Rhubarb, rhubarb, rhubarb

It might just be under-reporting, but the University of Nebraska-Lincoln is investigating oxalic acid as a varroa treatment. Oxalic acid has been used by some beekeepers in Europe for many years now (it's not that popular though).

In fact as far as I understand it, the UK authorities are turning a blind eye to its use even though it's not regulated. I'm not sure if the American researchers have a new angle on its use.