my strongest and most productive
colonies are rarely infected above a trivial level, whereas my weaker and
less productive frequently are infected.  But I don't know whether N
ceranae is causal

This is the point. Is nosema an opportunistic infection, symptomatic of some underlying condition. You alluded to the fact that these colonies may have some underlying condition. They may be overburdened with viruses, or there may be some other immune disorder as yet unidentified. 

I find it ironic that beekeepers are quick to attack other folks who use pesticides for conditions that may not warrant them, and yet they want to have free reign to use whatever they can get their hands on. By the way, the fact that people buy a product by the truckload is absolutely not proof that it does what they say it does.

Pete

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