The variation in results using fumagillin could be related to two distinct causes. One, degradation in syrup:

The high rate of fumagillin degradation detected in samples stored in syrup at 22°C, as well as that observed in the more diluted mixtures, must clearly be taken into account when comparing studies on fumagillin treatments. These data indicate that a diluted mixture of fumagillin maintained in the hive for a long time cannot be considered the correct posology to treat nosemosis.  

In the light of the instability of fumagillin, it is very important to ensure the complete and rapid consumption of the medication to optimise the effectiveness of this treatment.  It is also important to ensure that the syrup with fumagillin is correctly prepared (as suggested by Webster 1994), by adequately dissolving the fumagillin and protecting it from the effects of light and heat.

The stability and effectiveness of fumagillin in controlling Nosema ceranae (Microsporidia) infection in honey bees (Apis mellifera) under laboratory and field conditions. Mariano HIGES, et al. Apidologie. Published online 12 April 2011

The other could be that different haplotypes (genetic variants) of Nosema may respond differently.

Pete

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