Organic acids are among the chemical treatments available for controlling parasitic Varroa mites in honey bee hives. In March of 2015 the EPA approved oxalic acid for use in the U.S.; it has previously been used in Europe. Researchers at the Laboratory of Apiculture & Social Insects in Sussex, England report their findings on the use of oxalic acid: http://dx.doi.org/10.1080/00218839.2015.1106777. Varroa mites occur in the hive both inside the capped brood cells and on the bodies of adult bees. Oxalic acid only kills the phoretic mites, the ones on the adult bees. There are three methods for treating bee hives with oxalic acid: trickling or dribbling, spraying, and sublimation. The Sussex researchers found the sublimation method, which uses an electrical heating element to cause oxalic acid crystals to convert directly to a gas, the most effective killer of mites. Treatments should be applied when temperatures are between 39 and 61 degrees Fahrenheit and when no capped brood is present in the hive. The EPA states that oxalic acid should be used in late fall or early spring when little brood is present. The Sussex researchers explain that even a little brood can protect a lot of Varroa mites from oxalic acid. The Sussex researchers placed 2.5 ml of oxalic acid crystals (half a teaspoon) in a heating device and placed it inside the hives sealed with foam to confine vapors. After the crystals vaporized, they left the hives sealed for 10 to 15 minutes.
As a word of caution, the EPA states, “In addition to the standard beekeeping suit (veil, long-sleeved shirt, long pants and gloves) as personal protective equipment, a respirator and goggles are required.” While oxalic acid occurs naturally in foods, such as carrots, Brussel sprouts, cabbage, broccoli, parsley, and rhubarb, the chemical can be extremely dangerous if it is breathed or if contacted with the skin or eyes. I highly recommend using other methods than oxalic acid to control Varroa mites. Photo: honey bee on early December sunflower.--Richard