NIH Study: Oregano oil for the control of Varroa mites. A new study is out showing that vaporized oregano oil showed the highest efficacy of any treatment tested so far.
NIH Study: Oregano oil for the control of Varroa mites.
Unfortunately, it requires the release through evaporation of +/- 0.5g of oregano oil daily. This can only be achieved using electric vaporizers left in the hive for weeks at a time. According to the study, it is just not a feasible treatment currently for Varroa mites. With the advent of solar-powered vaporizers, this may change but the article is worth the read. Full text of the study is available on the NIH website, located here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5547185/