Oxalic vapor is probably safe for bees. It's more trouble than other methods, though

> Trickling oxalic acid dihydrate seems to be the most suitable application method, especially for large apiaries. The beekeeper needs only a syringe or a similar applicator, gloves and protective glasses to apply the substance. The application is quick, about 1 min per hive, cost-effective and easy to conduct.

> Beekeepers using the evaporation method will have additional expenses; e.g., safety equipment and an evaporator. They need to wear gloves, a face mask and protective glasses.

> Most authors reported that the evaporation of oxalic acid dihydrate (0.5 to 5 g crystals) with the Varrox evaporator had no impact on bee mortality and colony overwintering; no losses of queens were observed (Baggio and Mutinelli, 2003a; Ferrero et al., 2004; Radetzki, 2001; Radetzki et al., 2000; Radetzki and Bärmann, 2001). 

> Observations concerning repeated treatments using the Varrox evaporator did not result in colony damage (Radetzki, 2002).

Rademacher, E., and M. Harz. 2006. Oxalic acid for the control of varroosis in honey bee colonies -- a review. Apidologie 37: 98-120.

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