Highlights from "Nosema and Honey Bee Colony Health" Brenna E. Traver Richard D. Fell 

Our studies on N. ceranae have helped determine the need and the most appropriate time for the treatment of N. ceranae infections. Our observations on seasonal variation of N. ceranae infections have shown significantly higher levels in April-June and lower levels in the fall and winter.

Treatment with chemicals during the spring, as the major nectar flow starts, is not advisable because of the potential for honey contamination. Disease levels are low during the winter, limiting the need for fall treatment, and the use of fumagillin is expensive and can be time consuming with large numbers of colonies. 

However, there are times when the treatment of colonies may be beneficial, particularly when dealing with weak, non-productive colonies in the early spring. Such colonies should be examined for Nosema and then treated if necessary, although studies have shown that fumagillin is only marginally effective at reducing N. ceranae levels. 

We no longer recommend treatment for N. ceranae in the fall since winter infection levels are low and do not appear to impact colony survival. 

Colony mortality due to N. ceranae infections does not appear to be a major factor in colony losses throughout the mid-Atlantic region. We do not feel that N. ceranae is a major factor associated with colony decline.
 
http://www.pubs.ext.vt.edu/ENTO/ENTO-66/ENTO-66-pdf.pdf

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