Meghan gave a very clear explanation.

Rinderer made this very clear in his experiments with Russian bees--placing
them near hives containing commercial stocks appeared to cause mite levels
to increase in the Russians.

I've observed the same in my own operation--a colony that consistently
controlled varroa over a long period of time, with exhibit rising mite
counts when that colony is moved into a crowded apiary.

Similarly, I sell many hundreds of nucs each season, as well as keeping
around 1500 for myself.  Those that I keep would nearly all crash from
varroa by October if not treated; but a great number of them that I sell to
hobbyists manage to live for years without any treatments.  Same bees,
different mite immigration loads (I can't vouch for whether the hobbyists
allowed their colonies to swarm).

Pete's observation re Africanized bees occupying cavities underground is of
interest.  Both the Cape honeybee in Africa, and *Apis cerana* in Asia will
nest below ground.  This habit may be an ancestral trait that the European
honey bee tended to drop.

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

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