A New Zealand research team claims to have developed a natural control of Varroa mites based on a fungus (Metarhizium) that is harmless to bees, but lethal to the mites. Sounds promising!
http://www.odt.co.nz/news/national/16062/scientists-find-way-treat-hives-with-varroa-mites
Very old news. It has been talked about now for several years and nothing has come of it. Try a search here and on beesource for "Metarhizium" and you'll find MANY discussions several years ago.