The synergistic effect of weak oxalic acid and thymol aqueous solutions on Varroa mites and honey bees

Abstract

In this study, the effects of a weak oxalic acid (OA) water solution (0.5%) and a mixture of oxalic acid (0.5%) and thymol (0.12%) (OA/TH) on Varroa mites and honey bees were investigated by trickling method. Prior to the experiments, queens in colonies were caged for 21 days to prevent egg laying and hives were provided with screened bottom boards. In experiments with caged bees in laboratory and brood-free colonies in field conditions, mite mortality and bee tolerance were assessed. Trickling caged bees once with OA resulted in 46.8 ± 4.4% effectiveness, while OA/TH had an effectiveness of 91.6–98.7%. Aqueous thymol (TH) solution had no acaricidal effect, but if mixed with OA doubled the effectiveness. Sugar solutions with low concentrations of OA and OA/TH (1:1) were considerably less effective than aqueous solutions. OA/TH was more harmful to bees than OA alone: respectively 3.2 and 1.2% of bees perished in trials. In field conditions, trickling with OA/TH (25 mL/comb space) was highly effective in autumn experiments: 97.6–99.0% for single and 99.2–99.8% for double trickling. Trickling once in spring was considerably less effective (77.0 ± 3.0%). The acaricidal effect of OA double trickling in autumn appeared more slowly, but had similarly high effectiveness: 99.6–99.8%. OA/TH double trickling in autumn was shown to be toxic to bees: colonies weakened considerably the following winter. Water provision reduced weakening of colonies in winter. Trickling of colonies with OA/TH or OA at a lower dosage (15 and 20 mL/comb space, respectively) earlier in autumn ensured high effectiveness and good bee tolerance.