What is oxalic acid (OA)? 

Oxalic acid dihydrate (OA) (also known as wood bleach) is an organic acid naturally found in plants including spinach, rhubarb, black tea, and honey. Although it is considered an organic chemical, it is highly corrosive and causes severe burns if used improperly.

OA is becoming a popular method for varroa mite control in the US. It was legalized in 2015 but, it has been legal in Canada since 2010 and in Europe for over 20 years. OA kills phoretic mites in the colony but it cannot penetrate capped cells. It is therefore recommended for broodless colonies, during winter months or in the spring before the queen starts laying eggs.

How should I apply OA? 

The three most common modes of oxalic acid (OA) applications are the trickle (also called dribble), spraying and vaporization methods. The trickle method is the most popular one in Europe and Canada because it is cost effective and easy to use. However, results from our research group have shown that the vaporization method leads to higher mite drops than the trickle method (Fig. 1).

Before applying OA to colonies, please keep in mind the following recommendations:

  1. OA is most effective during periods of no brood and when there are no honey supers present
  2. The best time to apply OA is late fall or winter
  3. It is important that OA is used at recommended dosages (see below)
  4. OA  must be certified for use in honey bee colonies. Be sure to buy OA from a reputable beekeeping supply company. The OA or ‘wood bleach’ that is sold in hardware and other stores may not be pure oxalic acid dihydrate and using it in honey bee colonies is not included on the label. Using a product on something not listed on the label is illegal, including OA from hardware stores.
  5. Follow instructions stated on the OA label
  6. Wear protective gear

For both trickle and spraying methods, the oxalic dihydrate crystals must be dissolved in warm sugar water (one part sugar to one part water by volume or weight). Granulated sugar should be used, not powdered sugar. 


The trickle method is applied to colonies through a 60ml syringe using a solution with 3.5% oxalic acid dihydrate crystals to a 1:1 sugar to water ratio.


Preparation of 1:1 sugar syrup by volume: Half fill your container with sugar, add warm water to fill the container, and stir or shake until the sugar is completely dissolved and the solution is clear. Sugar syrup solution can be prepared using a microwave or stove top. However, heat the water before the sugar is added to avoid heating the sugar. Heating fructose at high temperatures releases a chemical (hydroxymethylfurfural) that is toxic to bees. Click here to read more

Preparation of 1:1 sugar syrup by volume Dissolve 1/2 liter of sugar (17 ounces) in 1/2 liter of water (17 ounces)

    Total volumeGranulated Sugar         Water
1 liter (34 ounces)1/2 liter (17 ounces)1/2 liter (17 ounces)


Preparation of 1:1 sugar syrup by weight: Dissolve one pound sugar (2.27 US cups) in 1 pound of water (16 fluid ounces).

Total volumeGranulated SugarWater
1 liter (34 ounces)1 pound (18 ounces)1 pound (16 ounces)




Preparation of 3.5% OA solution: To prepare a 3.5% OA solution, measure one liter of sugar syrup and stir in 35 g of OA crystals until it is completely dissolved. The crystals dissolve best if the sugar syrup is warm. All the crystals must be completely dissolved, which may require vigorous shaking. Be sure to place stored OA solution in a sealed and labeled container. The solution can be left at room temperature for several days if used promptly and kept refrigerated for several months. Prepare the appropriate amount of solution based upon how many colonies you have. For example, one liter of oxalic acid solution will treat 20 colonies. Evidence shows that concentrations higher than 3.5% do not necessarily result in increased efficacy against varroa mites and could cause harm to the bees.

Sugar Syrup volumeOA crystals (grams)Colonies treated
1 liter (34 ounces)35 g (1.23 ounces)20
½ liter (17 ounces)17.5 g (0.6 ounces)10
¼ liter (8.5 ounces)8.5 g (0.3 ounces)5



No colony should receive more than 50 ml of OA. Using a syringe, OA solution is trickled directly onto the bees along the top of each bee space, about 5ml of OA solution per bee space. OA solution is applied during cooler months so the bees will typically be clustered inside the colony, making the application of the solution simple and quick. Use only as much as needed, 5 ml per bee space; many times it will be less than 50ml.


Rubber gloves and eye protection should be worn during solution preparation and application to avoid contact with skin and eyes.


The spray method is similar to the trickle method, but instead of trickling the solution onto the bees in the spaces between combs, a spray bottle with a 3% OA solution is used to spray onto each frame of bees inside the colony. This technique is effective, but time consuming because each frame covered with bees must be removed and sprayed. This also is more stressful for the bees because the colony must remain open for extended periods of time while frames are removed and sprayed. This method of application may not be feasible during the winter.


Rubber gloves and eye protection should be worn during solution preparation and application to avoid contact with skin and eyes.  

VAPORIZATION METHOD (also known as fumigation or sublimation)

The vaporization method uses a ‘vaporizer’ (heating apparatus) to evaporate OA dihydrate crystals inside the colony.


There are multiple vaporizers on the market (see below for a list). Regardless of where you get it from, it is always recommended to follow the manufacturer’s instructions, which include the use of appropriate protective gear. Here is a general protocol of the vaporization method.

Equipment required

  • Vaporizer
  • Power supply
  • Safety equipment: respirator, nitrile gloves, protective eyewear, and dampened cloths.


Place the prescribed amount of OA dihydrate crystals onto the metal plate of the vaporizer and insert it into the hive’s entrance. The recommended dosage for this type of application is 1g (3/4 teaspoon) for a single-story colony and 2g (1.5 teaspoons) for a colony with more than 1 hive body. After insertion, the hive entrance should be closed with a damp cloth or otherwise restricted to about an inch while the vaporizer is turned on. The metal plate on the end of the vaporizer reaches temperatures of 350-375°F, so please be careful when you use it! The crystals sublimate (go from solid crystals to vapor) and disperse within the hive, covering the bees and hive interior. All other entrances and openings such as cracks must be closed or taped shut so the fumes don’t escape and reduce treatment efficacy. Make sure screened bottom boards are covered with a plastic slider. It takes approximately 3 minutes for the OA to sublimate. Afterward, remove the vaporizer from the colony and fully close the entrance with a damp cloth. It is recommended that the hives remain closed off for 10-15 minutes after treatment.


An authorized Vapor/Acid Gas respirator, rubber or nitrile gloves, and eye protection should be worn during application to avoid inhalation and contact with your skin. DO NOT inhale the OA fumes and if possible keep the wind at your back. Most common vaporizers include those found at:

Contributed by Katy Ciola Evans; Edited by Robyn Underwood

Pictures by Nick Sloff & Katy Evans