Dead bamboo will not be full of insects because there is nothing much to attract them. Larger culms may be cracked from air drying...or not. The time to harvest culms is when you want them. If they are alive, they will dry out after cutting, though this will take a while. If they are dead and dried out (usually tan) they are ready for use. If you want to put a finish on them, you can. They don't take stain very well because they have a hard, exterior layer that is not porous.

Thank you for the fast and informative reply! I will ask permission.