Negative evidence for effects of genetic origin of bees on Nosema ceranae, positive evidence for effects of Nosema ceranae on bees

In two tests, honey bee colonies of different origins were sampled monthly to detect possible differential infection with Nosema ceranae; colony sizes and queen status were monitored quarterly. One experiment used queens crossed with drones of the same type obtained from colonies which had previously exhibited high and low infections. A second experiment used queens from ten commercial sources. No clear genotypic (P = 0.682) or phenotypic (P = 0.623) differences in infection were evident. Colony deaths and supersedures did not relate significantly with infection except for deaths of colonies in the autumn (P = 0.02). Significant effects on colony growth were found in all seasons: average 3-month decreases in population ranged from 0.4 to 1.4 frames of bees per million N. ceranae per bee. These results confirm that N. ceranae can be involved in weakening of colonies even in warm climates and suggest that breeding for resistance may require more intense selection, larger base populations, or different screening methods.