The efficacy of the antimicrobial Fumagilin-B® against nosemosis was evaluated in both spring and autumn feeding treatments following label directions in seventy-two honey bee (Apis mellifera) colonies across three apiaries in Nova Scotia, Canada. The seasonal trend of Nosema spp. spore loads was also tracked in these same colonies throughout a thirteen-month period (February 2018 – March 2019). We found the spring Fumagilin-B® treatment to be effective at significantly suppressing Nosema spp. spore levels below the recommended treatment threshold. There was no effect of Fumagilin-B® treatment in the autumn based on low spore levels at this time. We detected a drastic increase in Nosema spp. spore loads as May progressed but a decline in spores in summer (June–September). By October, there was another increase in spore levels, but this increase did not exceed the economic treatment threshold. Across seventeen collection periods in both control and Fumagilin-B® colonies, 74% (25) of samples tested positive for Nosema ceranae, while 26% (9) contained no Nosema spp. spores. No Nosema apis spores were detected during this trial. Our results indicate that Fumagilin-B® is an effective management practice in the spring, but colonies should still be monitored in the autumn. Our data also support that the Nosema species profile is shifting to be exclusively N. ceranae and the treatment threshold for Fumagilin-B® may need to be updated to reflect this, as the threshold was originally developed for N. apis.
