I consistently observe colony mortality to be grouped,
strongly suggesting that the parasites causing such mortality disperse by
the drifting of bees to adjacent hives.

This is why I always suggest that one must regard the apiary as the unit, not the hive. As a state inspector we inspected 10% of the hives in an apiary and made an assessment based upon that. If you didn't find AFB, you went to the next yard. If you did, all the hives in the yard got inspected. 

Beyond that, one might as well regard the bee population of a given area as the unit. I would like to keep bees without worrying about varroa, like many of the people I have spoken with who live in other areas. But the plain fact is that in this area, we have real trouble with varroa. They spike in August. 

So the baseline is drifting, not only between hives but between yards. That's why I state that its importance is exaggerated, because it is the status quo. It constitutes a level of background noise that cannot be eliminated. If your aim is to eliminate drift, you need to model your apiary after feral colonies, which are much more widely dispersed.

In a temperate forested area the concentration of feral colonies has been found to be about 4 per square mile (Morse). In commercially exploited areas such as the eucalyptus forests, the concentration can go to 100 per square mile (Oldroyd).

PLB

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