>
> >Experiments with isolated vs. grouped colonies indicate that the
> cumulative number of mites acquired by colonies from outside sources is
> relatively small compared to the numbers that can potentially be produced
> by reproduction within colonies.


My informal checking of the success of beekeepers to whom I sell nucs
strongly suggests that colonies kept in apiaries of only 1-2 hives some
distance from other apiaries often survive for years without treatment.
The same nucs, placed in my apiaries of 25-30 hives invariably build up
unacceptable mite levels by the end of summer.  The surprisingly high level
of day-to-day drift among hives in a yard makes me suspect that a few mite
bombs in a yard make it harder for all the rest of the colonies.

That said, I also often find side-by-side colonies with huge differences in
mite levels, suggesting that either the low-mite hive is very efficiently
thwarting mite reproduction (which I doubt), or that it somehow is
preventing mite-infested bees from the neighboring hive from drifting in.
I have definitely not figured this out!

Of interest to me is that when I develop Excel models to predict mite
populations in hives, I can very closely match field observations.  Of
great interest to me is when I factor in late summer immigration at
measured levels.  Such immigration of a few to several hundred mites can
easily tip a colony that was in good shape mite-wise into one that is
unlikely to survive the winter.  These days, colonies containing over 1000
mites typically exhibit poor winter survival.  Thus, the addition of a few
hundred mites in fall can make a big difference.


> >If one is measuring "number of mites" in a colony by density of mites on
> adults, that "number" can dramatically increase when brood production
> decreases or stops in the fall.


One must carefully interpret both natural mite fall counts, as well as mite
wash counts to account for the amount of worker and drone brood present,
and in the case of natural mite fall, the strength of the colony.

Personally, since we monitor our colonies regularly for mite levels (via
alcohol wash), we are rarely surprised.  That's not to say that mite
management is necessarily easy--it's just that levels are predictable based
upon the trend of previous counts, weather and forage (affecting
broodrearing), and exposure to poorly-managed colonies in the vicinity.
-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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