If the hole is "round" They are carpenter bees. They do not sting but as Sawdust said they are very destructive. There several ways to control these bees but every way that I know requires a watchful eye to retreat. The most effective method I have used is to caulk every hole, every time one starts a new hole. I have used diesel squirted in the hole, effective but messy. foam, any insecticide, but caulking and painting is the best way I have found.