Bob writes:
> Conclusion: I believe the future of varroa control will be in thymol, formic and oxalic

Comments:

> Essential oil-based varroacides were exempted from extensive testing for EPA registration because they are common food additives and "generally recognized as safe" for human consumption. However, monoterpenoids such as thymol and menthol may not necessarily be safe for honey bees, since these compounds play a role in plants as broad spectrum pesticides. Indeed, thymol and menthol were found to be among the most toxic of all terpenoids tested when applied to honey bees as a fumigant. Despite being naturally derived, these compounds may harm honey bees: thymol treatment can induce brood removal and may result in increased queen mortality.

> Repeated treatment of colonies with oxalic acid can result in higher queen mortality and a reduction in the amount of sealed brood. The midguts of honey bees fed oxalic acid in sugar water exhibited an elevated level of cell death, though in field conditions bees will generally avoid consuming syrup with oxalic acid.

> With the large number of varroacides available to beekeepers in the USA, there is potential for interactions between compounds, a problem compounded by the fact that many synthetic varroacides are lipophilic and *may remain in the wax component of hives for years following treatment*. The overlapping modes of action and mechanisms of tolerance in honey bees are also cause for concern.   Fenpyroximate and the organic acids all interact with components of the mitochondrial electron transport chain, where interactions could also be possible. 

> Interactions between in-hive varroacides and out-of-hive insecticides and fungicides are also of concern, particularly interactions between the P450-detoxified varroacides and the P450-inhibiting ergosterol biosynthesis inhibiting fungicides

> Beekeepers searching for the primary source of pesticides contaminating bee hives need only to look in a mirror. Unfortunately, the regulatory system governing the veterinary use of pesticides in bee hives in the USA may be perversely contributing to the problem. The use of unregistered products is a serious threat to the beekeeping community and should not occur.

Pesticides and honey bee toxicity – USA
Reed M. Johnson, Marion D. Ellis, Christopher A. Mullin, Maryann Frazier
Apidologie 41 (2010) 312–331

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