Selective Breeding of hygienic behavior (HB). In an initial breeding experiment, colonies were selectively bred for HB over three generations to test the efficacy of selection. For each generation, colonies expressing the highest level of HB estimated by the fraction of freeze-killed pupae removed after 24 hours (R24) were selected as breeders and their virgin queens and drones close mated in an isolated apiary. F0 bars represent the natural frequency distribution of HB in the starting population, while the F1 and F2 bars represent the distribution after one and two rounds of selection, respectively. The frequency distribution in the F1 and F2 is significantly different from the F0 (*P = .0002, one-way ANOVA).