Effect of spraying bees in brood-free colonies in April 2003 with 0.5, 1.0 and 1.5% oxalic acid dihydrate in aqueous solution four times on the mortality of Varroa mites (dose 25 mL per comb, n = 3 experimental series). Mean values marked with same letters do not differ significantly. The 0.5% concentration was significantly less effective than the 1.0 and 1.5% concentrations (ANOVA one-way, LSD test, F_3;8 = 4466.64, P = 0.000).