?> I, too, have intentionally done the "set it and forget it" experiment. 
Those bees generally lived into a second season, but the third season they 
were wax moth food.

This is quite widely observed if varroa are not managed.  It also 
corresponds to the Spanish observations about uncontrolled nosema.

We don't really know for sure what is happening, but it seems that if one or 
the other -- or even more so, both  -- gets out of hand, eventual loss 
results.

The mortality is not always immediate as one would expect, but  can be 
delayed by months.  It seems that once these agents get above a certain 
level, they enable whatever it is that causes colony death to cross a 
threshold from which there is often no return.

We do see instances where either of the two an get above what we know to be 
dangerous levels without that result, and can only conclude that in those 
cases whatever is finishing the colonies off is either not present or 
somehow unable to take advantage due to some interfering factor.

Medhat, in Alberta, demonstrated that by simply maintaining low levels of 
both nosema and varroa, winter losses -- so far at least -- return to the 
normal low background winter losses which we expected before coumaphos lost 
efficacy.  There is a very observable correlation between the degree of 
control achieved by a beekeeper and wintering success.  That correlation is 
not direct, but it is quite obvious.

So, it seems that these two agents, nosema and varroa -- each of which 
wounds bees -- enable some other agent to achieve "critical mass" and to 
balloon towards colony destruction.  Last minute interventions can work if 
things are right and luck is on the beekeeper's side, but more often than 
not, the damage is already done and the seeds of destruction are already 
sown. 

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