>There's been a stack of evidence growing for the last 20 years showing
that robbing is far more a force in varroa reinfestation than drift within
a yard.

When I first developed my mite infestation rate spreadsheet
http://scientificbeekeeping.com/sick-bees-part-12-varroa-management-getting-down-to-brass-tacks/
, the single thing that most jumped out at me was the importance of this
fall invasion of mites, ostensibly due to the robbing of, or bee drift
from, other highly infested or collapsing colonies.

When I factored in published figures from studies that quantified the
amount of mite invasion, I found that such invasion could easily overwhelm
both innate mite resistance of the colony (which had held mite levels down
up 'til then), or the previous successful treatment for mites.

We learned this in California the hard way when Apistan was still young.
We could knock the mite pops down to near zero, but when the feral
population around us crashed, our colonies were (without our being aware of
the fact) overwhelmed by mite invasion, and subsequently crashed.

For this reason, in my operation, we continue to monitor mite levels until
cold weather has killed off collapsing colonies.


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

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