Carpenter bees resemble bumblebees, but have a bare black abdomen. The females build their nests in wood, drilling a perfectly round hole about 3/8 inch in diameter. They drill straight for an inch or two, then make a 90-degree turn and drill several more inches. Carpenter bees sometimes reuse nesting sites, enlarging old nest holes. They do pollinate flowers, but they can cause extensive damage to wood if allowed to use the same holes year after year.
Paint the wood. Carpenter bees are attracted to unpainted wood surfaces. Staining wood does not work as well as paint for repelling carpenter bees, but staining provides better protection than bare wood.
Close doors to garages and outbuildings to prevent carpenter bees from accessing unpainted wooden beams inside.
Mix an insect-repellant stain additive, such as Outlast NBS 30 or Bug Juice, with water and apply to wood surfaces as a temporary repellent. It can be mixed with oil or water-based stains or paints as a permanent repellent that lasts up to two years. You may need more than one coat to repel carpenter bees.
Apply insecticidal dust or liquid to wood as a repellent in the spring when carpenter bees are building nests. If you find a nest, puff insecticidal dust into the opening, then wait several days for the bees to spread the dust into the tunnel galleries before sealing the opening.
Wrap exposed wood with metal flashing to exclude carpenter bees. They cannot chew through metal. Plastic siding also will keep them out.