Referencing an article from Randy Oliver's Scientific Beekeeping website, it is worthwhile to consider using oxalic acid vapour to treat beehives for varroa mites.
This article, amongst many others, details at the end that mite kill is about 95% on a single treatment.
An additional benefit is that oxalic acid is less likely to have a substantial impact on the queen bee and bees in the hive.
Medhat Nasr, former Provincial Apiarist of Alberta, confirms that vaporized oxalic is very gentle to the bees.
BC Bee Supply carries both oxalic acid and a vaporizer that can be powered with a standard 12V. Please click below or contact us to order.