This is completely speculative based on a what I've read & my  
experience with bees in the lab.

A number of years ago we were working with bees in the lab- testing  
dose mortality on recently emerged workers against Beauveria bassiana  
-an entomopatthogenic fungus.    Bees being beneficial--we were  
testing potential for infection at doses a bee might encounter in the  
field.  We figured that this fungus would be inactivated at normal  
hive temperatures.  I was unable to infect bees at field doses but  
the longer I held bees away from the colony the higher Nosema levels  
became.  Every bee, upon dissection, had a certain level which  
increased the longer they were held in an incubator below hive  
temperature.

So if worker bees get  Nosema through tropholaxis upon emergence,  
meet an environmental, chemical or multiple stresses this would  
initiate high levels & sicken bees---or produce the onset of disease.

OK now we have a stressed colony --add a small level of imidacloprid  
(PPM or PPB) which has been reported to synergize with some  
pathogenic fungi.  If it were to weaken bees & synergize with  N.  
ceranae  (a fungus) it would provide a  further opportunity to grow &  
cause disease.  Only young as yet uninfected bees would remain.   
Larvae might be ok--I admit that I do not know the specifics of  
Nosema--do larvae become infected or is it a adult bee disease?

Depending on where bees are (or where they have been)  certain bees  
or apiaries may end up with collections of Imid which would cause  
increased synergy.  This might result in the variability amongst  
sites & affected colonies.

Now the synergy with certain fungi has been shown--it needs to be  
verified & the mechanism needs to be elucidated.   In our lab we are  
trying to utilize this synergy to reduce duel treatment costs.  Can  
we cut the use of the Bb in half or 1/10 by pairing it with an  
insecticide?

This may prove to have negative non-target effects in the wild!

Mike Griggs
FLBC

******************************************************
* Full guidelines for BEE-L posting are at:          *
* http://www.honeybeeworld.com/bee-l/guidelines.htm  *
******************************************************