Comparison between two thymol formulations in the control of Varroa destructor: effectiveness, persistence, and residues.

An apiary trial on the use of two acaricide formulations (gel-Apiguard and vermiculite and Api Life VAR) in the control of Varroa destructor (Anderson & Trueman) was conducted in summer 2001 in Sardinia (Italy). The main goals were 1) to determine their effectiveness against V. destructor, taking into account natural mite mortality in control hives; and simultaneously 2) to determine the persistence of both formulations and residues in honey and wax, by using a new extraction method. Both thymol formulations, after the treatments, reduced significantly the levels of mite infestations of adult bees and sealed brood, but their efficacy, expressed as percentage of mortality, was lower for both products (Api Life VAR 74.8 +/- 13.1 and 81.3 +/- 15.5, Apiguard 90.4 +/- 8.3 and 95.5 +/- 8.7 for sealed brood and adult bees, respectively) than the efficacy previously obtained with the same products in other experimental conditions. Moreover, a considerable colony-to-colony variability was recorded, and a significant negative effect of the thymol treatments on colony development was observed. During 2 wk of treatment, the bees removed nearly 95% of all the applied product (gel or vermiculite). Residues found in honey collected from the nest varied from 0.12 to 4.03 mg/kg for Api Life VAR and from 0.40 to 8.80 mg/kg for Apiguard. The residues were relatively higher in wax (Api Life VAR = 21.6 +/- 13.0; Apiguard = 147.7 +/- 188.9) than in honey, because thymol is a fat-soluble ingredient.