Charlie, please understand that evolutionary change is all about math.
That is, the proportions of different genes that get passed to the next
generation.  It doesn't make any difference how large (the total number of
individuals) that there are in the next generation (think varroa
here)--what's important is the proportion that are resistant.

 Imagine that one has 1000 hives in a population.  Now treat with a
miticide that kills every damn mite save one in one hive--that mite being
completely resistant to the treatment.  Keep treating every year with the
same treatment.  How many years do you think it would take until your
entire operation crashed from the descendants of that single mite?

The answer: two years for the offspring of that single mite to build up and
crash the first hive, which then gets robbed by the other 999 hives.  Two
years later, the entire operation crashes.  So much for a 99.99% kill.
Just buys you four years.

We've seen this happen in every country, over and over.

In my own operation, we don't normally try for a high efficacy kill, as I
want to prolong the effectiveness of the organic acids and thymol for as
long as possible.  I rotate my treatments, and apply treatments often
enough that I only need a 50% kill.  This strategy is optimal for pest
resistance management, since it minimizes the selective pressure for the
mites to develop resistance to any particular treatment.

Of course, I consider ANY treatments to be temporary bandaids until we
manage to shift the managed bee population to mite resistant stock.

-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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