If I recall Seeley's conclusions correctly, distance was the factor for
survival. Varroa "resistance"  was couched in  a "vertical" instead of
"horizontal" (i got a bit lost here) relationship between the bees and
Varroa.

 

In a vertical relationship the Varroa depend on the survival of their host colony for their own survival.  This is what happens when the colonies are naturally spaced. When the beekeeper congregates the colonies in apiaries, it is comparatively easy for sufficient mites to find their way into the colony next door (horizontally) to continue to thrive even though the colony they have just left is on its last wings.

Chris

 

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