Charlie, I've pondered the same question myself.  One would think that
drifting and/or robbing would tend to equalize mite populations in an
apiary.  But to the contrary, there can be huge colony-to-colony difference
in mite levels, even with hives placed side by side, so it is difficult to
extrapolate meaning from means.

The dynamics of varroa infestation have much to do with the typical
decrease in the adult bee population in late summer, followed by the normal
sudden decrease in fall.  These decreases tend to concentrate the mite
infestation rate in colonies.  It may be the sky-high phoretic mite
infestation rates in colonies just prior to collapse that allow such
substantial invasion.

When I developed my Excel program for predicting mite populations in
colonies over the course of the year, one of the variables that I included
was a fall invasion rate, based upon published data.  To my great surprise,
I found that the fall invasion rate was one of the most important factors
in predicting mite populations in fall.  No matter how resistant the bees,
or how good the IPM program, fall invasion of moderate numbers of mites
could undo everything!
-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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