It took ages to find this old post of mine but it very much needs a followup. Thanks very much to Webclan for the reminder over coffee on the weekend.
There may be a few people following this thread and copper naphthenate is very widely used to combat fungal woodrot in bee boxes. Normally, I leave timber treated with copper naththenate to cure for several months before painting over it with plastic paint. However late last year I needed to use a couple of boxes which hadn’t been painted over. The colonies in the boxes were fine for a couple of months but both have now had serious problems. It might be a coincidence because two uncontrolled experiments isn’t very strong evidence. To be on the safe side of prudent caution, I won’t be putting copper naphthenate treated timber in direct contact with my precious bees again. I will still continue to use it as a preservative but will be painting over it before use in future.
The point may be a bit moot because I can no longer obtain the 20% concentrate I had been using. I still have a stock of diluted preservative but once it runs out, I may need to find something else to preserve my woodware.