>The question would be, are those live mites "healthy"?  At least one study found that fallen mites tended to die soon

I agree with this. This is probably the explanation as to why screened bottoms are not worth much, too. Mites that let go and fall off the cluster are no doubt old and infirm. A healthy mite hangs on tight, and isn't likely to cascade all the way down through the cluster without grabbing on to anything. To suppose that healthy mites can't crawl a meter or two, is also implausible. 

PLB

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