In the decades long fight against varroosis in honey bees (Apis mellifera), caused by Varroa destructor, oxalic acid (C2H2O4) has been suggested and investigated early on as an efficient active compound. It was first used to treat varroosis in Asian countries (i.e. Japan, Korea) and the UDSSR. (Ref 1-2) In Europe, oxalic acid quickly became one of the most important organic active ingredients to fight varroosis since the Varroa mite has arrived in Germany in 1977. (Ref 3) Although the mode of action of oxalic acid as a miticide is not clearly understood, it seems that the low pH of oxalic acid solutions has a deleterious effect on the mites when they get in contact with it. (Ref 4)
What makes oxalic acid interesting as a treatment against varroosis – apart from its high efficacy of more than 90% (Ref 5) – is its classification as an organic treatment by EU regulations (EU Council Regulation, No. 1804/1999). (Ref 4) The compound also occurs naturally in honey, albeit in very low concentrations of up to 65 mg/kg in untreated and up to 67 mg/kg in treated colonies (in the following year after winter treatment). (Ref 6) This makes oxalic acid especially attractive for organic beekeepers, but also for conventional operations practicing IPM (Integrated Pest Management) with rotating treatment schedules. Moreover, as it is only recommended for use during the brood free period in honey bee colonies, oxalic acid is generally considered the preferred active ingredient for the winter treatments. In regions with a natural brood break in winter due to low temperatures, this treatment is often administered in December, ideally a few weeks after the first frost. However, oxalic acid treatments can also be administered in summer, if the colonies are brood free. Beekeepers can create brood free colonies in summer by taking specific actions like caging the queen or working with removing brood frames. (Ref 7)
Oxalic acid is used in three different ways in beekeeping: trickling, spraying and sublimation (vaporization). (Ref 4) For trickling, the standard dose of ca. 35 g oxalic acid dihydrate (a white, crystalline powder) is added to a sucrose solution (1:1) and then applied (trickled) onto the bees with a syringe. The exact dose varies depending on the specific recommendations for the oxalic acid product used by the beekeeper. Usually, 5-6 ml of oxalic acid dehydrate solution are trickled directly onto the worker bees occupying a single bee space (the space between two frames) in brood free hives. (Ref 8)
The method of trickling is – next to sublimation – by far the most common form of application. Research on toxicity and safety of oxalic acid treatments against Varroa mites has demonstrated that trickling is a highly efficient treatment method that is well tolerated by the bees and safe for beekeepers. (Ref 5) The sublimation method (using a vaporizer to treat colonies with oxalic acid dihydrate), requires more safety measures by the beekeepers which make the treatment more time consuming and expensive overall. (Ref 5) Oxalic acid crystals that can be taken in through the air by beekeepers using the sublimation method, can manifest in the lungs for long periods of time. (Ref 8) Therefore, beekeepers must wear protective masks during treatment application. Spraying oxalic acid dihydrate solution on the frames is more time consuming than trickling and sublimation, because every single frame must be pulled and sprayed from both sides. (Ref 5) This application method is more suitable for newly created nuclei (young colonies).
A new and exciting addition to the group of registered oxalic acid treatments in Europe is Oxybee®, which is now available by Véto-pharma. The company has received the distribution right for Oxybee in 27 European countries as of February 2018. Oxybee contains some unique ingredients and offers a few distinctive advantages to beekeepers. Apart from the standard ingredients oxalic acid dihydrate and sucrose, Oxybee also contains glycerol and essential oils. Glycerol helps to increase the adhesiveness of oxalic acid to the bees (Ref 9), thus increasing the potential for contact of oxalic acid molecules with Varroa mites in the hive. The essential oils were included to the formulation for an extra boost in efficacy, as anise and eucalyptus oil have been demonstrated to act effectively against Varroa destructor. (Ref 10) In comparison with other oxalic acid treatments, Oxybee can be stored longer due to the stabilizing properties of glycerol in the formulation. The shelf life of the packaged product (= 2 years) and of the readily reconstituted solution (= 1 year at 2°C – 8°C) provide a significant economic advantage to beekeepers. Therefore, Véto-pharma is proud to provide Oxybee as a new solution against varroosis to beekeepers, adding significant improvements to a tried and tested active ingredient.
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