Recommendations for oxalic acid application method and treatment intervals for reduction of Varroa destructor populations in western honey bee (Apis mellifera) colonies

Oxalic acid (OA) is a popular miticide used to control Varroa destructor in western honey bee (Apis mellifera) colonies. Our aim was to investigate which method of OA application (dribbling, fogging, or vaporizing) was the most effective at reducing V. destructor infestations (Experiment 1) and to improve upon this method by determining the treatment that resulted in the greatest V. destructor control (Experiment 2). In both experiments, we used the product Api-Bioxal (97% OA) and maintained 40 honey bee colonies (10 per treatment) for all treatments. In Experiment 1, our three methods of application included: 1) dribbling 50 ml of 3% OA solution, 2) vaporizing 4 g solid OA, and 3) using an insect fogger supplied with 2.5% OA dissolved in ethyl alcohol. After three weeks, only the vaporization method reduced V. destructor infestations (from 9.24 mites/100 bees pretreatment to 3.25 mites/100 bees post treatment) and significantly increased the amount of brood and number of adult bees. In Experiment 2, all colonies were treated with four applications of OA via vaporization at a constant concentration of 4 g OA/colony. In this experiment, the groups were separated by treatment intervals at either 3-day, 5-day or 7-day intervals. We observed that both 5-day and 7-day intervals significantly reduced V. destructor populations from pretreatment levels over that of the controls and 3-day intervals. Our data demonstrate the efficacy of OA in reducing V. destructor infestation, particularly vaporizing 4 g every 5 - 7 days as the most effective method of application.