Carpenter bees may have been misnamed. Instead of building things, they cause damage to the wood where they make their nests.
Carpenter bees make their nests in wooden decking, porch railings, siding, eaves, windowsills, shutters and trim, and even patio furniture. They also make nests in utility poles and bridges.
A female carpenter bee makes a 0.5″ hole in the surface of the wood. After the hole is an inch or two deep, the bee turns and begins to dig a tunnel. The tunnel follows the grain of the wood. It can extend 5″ or 6″ into the wood. The bee deposits an egg and places food for the larva before sealing the chamber. She makes a second chamber with another egg and a supply of food next to the first chamber and seals them in. She continues making egg chambers until the tunnel is filled. When she has finished, the female bee has made five or six egg chambers in the tunnel.
Carpenter bees can use tunnels that were built during the previous year. They often extend these tunnels before placing eggs in them. Sometimes several bees excavate in the same tunnel. Scientists have found tunnels that were more than ten feet long. They had been excavated by a group of bees over an extended period of time.
Many times, female carpenter bees will make nesting tunnels next to each other. This tunneling can create weakness in structural lumber. If the bees are tunneling in siding, they can spoil the appearance of a home. Buildings that are seldom used, like hunting cabins, can sustain extensive damage from carpenter bee tunneling.
The most effective treatment for carpenter bees involves injecting insecticide into each tunnel. The insecticide should eliminate the bees that are actively tunneling. It should also eliminate the bees that come out of the tunnels in a month or two. Dust or wettable powder insecticides are most effective because they do not soak into the wood. They remain on the surface of the tunnels so the bees can crawl through it when they move in and out of the tunnels.
Insecticide injection requires special equipment. It is important to make sure that no insecticide is misapplied onto the surface where children or pets could be exposed. There are also safety concerns to make sure no insecticide splashes back into the face of the person who is making the application. Because of these concerns, many people prefer to have a pest control [professional make the application.
After the insecticide treatment, the tunnels should be left open for a few days so the bees that are tunneling can be exposed to the insecticide. After that, the holes can be sealed with caulking or wood dowels.
Scientists disagree about the value of painting to prevent carpenter bees from tunneling. Most scientists agree that carpenter bees prefer to nest in unfinished wood. Paint or a similar finish on exterior wood may be able to prevent damage by causing carpenter bees to move to another nesting site.