Seasonal variation in the infection of honeybee colonies with Nosema apis Zander

Seasonal Nosema apis infection levels have been assessed in terms of percentage workers infected, mean number of spores/ventriculus with small intestine, and mean number of spores/rectum. Percentage worker infection is found to be a good indicator of the extent of colony infection. Separate rectal spore counts facilitate the recognition of falling infection levels resulting from defecation flights rather than diminished ventricular infection. Samples of bees from central broodcombs were found to be far less useful for determining colony infection levels than samples taken at the hive entrance. Infection levels >50% were associated with colony death or observable colony debilitation. Where some colonies were split during the experimental period, the new daughter colonies that raised replacement queens did not develop different infections levels in the short term (5 months) compared with the daughter colonies that retained their original queens. The seasonal variation in infection levels was very similar to that found in other reported studies.