>or low mite loads (usually after
>treatment) with major loss (look at US commercial operations ).

I suspect the leading explanation of the above circumstance is treatment 
that comes too late and/or hard side effects from the treatments 
themselves.  Tens of thousands of malformed bees with deformed wings, for 
instance, aren't going to get a colony through the winter even if every 
last mite in the hive has been killed.  The mites are still to blame.  I 
suspect treatments/mite manipulations that don't take care of the mite 
problem a full two to three months before hives go broodless (or near 
broodless) lead to a lot of "major losses" with "low mite loads."

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