It seems the most prime targets are wooden decks, overhangs and unfinished wood inside shop buildings. They also love old, wooden ladders.
Take special notice if you have a house or structure made of redwood, cypress, cedar, white pine or Southern yellow pine. Those are carpenter bees’ favorites.
Though the bees prefer bare wood, they will attack seasoned or treated wood that has been softened by weather exposure. It used to seem that pressure-treated or painted wood was spared from attack, but this has not always been the case. It’s just that it is less susceptible from being attacked.
The holes carpenter bees bore are for making nests. Inside her gallery, the female builds a large pollen ball that serves as food for her offspring. She deposits an egg near this pollen and then seals this section of the tunnel with a petition made of chewed wood. She then constructs additional cells until the tunnel is completely filled, usually with six to seven cells. The adults then die in a matter of weeks. The immature bees will then stay in their tunnels until next spring.
The females do all the boring. The male stays outside and hovers around the head of the intruder. They are frightening because of their large size, but the males cannot sting. The females can sting but only when they are handled. Males can be distinguished from females by a whitish spot on the front of the face.
To cut down on their numbers, you need to take some action on the females. It is difficult and can be time-consuming, but here is what you can do.
Discourage nesting by keeping your garage and outbuildings closed when carpenter bees are searching for nesting sites. This lasts about four to five weeks in the spring starting sometime in mid to late March.
To kill and discourage the bees from boring, you can spray all the exposed wood surfaces that they attack with an insecticide such as carbaryl (Sevin), or a synthetic pyrethroid (i.e. permethrin, cyfluthrin, bifenthrin, etc.). These only last about one to two weeks.
Your next step is treating the nesting holes after mating season is over. Spray or dust Sevin into the holes. A good can of Wasp and Hornet Spray (contains pyrethroid) works well. Come back around dusk and squirt or puff the insecticide into the holes. You might hear a buzzing sound (a good sign). Come back later and caulk the holes with wood putty. Wait at least 36 hours (a few days is even better) after using the insecticide to caulk the holes.