>  find that bees from unculled sick colonies tend to drift to adjacent hives, taking those colonies down with them.

Good point. I think historically, this refers to colonies that are below the strength needed to survive winter. The idea is it's easier and cheaper to ditch them at the end of the season and make new ones. When beekeepers only had overt disease to deal with, then no diseased hives would be tolerated in a healthy apiary. 

In these days of hives that are weakened or sick due to mite infestation, it probably makes more sense to haul the sick ones away, as you said. The old practice of combining marginal colonies does what you say, only quicker; it saddles the healthy colonies with a burden they don't need (or deserve). 

Reminds me of Richard Taylor's suggestion of putting AFB onto the best colony in the yard. Taylor, may he RIP, was flat wrong there. Funny, he cited Charlie Mraz for the concept, and the notion has been passed on to this generation in various forms. Since it is so easy, the idea is attractive to folks who forgot the hygiene classes they took in grade school.

PLB

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