... after all the other experts weighed in...
I guess none of us are carpenter bee experts.
A few general "facts" I've come across are:
Carpenter bees prefer deteriorated wood because it's easier to bore into.
Carpenter bees don't eat wood. They drill nesting tunnels for cells,
and fill them with multiple layers alternating
separator//food+egg//separator...
They're supposed to hatch out next year.
Some species are semi-social, some are solitary.
A good layer of paint keeps the bees from boring into the wood.
carpenter bee holes sometimes attract woodpeckers.
The bore holes are generally shallow. superficial damage.
Read the Wikipedia article for starters.
Since they don't eat the wood, a pesticide treatment of the wood might
not help much.
If they're going into a cavity, screen them out.
A good layer of paint. Stain probably won't hold 'em back.
Pest control companies probably have their own advice.
Sorry this isn't very helpful