I've heard about feeding hives with syrup with essential oils added to it -- does anyone have a recipe for this? It sounds like it might be really helpful vs. varroa -- would anyone talk about their experience with using the stuff?

I'm interested in keeping my hives healthy without using poisons. I noticed this past September that my hives had contracted varroa, and so I panicked and put formic acid pads on them. This seems to have helped, but in future I'd much rather use a natural cure like essential oils than a harsh acid.
I found some recipes for grease patties that contain essential oils, but not the proportions for the feeder syrup with essential oils added.