Weak solutions of organic substances are very effective against Varroa mites

What is it about?

The parasitic mite Varroa destructor is the most detrimental pest of honeybees and the main cause of current colony losses. The use of chemicals for Varroa control has led to the development resistant mites and increased residues in beeswax and honey. Organic acids and essential oils are usually less toxic to bees and do not contaminate the beeproducts. Oxalic acid and thymol have shown a good effectifveness against Varroa, but several studies have revealed toxicity of both to bees at commonly administred doses and concentrations. The aim of this study was to investigate the use of lower concentrations of oxalic acid and thymol and their possible synergistic effect against Varroa mites. Additionally, bee tolerance was assessed. Aqueous solution of thymol had no acaricidal effect, but if mixed with oxalic acid doubled the effectiveness and it reached 91.6-98.7% by trickling once in laboratory conditions. Sugar solutions with low concentrations of oxalic acid and thymol were considerably less effective. In field conditions trickling at the dosage of 25 ml per comb space achieved the effectiveness of 97.6-99.0% for single and 99.2-99.8% for double trickling. However, double trickling in autumn was shown to be toxic to bees. Water provision reduced weakening of colonies in winter. The toxic effect was reduced or eliminated by reduced dosages of the applied solutions and by treating the bees earlier in autumn that enabled the bees to fly outside following treatment.

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Why is it important?

In this study, low concentrations of used substances and the synergistic effect of them showed the high effectiveness against Varroa mites (99% and more). It is important to note that the high rate of efficacy was achieved by the water solutions that are less harmful to bees, and not by the sugar solutions as commonly used in beekeeping practice. Using of treatments less toxic to bees in autumn helps to improve overwintering of honey bee colonies.

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