Video of Dr. Greg Hunt of Purdue University, Dept. of Entomology speaking to the Southeastern Indiana Beekeepers Association. He talks a little about pesticides, but what really caught my interest is that he's been selectively breeding for bees that will chew the ankles of varroa mites.
His experiences seem to suggest that finding mites with chewed off legs on the bottom board may correspond to hygienic behaviour in honeybees. After selecting for this behaviour, very few of his colonies require mite treatments.