Charlie,  several factors at play.  One is that as mite/virus infested
colonies in late summer begin to collapse, they reduce broodrearing, which
forces the mites out of the brood and onto workers.

Somewhere I read that the alarm pheromone associated with robbing releases
a behavior in the mites to shift to another bee, but I do not have a
reference, so don't know if true.

In my own studies, I have found that bees collected at the entrance have
far lower mite levels than those in the broodnest, so I suspect that
robbers going through the broodnest pick up mites.

A study from Germany several years ago documented that colonies up to a
mile away (the furthest distance in the study) picked up mites via robbing
at at least the same rate as colonies in the same apiary.

So as Pete and others have said, one's neighbor's "mite bombs," or
collapsing escaped swarms, can load your colonies.  I make the point when I
speak that beekeepers have a responsibility to both their beekeeping
neighbors and to the feral population to not allow their colonies to
collapse from either varroa or AFB.

That said, let's do some math.  Nowadays, the economic threshold for mites
is about 1000 mites total going into winter.  For a 10-frame cluster of
about 20,000 bees, that works out to be about a 5% infestation rate for
broodless bees.

A collapsing colony can easily have 25-50 mites per 100 adult bees, which
works out to about 5000 - 10,000 mites.  That is enough mites, if all
transferred via robbing, to tip a lot of colonies past the economic
threshold.

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

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