Beech seems to be the most attractive to them. You might try a treatment with a borate. You might also be able to tent off the frame and have it fumigated. High heat like a kiln will also kill them. They need about 15% moisture to survive so the drier the better. If you can get rid of the current bugs try oiling with linseed oil generously. This should make it less palatable to them in the future. Good luck with it. I've seen a lot of old beech barn timbers in my area destroyed by them. Here is a link with some nasty chemicals which might help.
http://www.pestmall.com/powder-post-beetle-control-products/