More evidence that Nosema is simply not a critical factor in honey bees in temperate climates:

> the results obtained with the German beemonitoring project did not reveal any relation between infection with Nosema spec. and winter losses although both Nosema species are prevalent in Germany (Klee et al., 2007). Since no losses occurred during summer although colonies were infected by Nosema spec. it can also be ruled out that infection with Nosema spec. killed colonies between spring and autumn as described in the Spanish studies  

> infestation with Varroa destructor turned out to play the key role. Based on the results presented it is safe to state that Varroa destructor is the dominant killer of honey bee colonies during winter. In addition to high varroa infestation levels, DWV and ABPV infections in autumn significantly lower the winter survival of honey bee colonies as do old queens heading overwintering colonies. That a weak colony has not the best chance to survive the winter is rather trivial but the fact that we observed such winter losses due to colony weakness shows that beekeepers still winter weak colonies. 

> A negative effect of pesticide residues in bee bread from spring on the survival of the bee colonies in the subsequent winter could not be proven 

Elke Genersch, et al. 2010. The German bee monitoring project: a long term study to understand periodically high winter losses of honey bee colonies. Apidologie

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