If you see large, black bees hovering around eaves, decks and wood siding of your home or and outbuildings, then you are familiar with the carpenter bee. These insects are searching for mates and nesting sites. Carpenter bees cause cosmetic and structural damage to wood. They can also be quite intimidating and have the potential to inflict painful stings. Males aggressively patrol their territory and inspect intruders. The males are harmless, however, since they lack stingers. Female carpenter bees can inflict a painful sting, but seldom will unless handled or molested. The males are easily distinguished from females by a large white marking on their “face."
While carpenter bees are pollinators for several species of plants including blueberry, wisteria and holly, they are considered pests when they bore into wood. Carpenter bees are similar in appearance to bumblebees, but have different nesting habits. Bumblebees generally nest in the ground, whereas carpenter bees tunnel into wood to lay their eggs. Bare, unpainted, weathered softwoods are preferred especially redwood, cedar, cypress and pine. Painted or pressure-treated wood is much less susceptible to attack. Common nesting sites include eaves, fascia boards, siding, wooden shake roofs, decks and outdoor furniture.
Carpenter bees overwinter as adults in old nest tunnels. Females excavate galleries in wood, laying their eggs within a series of small cells. The larvae emerge as adults in the summer. The entrance hole and tunnels are perfectly round and about the diameter of your finger. Coarse sawdust, the color of fresh cut wood, is often seen beneath the entry hole, and burrowing sounds may be heard within the wood. Female carpenter bees may excavate new tunnels or enlarge and reuse old ones. Serious damage can result when the same piece of wood is worked year after year.
Insecticidal dusts such as Tempo 1D (cyfluthrin), DeltaDust (deltamethrin), Bonide Termite & Carpenter Ant Dust (deltamethrin) and Apicide (carbaryl) can be puffed into nest holes in the evening when the carpenter bees are at rest. An insecticidal dust fills the void very well and will
not soak into the wood as a liquid might. The bees should have access to the nest for at least 24 hours to allow them to spread the dust through the galleries.
The hole is then sealed with a wooden dowel coated with suitable sealants, such as carpenter’s glue or wood putty, to prevent reinfestation, moisture intrusion and wood decay. Painting the wood after the dowel is sealed may prevent woodpeckers from banging against the wood in search of the bees. Carpenter bees overwinter in previously used galleries, so the structure should also be inspected in the fall and any holes that may have formed should be treated and sealed.
Homeowners may not have access to insecticidal dusts labeled for wood treatment and thus may need to apply sprays (bee/wasp killer aerosols, Ortho Home Defense Max Termite and Destructive Bug Killer Concentrate, Bayer Advanced Home Pest Control Indoor and Outdoor Insect Killer ready-to-use pump) into the nest opening. The broadcast spray approach is often warranted when carpenter bees are riddling siding on a barn, wood shake roofs, decking or similar large expanses of wood.
Broadcast treatment is best accomplished with a pump up or hose end sprayer, targeting wood surfaces that are most favored by the bees (fascia boards, joist ends of redwood decks, etc.).
Residual effectiveness of such applications is only about one to three weeks, so the treatment may need to be repeated. Individual holes which are already present can also be treated with a wasp and hornet aerosol spray or insecticide dust (e.g., Sevin), directed into the nest opening. Although carpenter bees are less aggressive than wasps, female bees in their nests will sting. It is advisable to treat at dusk or while wearing protective clothing.
Leave the holes open for a few days after treatment to allow the bees to contact and distribute the insecticide throughout the nest tunnel. Then plug the entrance hole with a piece of wooden dowel coated with carpenter's glue, wood putty, or other suitable sealant. This will protect against future bees using the old tunnels, as well as moisture intrusion and wood decay.
Carpenter bees normally will not tunnel into painted wood. Therefore a more permanent solution is to paint unfinished wood surfaces, especially those with a history of being attacked. Wood stains and preservatives are less reliable than painting, but may provide some degree of repellence versus bare wood.
To further discourage nesting, garages and outbuildings should be kept closed when carpenter bees are actively searching for nesting sites.
Call Becky Muller at 476-0231 or e-mail at firstname.lastname@example.org with questions.
Karen Vail, John Skinner, Adam Taylor, and Paul Rhoades from UT Extension contributed to this article.