, would be considered adulterated. The sale and distribution of adulterated honey would be a violation of the FFDCA.“

Again, I’m no attorney, but  since both glycerin and cellulose are on the EPA’s Minimal Risk Inerts list [2], and thus exempt from a tolerance, so as long as you don’t include any other contaminates, it appears that although the EPA would place no restrictions upon one’s creating and applying their own uncontaminated oxalic/glycerin strips on an uncontaminated cellulose matrix to their own hives (but couldn’t sell them).

Note that the above applies only to the use of a registered source of the OA (we are working on trying to get EPA to put it on the Minimal Risk list).  It wouldn’t count for use of generic oxalic acid.

Practical application:  The advantage of making your own sponges or strips using EZ-OX (if bought in bulk) is that you could cook up a 50-g dose of strips or pads for about $3.00, compared to $12 for the same dose of VarroxSan.

Action item:  We need beekeepers in every state to ask their State Lead Agency to allow them to formulate their own extended-release oxalic acid in glycerin strips, using either EZ-OX or generic 99.6% oxalic acid.

Disclaimer: I am not giving legal advice!  Enforcement is up to your State Lead Agency, as is the decision whether they will allow you to use unregistered off-the-shelf OA for varroa control. I experiment with mite treatments under a Pesticide Research Authorization, and do not promote the use of any treatments restricted by your State.  I have been researching OAE for some years now, always under a PRA, and am continuing to improve the method.  The information below is only a “current report” on an application method that has to date shown the best efficacy and least adverse effects.

[1] The FEDERAL INSECTICIDE, FUNGICIDE, AND RODENTICIDE ACT

[2] Inert Ingredients Eligible for FIFRA 25(b) Pesticide Products

The Concept

Oxalic acid applied by dribble or vaporization is highly efficacious if a colony does not contain any sealed brood, but not so if there is brood present.  By extending a slow release of OA on a pad or from hung strips, there is a brief drop of mites over the first few days, but then a slow reduction of the mite population over the next two months (we’re not completely clear on how it works).  There is no buildup of OA in the hive, and it does not contaminate the honey.  Although it can usually bring down high mite levels, it takes two months to do so.  So it is best applied proactively in springtime or early summer.  The purpose of the glycerin is to absorb moisture from the air within the cluster, which then dissolves and activates the OA in the matrix.  Once a pad “goes dry,” it is no longer efficacious (if you touch a pad lightly with a dry fingertip, your fingertip should taste like you’ve touched lemon juice).  Caution: do not perform more than 1000 taste tests per day, or you may exceed the amount of OA found in a serving of spinach).

Safety

Although oxalic acid is not as reactive as stronger mineral acids, it can still cause eye damage, and if left on the skin, tissue damage.  Always wear safety glasses and waterproof gloves (we find 5 mil vinyl or even disposable restaurant gloves to be adequate) during preparation, and be careful to avoid splashing.  Tip: when handling acids, rub dry baking soda on your hands before putting on gloves.

Always keep a jug of  strong baking soda solution at hand! Mixed at the rate of 1 cup of baking soda per gallon of water.

After preparation or application, wash your hands and equipment with soap and warm water to remove any acid residues, or better yet, neutralize any acid on hands, hive tools, or smoker with the baking soda solution, which will immediately neutralize any acids.

After preparation or use of oxalic acid, I always taste my fingers to see whether there are any acid residues on them.  If there are, they will taste sour like lemonade. Wash or neutralize them before touching anything else (door handles, your eyes or nose, or taking a leak).

Dosage and delivery matrices

For extended-release application of oxalic acid, it can be dissolved into glycerin (I prefer soy rather than palm oil, since it does not involve the destruction of orangutan habitat), and applied to the hive by either laying pads across the top bars (if applied between two brood chambers), or by hanging strips over the top bars, extending down into the interspaces between the frames.  The delivery strips or pads must be applied so that bees freely contact the surfaces.

Biodegradable cellulose matrices such as cardboard (chipboard), Swedish sponges, or cotton absorbent fabrics may be used (https://scientificbeekeeping.com/8466-2/).  For full efficacy, roughly 55 – 60 square inches (365 cm²) of delivery matrix must be used if applied across the top bars, or 100 square inches (645 cm²) if hung between the frames.  The instructions below are for moisturizer-free Swedish sponges, which hold 100 g of 1:1 (weight to weight) solution of oxalic acid dihydrate to glycerin.  Other matrices, or different ratios of acid to glycerin, will require different preparation.

Preparation

For extended-release application, oxalic acid can be dissolved in glycerin, absorbed into any number of absorbent matrices.  Field data suggest that for a double-deep hive, there should be roughly 60 square inches of matrix, holding roughly 100 g of 1:1 OA:glycerin (weight to weight), although other ratios may be used (our testing indicates that the 1:1 ratio gives better results than a higher glycerin ratio).  For cardboard strips to be hung over the top bars, it will require more total surface area to hold the same amount of solution.

To prepare enough Swedish sponge pads to treat 10 full-sized colonies in double deep boxes (one full sponge per hive):

• First prepare the sponges by cutting them in half (into 3½” x 8” pads), each half of which will absorb 50 grams of the solution (50 g oxalic acid dose per hive), or 1.5” x 15” chipboard strips (each will absorb ~20 g of solution).
• Wear safety glasses and waterproof gloves when preparing the solution. Have a neutralizing solution of baking soda dissolved in 1 gallon of water on hand, to neutralize any spills.
• Place 500g OA dihydrate into a stainless steel pan (for 20 sponge halves), then add 500g (400 mL) vegetable glycerin (add the glycerin second in order to avoid splashing of the solution).
• Place the pan over a low/medium heat (preferably using a double boiler), and heat the ingredients while closely monitoring the temperature, not to exceed 160°F (70°C)  (the acid crystals will dissolve at as low as 110°F, the solution will start to bubble if the temperature exceeds 170°F due to a chemical reaction for which it is unclear whether it is of benefit.).
• Continually stir gently until the acid crystals are completely dissolved (it is important to stir, since the oxalic crystals will melt and form a clear layer under the glycerin, which will float above, thus resulting in the oxalic acid overheating).  Once the solution is completely clear, remove the pan from heat.
• The best way to prepare the pads or strips is to first determine how much hot solution a strip will absorb (use a scale).  Then multiply that amount by the number of strips and pour that amount over the pads or strips.  This will avoid excess solution from being on the surfaces of the matrix when you apply it in the hive.
• While the solution is still hot, either (A) add twenty (20) (3½” x 8”) absorbent cellulose pads (or fifty (50) 1.5” x 15”(6.35 x 38 cm) cardboard strips) on edge into the pan and allow them to absorb the solution, or (B) place the pads into a separate plastic container and slowly and carefully pour the still-hot solution over them. With either method, you may need to use tongs to carefully turn the pads over to obtain full absorption (which must take place before the solution cools).  Be careful to avoid splashing of the solution!  The hot solution will be absorbed rapidly, so long as the solution is hot.  You may need to reheat the pan, or as sugested by beekeeper Dave Gaetano, wrap the pan in insulation.
• Caution:  The chipboard strips should not be bent when placing them in the solution, since they can spring and splash!  Use a 20″ stainless steel steam table pan.
• Note if you are working in an area with high humidity: place an airtight cover over the the hot solution and the matrices to minimize absorption of moisture from the air, otherwise the glycerin will absorb considerable moisture.  One maker suggests working in a dehumidified room.
• If all the solution does not absorb, the excess should be drained off before allowing the pads to completely cool.
• For easier handling, allow the pads to cool (sealed from the air if you are in high humidity) for at least a day before application. The oxalic acid will recrystallize during this time, and make the pads easier to handle and apply, with no dripping of solution.
• Storage:  The pads can be stored in a labeled sealed container for up to 2 months, by which time the cellulose will slowly start to degrade. If exposed to of high humidity, the pads will absorb moisture and be much messier to apply.  Once inserted within the cluster, they quickly absorb moisture and “wet up.”
• Checkbacks during the course of treatment:  We spot-check regularly to make sure that the pads or sponges are moist on the surface, and if touched with the fingertip, have a strong sour taste.  If they dry out and don’t pass the taste test, they are no longer active.

Notes on crystallization

Sources for Swedish Sponges

I prefer to use Swedish sponges that are free of moisturizers or plastic mesh, and that are biodegradable in my compost pile.

If You Care brand fits the bill, and are readily available on Amazon.  They are cheaper in orders of 500 or 1000; contact Paul Bliss <paul@blipservices.com>

Another company in Colorado sells seconds to beekeepers;  go to https://swededishcloths.com/pages/bees

Using maximizer lightweight pads

For commercial beekeepers, Maximizer pads are cheaper than sponges, and easier to scrape out at the end of their lives.

We get eleven (11) – 3.25″ x 7.375″ pads per Maximizer (lightweight, part number GPC 100S) sheet.  We apply two pads per hive between the brood chambers.

Each pad holds ~60 g of 1:1 solution (67g fully saturated; 57 g lightly saturated for easier handling).  That means ~30 g each of OA and glycerin per pad, 60 g each per hive.

Note: these pads contain a fire retardant.  I can’t disclose what it is, but it’s not one that causes me concern, but may be of concern to regulatory agencies.

Other Matrices

I am continually experimenting with other matrices, limiting myself to biodegradeable ones without added chemicals.

Application

• Optimal timing of this treatment in treatment rotation is to apply the pads into the brood chamber at time of placement of the honey supers. This treatment should only be used once while colonies are rearing brood, rotating with miticides with other modes of action, such as formic acid, thymol, amitraz, or fluvalinate.  An oxalic dribble or vaporization can then be used during the winter brood break.
• Wear waterproof gloves.
• Using gloved hands or tongs, apply two pads between the brood chambers, placed so as to be within the cluster (avoid placing under a top feeder where syrup may spill, or directly against pollen substitute). Placement of the pads must allow for movement of the bees over both surfaces.  The pads or cardboard strips can also be hung over the top bars, inserting them spread between the two brood chambers.  For cardboard strips, 3-4 will be needed per brood chamber.
• The pads or strips must be placed within the cluster.  If placed where bees don’t walk over them, no distribution of acid occurs.
• A tip from a user of the strips: use strips that do not reach fully to the bottom of the frames, so that the queen can walk around them.
• For optimal efficacy, the pads or strips must remain in the hive for 60-75 days, or until most of the acid has been distributed. After treatment, remove the pads, handling them carefully, since they will still contain acid.
• After removal, place the spent pads into a plastic bag or container for transport, and you’re your hands, hive tool, and smoker with neutralizing solution.
• Dispose of the spent pads in a landfill, or compost them.