Hi David. Welcome. Depends on wood. Red cedar stands up to the environment well on its own but most timber benefits from a protective coat. Cuprinol wood treatment has been advertised as 'bee friendly' but there has been some concern aired elsewhere recently on this forum, as the manufacturers have apparently changed the ingredients.
I personally have used teak oil to good effect (making sure that it is dry and weathered before bees are intoduced) although I could get no guarantee that it was bee-safe. Linseed oil has been used. Paint can be used, if you are one for pretty colours. Paint may be a poor choice (especially with cedar, as the wood should 'breathe').
Don't coat the insides. Avoid anything that has insecticide, obviously. I would suggest that most wood-treatments that are free of insectiides may be suitable and a call to the manufacturer *may* confirm this (but don't build your hopes up)! Manufacturers' toxicity data is informative.
