Evaluation of oxalic acid with glycerin efficacy against Varroa destructor (Varroidae): a four year assay

Beekeepers in order to control the Varroa population, apply oxalic acid (OA) by trickling, spraying, evaporating, or fumigating, onto broodless colonies for higher efficacy. Recently, a new method was developed using strips with OA and glycerin during brood-rearing periods. However, there has been little research regarding the optimum concentration of the active ingredient and the suitable solvents to reach maximum efficacy. Thus, in the present study, we applied strips with different concentrations of OA in different mixtures of glycerin, assessed the efficacy of the method throughout a 4-year experiment, and compared it with others, commonly followed by beekeepers. Among the tested concentrations of OA, the optimum was 21% (w/v) (60 g OA, 130 mL glycerin, and 100 mL water), while the presence of water made the strips more manageable. The corrected efficacy of the method ranged from 90.4% to 94.5%, despite the different climate conditions prevailing each year and the presence of brood. All the tested concentrations presented a high number of fallen mites in the first 15–18 days, as glycerin releases OA slowly into the hive. On the other hand, synthetic acaricides or other methods (fumigation, essential oils) presented a corrected efficacy range from 10.1% to 85.6%, requiring more in some formulations repetitions. No bees’ toxicity was observed in all study cases. The results showed that the application of strips with 21% (w/v) OA diluted in glycerin and water seems to be more practical, leading to a successful Varroa control even in seasons with brood presence.