They are a large bee usually around 1 inch long with a colored metallic blue-black with green or purplish reflections. A lot of people think they are a black bumble bee.
Carpenter bees are beneficial insects. They help pollinate plants. The males do not have a stinger and females rarely attack. Females that have overwintered emerge in the spring to establish new nests. However, they will still use old tunnels. Nests usually are 1/2” in diameter and between 6” to 10” deep and have several chambers.
Within each chamber she will lay an egg and leave pollen for its development. Sometimes several bees will use the same entry hole connected to different chambers. Development from egg to adult takes around three months. There is only one generation per year and it is common for adults to overwinter in old tunnels.
Carpenter bees bore into sound wood or sometimes into decaying wood to make their nests. Places like this fence railing. Nests weaken structural wood and leave unsightly holes.
Carpenter bees are easily controlled by treating their nests with desiccant dusts or liquid pesticides.