Carpenter Bees look a whole lot like Bumble Bees except that they have a bare shiny black abdomen without yellow stripes. The females are relatively docile; they have a very functional stinger that is good for multiple stings without coming out … much like a Yellowjacket. The males are normally more aggressive, but they have no stinger! This is truly fascinating; the more aggressive male has no stinger, and the female, who has a stinger, is relatively docile unless she is defending a brood tunnel. Here in California, the male Carpenter Bee is a lighter color than the female and has large green eyes and a fuzzy bear appearance.
Carpenter Bees do not bore holes into living wood, but rather create tunnels in dead branches or cut wood in which to raise their young. The entry hole is 3/8 to 1/2 inch in diameter and is perfectly round. Entry holes are usually located in well-lit and sheltered areas, such as headers, roof eaves, porch ceilings, fascia boards, decks and window sills. If you see shiny black bees flying around your structure, watch closely until you find the opening for the brood tunnel. Treatment is simple; put insecticide dust in the hole, caulk it shut, and watch closely for new holes. Ideally you would do this treatment after dark to catch them all inside. If you neglect the dust, they will open the hole or make a new one and continue tunneling. The female will protect the brood tunnel, so be careful when you approach it.
When a female Carpenter Bee is ready to breed, she will return to the same brood tunnel that she used last time, or she will begin boring a new tunnel near her last home. Her offspring will often frequent the same area, so it is not unusual to find a lot of bees hovering around the same woodpile, structure, or dead tree. Whenever possible it is a good idea to not delay Carpenter Bee treatments because, once they build up a large population, they become very persistent and difficult to control.
There is another species called the Small Carpenter Bee that is relatively uncommon in North County. They like to build small broods in window sills and are not very aggressive.