Reading papers on corn pollen I came across papers showing that Bt, Bacillus thuringiensis, kills varroa but is harmless to bees.
GMO corn with Bt genes, created to kill pests that eat corn, has Bt in the pollen that bees collect and feed to larvae. Fortunately, this seems to have no negative effects on the bees at all. Looking at some of the papers linked in this study I happened upon several that studied the effects of Bt on mites, and it appears to be very effective in vitro (in the lab). Was not able to find much other info or whether anyone had tested this in the field.
Effects of Bacillus thuringiensis strains virulent to Varroa destructor on larvae and adults of Apis mellifera