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 has become 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 only phoretic mites in the colony; it cannot penetrate capped cells to kill developing mites and therefore it is recommended for use during broodless periods, for example 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 include trickle (also called dribble), spraying and vaporization methods. Before applying OA to colonies, it's important to keep in mind the following recommendations:
Trickle Method The trickle method is the most popular method in Europe and Canada because it is cost effective and easy to use. It is applied to colonies through a 60ml syringe using a solution of 3.5% oxalic acid dissolved in a 1:1 sygar to water ratio (ie. sugar syrup). The sugar syrup can be prepared by weight or volume by dissolving sugar in warm water. To prepare a 3.5% OA solution, measure one liter (4 cups) 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 or 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. For 20 colonies: 1 liter (34 ounces) sugar syrup & 35 g (1.23 ounces) OA crystals (grams) For 10 colonies: ½ liter (17 ounces) sugar syrup & 17.5 g (0.6 ounces) OA crystals (grams) For 5 colonies: ¼ liter (8.5 ounces) sugar syrup & 8.5 g (0.3 ounces) OA crystals (grams) Regardless of the size of the colony, no colony should receive more than 50 ml of OA. Using a syringe, The 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. Spraying Method 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 3.5% 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 and this 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. Both the spraying and trickle methods can be used on package bees. It is important to remove the queen prior to treatment. Vaporization Method (aka 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. The equipment required include: vaporizer, power supply (for example from a battery), and safety equipment (authorized respirator, nitrile gloves, protective eyewear). When the equipment is ready, 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! 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. This post has been modified from an original article that I wrote as a technician in the Lopez-Uribe Lab, Oxalic Acid Treatment Protocols
Article and photos contributed by Kathleen Evans. Comments are closed.
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