 They know where to put it, how much to put there, and they also can use pesticides that can stay around a little while longer than the stuff you can buy and therefore give you some long-term protection. You are doing the right thing by filling the holes. What I also do on properties that I have where I've got carpenter bees that attack, like the fascia or the soft wood, is I replace it with AZEK, A-Z-E-K, which basically looks like wood, but it's made of PVC.