Did you realise that Thymol can kill brood and bees at concentrations just twice that needed to deal with varroa.
There is some limited and 'dated' information on the Dave A Cushman site. 1999 seems to be the most recent of the references.
Quoting the Conclusions
Establishing the toxicity of these potentially useful botanicals is an important step to understanding dosage response, and the risks of administering these materials orally to bees for their protection against mites. Although a given compound may be efficacious against mites, if its toxicity to bees is high then there is great risk in using it.
Materials such as thymol may continue to be useful as fumigants and in topical application, but would have to be used with care as an oral medicament. Pinene, on the other hand, may be valuable in killing mites on and in the bodies of honeybees by its relatively low toxicity to bees when they ingest it.
Phase I of this research was unable to proceed with toxicity trials against mites and micro encapsulation because matching funds were still pending. In Phase II of this research, it is intended continue the work on the toxicity of the botanicals to mites and investigate the potential for micro encapsulated formulations being used as medicaments.