MICRO FARMING HOW-TO: 
Researchers at West Virginia University created a natural bee source of supplemental food that appears to interrupt the reproduction cycle of varroa bee mites without harming bees if done correctly.
It was fed at the entrance and broodnest. It’s believed the nurse bees feed it to the larvae, and it eventually gets into all the communal food supply and then to the mites. Experiments that seemed to do the best at this writing used 25 drops of wintergreen, spearmint, rosemary or peppermint essential oils mixed into a pint of honey put into a quart jar, then filling the jar with very warm water (too hot evaporates the oils), and mixing well. More experiments are underway.
Problems for the honey bees occurred when the essential oils at the bottom of the feeder which hadn’t completely dispersed were eaten. True essential oils (vs. culinary or fragrance oils, which can kill bees), must be used. Straight essential oils can also kill and are too strong for the bees. Timing is critical and other ways of killing mites directly with essential oils for highly infested hives are being experimented with, and some results were reported. This url leads to further information: http://www.wvu.edu/~agexten/varroa/varroa2.htm#Revised. – www.microecofarming.com