I have looked over the scanty materials on copper salts and make the following observations. It seems that Bounias, Andre, Nectoux, and Popeskovic applied for a US patent on the copper salt treatment back in 1990, received a patent in 1991 and then let it expire in 1999.

The study by Bounias, Andre, Nectoux, and Popeskovic is the sole support for the efficacy of this treatment. A close look at Bounias paper shows a reference to cupric salts being tested on 1500 hives but the charts refer to very small numbers (6 to 10 hives). The authors have continued to publish their ideas in various trade journals but this treatment is not mentioned in any of the mainstream summaries of varroa treatments.

I can't find any corroboration of it by other researchers.  Antonio Nanetti at Istituto Nazionale di Apicoltura in Bologna, Italy tested copper gluconate against Varroa without good results. Anecdotal information from France indicates a lack of success there.

Peter Borst
Ithaca NY
U S A