Effect on colony size of spraying bees four times in brood-free colonies in Spring 2003 with 0.5, 1.0 and 1.5% oxalic acid dihydrate in aqueous solution (dose 25 mL per comb, n = 3 experimental series). Mean values (±SE) marked with different letters are significantly different. The 1.0 and 1.5% concentrations weakened the colonies considerably (t = 2.3206, P = 0.05 and t = 9.3228, P = 0.0007, t-test) whereas the 0.5% solution had no noticeable toxic effect (t = 0.2579, P = 0.8092, t-test).