If you have 20 hives and 2 are crashing from mites, those hives die and
increase the mite loads on the other hives because many of their mites
don't die with them.  Or say you have no crashing hives, and you have done
varroa treatment, but a couple of hives still have a high mite count
(resistance or misapplication or dumb luck or whatever).  Drifting and
robbing will spread out those "excess" mites (especially as more become
phoretic) more evenly throughout the apiary, possibly putting your hives
that had adequate treatment above the economic threshold.  Think one hive
with 20 mites/sample and 3hives with 1mites/sample.  You disperse even half
those from the high load hive and suddenly your hives with one now have 4.
 Or something like that.

It's just probability or diffusion, hives with more mites are likely to
donate more to other hives even given equal robbing rates, but hives with
more mites are likely weaker so it compounds the robbing effect.

Jeremy
West Michigan

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