Hi all
Oxalic acid as been used on bees for decades, but I have not used it. Charles will say this is old news:

> A long-term study of the colonies for 3-4 months after the last application of oxalic acid showed a statistically significant negative effect of the acid on brood development.

> Reports agree on the use of oxalic acid when no brood is present in winter. Its efficacy when brood is present is around 60%.

> Although no effects of the treatment were seen in the colonies during the application period, the long-term study of the colonies clearly shows a negative effect of the acid, at least when brood was present. 

> The most important finding in this study was that the increase in the number of brood cells was significantly higher in the control hives than in the treated colonies. 

Higes, Mariano, et al. "Negative long-term effects on bee colonies treated with oxalic acid against Varroa jacobsoni Oud." Apidologie 30.4 (1999): 289-292.

More recently:

> We studied the toxicity of water solutions containing various concentrations of oxalic acid dihydrate to bees and Varroa mites (Varroa destructor) using by spraying honey bee (Apis mellifera) colonies with no brood or little brood in beehive conditions in West-Viru County, Estonia. 

> A water solution of 0.5% OA gave effective control of the mite and was not toxic to bees whereas higher concentrations of OA (1.0 and 1.5%) were highly toxic to bees. 

> In autumn, spraying test colonies that had little capped brood once or twice with a 0.5% OA solution gave effective mite control [93%] with no noticeable toxicity to bees.

Toomemaa, K., et al. "Using oxalic acid in water solution in control of Varroa mites and its influence on honey bees." Agronomy Research 8.Special II (2010): 345-350.

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