What makes them fall off for any one of a number of reasons?
Story of two hives, both of which produce the same low mite counts during recent powdered sugar rolls.
Hive A has a drop rate of about 7-8 mites per week into an oil tray, and the last roll produced 2 mites, supposedly less than a 1% infestation.
Hive O had the same sugar roll rate as A, and a repeat last weekend produced 4 mites. However, the drop rate onto an IPM board has been steadily and dramatically increasing, to 174 mites last week (natural drop, didn't even sugar dust). The increase in drop rate started as soon as the queenless O was combined with a VSH nuc, whose brood is taking over the hive. So the question is, is this VSH breed doing something to drop mites (they supposedly will remove infested brood)? If this is an actual infestation, why are the sugar rolls producing such low rates? And while I'm cleaning the IPM board, I stick in a dry tray. The mites dropping into the tray are mostly live.
Somebody out there must have studied this mite dropping behavior. I expect there are a lot of factors involved, including mite population, but also grooming behavior. Africanized bees are supposedly better groomers. Is this also a trait of some VSH bees? I think the drop rate is an interesting observation but probably not great for hard counts. I'd hate to label a hive of good groomers as being prone to varroa infestation if, in fact, the very opposite is true. But I'd also hate to lose this presently very productive hive to varroa.
How effective a SBB is would have everything to do with what makes mites fall off of bees. For a natural drop rate, that could have everything to do with grooming, which could be very dependent on breed.
I may resort to an alcohol wash to get a really good count on Hive O.