Towards integrated control of varroa: 2)comparing application methods and doses of oxalic acid on the mortality of phoretic <i>Varroa destructor</i> mites and their honey bee hosts

Abstract

In the past two decades, the parasitic mite Varroa destructor has become harder to control with synthetic acaricide chemicals due to genetic resistance. We determined the efficacy of the natural chemical oxalic acid (OA) in killing phoretic mites on adult worker bees under field conditions in southern England. We compared three OA application methods (trickling, spraying, and sublimation) at three or four (sublimation) doses, using 110 broodless colonies in early January 2013. Treatment efficacy was assessed by extracting mites from samples of c. 270 worker bees collected immediately before and 10 days after treatment. All three methods could give high varroa mortality, c. 93–95%, using 2.25 g OA per colony. However, sublimation was superior as it gave higher mortality at lower doses (.56 or 1.125 g per colony: trickling 20, 57% mortality; spraying 25, 86%; sublimation 81, 97%.). Sublimation using 2.25 g of OA also resulted in 3 and 12 times less worker bee mortality in the 10 days after application than either trickling or spraying, respectively, and lower colony mortality four months later in mid spring. Colonies treated via sublimation also had greater brood area four months later than colonies treated via trickling, spraying, or control colonies. A second trial in December 2013 treated 89 broodless colonies with 2.25 g OA via sublimation to confirm the previous results. Varroa mortality was 97.6% and 87 (98%) of the colonies survived until spring. This confirms that applying OA via sublimation in broodless honey bee colonies in winter is a highly effective way of controlling V. destructor and causes no harm to the colonies.