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Oxalic acid is a natural component of honey
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What is oxalic acid (OA)? 

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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 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 only recommended for broodless colonies, during winter months or in the spring before the queen starts laying eggs.

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How should I apply OA? 

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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 should be used exclusively 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. Brushy Mountain Bee Farm is the only beekeeping company in the U.S. which sells certified, pure OA.  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 hydrate crystals must be dissolved in warm sugar water (one part sugar to one part water by volume or weight). Granulated sugar should be used over powdered sugar. 

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TRICKLE METHOD

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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

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 the and the solution is clear. Sugar syrup solution can be prepared via a microwave or stove-top however, be careful to NOT boil the sugar syrup. Heating fructose at high temperatures releases a chemical (hydroxymethylfurfural) that is toxic to bees. Click here to read more …

Total volume Granulated Sugar  (volume) 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 of water).

Total volume Granulated Sugar  (weight) Water
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 (chart 3). Evidence shows that concentrations higher than 3.5% do not necessarily result in increased efficacy against varroa and could cause harm to the bees.

Sugar Syrup volume OA 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

Application

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 bees space. OA solution is applied during winter months so the bees will typically be clustered inside the colony, making the application of the solution simple and quick. If the bee cluster is in multiple boxes, the 5ml treatment per bee space may be reduced. Use only as much as needed, many times it will be less than 50ml.

Safety

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

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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 since 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.

Safety

Rubber gloves and eye protection should be worn during solution preparation and application to avoid contact with your skin.  
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VAPORIZATION METHOD (also known as fumigation or sublimation)

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

Preparation

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
  • 12-volt battery
  • Safety equipment: respirator, nitrile gloves, protective eyewear, and dampened cloths.

Application

Place the prescribed amount of OA dihydrate crystals into metal plate of the vaporizer and insert it into the hive’s entrance. The recommended dosage for this type of application is 1g (0.2 teaspoons) for a single-story colony and 2g (0.4 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 sublime (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.

Safety

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:

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Pictures by Nick Sloff & Katy Evans
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