> if you accept that the Nosema is more suseptable to cold,  is the assumption that clustered bees are cool enough to cause it to die off?  

I don't think it has to do with winter conditions, but spring and summer conditions. Nosema ceranae may thrive in conditions that resemble their tropical origin...

> 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. -- Traver and Fell

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