Api-Bioxal®, with the active ingredient of oxalic acid dihydrate, has become a common tool used by beekeepers to treat colonies of bees for controlling the parasite Varroa destructor. It comes in a crystal formation and may be applied to bees in two main ways, either as a liquid or as a vapor. It is important note that oxalic acid (OA) is not effective at killing mites underneath cappings, therefore it is best to apply OA during a time of broodlessness. Going into winter is an ideal time to apply OA since this is a time when honey bees are naturally brooding down.

Oxalic acid is highly corrosive and can be very dangerous to the beekeeper. Be sure to wear proper Personal Protective Equipment (PPE) as directed by the label. You will need to wear chemical resistant gloves, goggles, and a half face respirator as well as long pants, long sleeves, and closed-toe shoes. Inhaling vaporous OA can cause significant damage to the lungs, and liquid OA can be highly toxic and corrosive.

A protective mask is required during application

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A protective mask is required during application

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Dribble Method

ApiBioxal® crystals can be mixed with sugar syrup to make a 3% oxalic acid solution. Directions are on the label, and it is important to always follow the label. After properly mixing, the liquid is then measured using a 50ml syringe and applied by dribbling 5ml over the adult bees between each of the frames where bees are clustered. No more than 50ml/hive is necessary.

This method works best when the bees are in a slight cluster, so applying on a cold morning before it warms up is ideal. Nighttime temperatures begin to drop in NC in late October or early November. This is often when capped brood becomes less common, so this is an ideal time of year to treat using OA.

If done correctly, and during a period of broodlessnes, it is only necessary to do one application. Although not as common, this 3% OA solution may also be used to treat packages before installation, or newly captured swarms.

Vaporization Method

The Api-Bioxal® crystals can be placed into a special device, connected to a power source, for the purpose of heating the crystals to the point of vaporization. The current recommendation is 4 grams of OA per brood box, but always read the label first as these recommendations have changed over time. It is also important to read the instructions for the device being used, as there are several types on the market, and each is slightly different. The wand of the vaporizer is placed inside the hive onto the bottom board while OA crystals are heated. The crystals begin to sublimate and the vapors travel throughout the hive. It is good practice to close any holes, or upper entrances, and to minimize the entrance to prevent the vapors from escaping. This process takes several minutes, and it is important to wait until all the crystals have completely sublimated before removing the device. Remember it is required to wear a respirator and other PPE, but it is also advisable to stand up-wind so as not to accidentally inhale any of the vapors.

This method will kill only the phoretic mites. Any mites under the cappings will not be affected, so this treatment is most effective when used during a period of broodlessness.

Application using the dribble method

Photo Credit: Veto Pharma

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Application using the dribble method

Photo Credit: Veto Pharma

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Vaporization method

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Vaporization method

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VarroxSan® is a relatively new product (first registered in 2025) for controlling Varroa mites. The active ingredient is still oxalic Acid, but the formulation is quite different. This product uses a slow-release mechanism over a period of 42 days (two complete brood cycles). VarroxSan® is sold as long cardboard strips which are wrapped over the top bars of brood frames. As the worker bees walk across the frames they will come in contact with strips and the phoretic mites are killed. One advantage of this is that it can be used at any time of the year, and it may also be used while honey supers are on. Preliminary research suggests that this is sufficient from keeping the mite population from growing in the late spring and early summer, but that it is not an effective knock-down treatment in the fall when the mite populations are already high.

It is not completely understood how OA kills the Varroa mites. Although it is believed that the Varroa mites will not be able to develop a resistance to OA, it is still best management practice to rotate treatments. Relying on one treatment alone may lead to resistance. Api-Bioxal® should be just one of the treatments used, but not the only one relied upon. Since it is most effective during a period of broodlessness, it may be best used as a ‘clean up’ going into winter to ensure low mite numbers throughout the winter months and a healthy colony of bees to start the spring.

VarroxSan strips

Photo Credit: Mann Lake

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VarroxSan strips

Photo Credit: Mann Lake

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For more information, contact the following:

NCDA&CS Apiary Inspection Program

NC Department of Agriculture & Consumer Services

Telephone: (984) 477-8030

North Carolina State University Apiculture Program
NC State University
Telephone: 919-513-3967

For more information on beekeeping, visit the Beekeeping Notes website.