Effect on colony size of spraying bees five times in brood-free bee colonies (dose 25 mL per comb, n = 11) with 0.5% oxalic acid dihydrate in aqueous solution in Spring 2004. Different letters indicate statistically significant differences. Test colonies were weakened slightly more than the control colonies after the third spraying and considerably more after the fifth spraying. They continued to be weaker 12 days after the last spraying (ANOVA one-way, LSD test, F_3;40 = 10.2377, P = 0.000).