Carpenter bees (Xylocopa virginica) are very active from early spring through summer around houses and other wooden structures. These insects bore one-half (½) inch wide holes that appear to be perfectly round on exterior wooden surfaces of house siding, eaves, window trim, porch ceilings, fascia boards, shingles, decks and even outdoor wooden furniture. The bees chew one half inch per day to open the entrance hole, and then move at a faster pace as they move with the grain to expand the tunnels.
Homeowners are often frightened by these pesky black bees that fly erratically around their homes. The male carpenter bee is very territorial and protects its nesting sites by hovering, buzzing, and attacking intruders. Although the male is aggressive, it does not have a stinger, making it harmless. The females do have a stinger but are docile and rarely sting and then only if handled or touched.
Homeowners often refer to these large, dark colored insects as bumble bees because of their similarities in size and appearance. Carpenter bees nest in excavated wooden tunnels. Bumble bees nest in the ground. Carpenter bees are robust, heavy-bodied bees that range ¾ to I inch in length. The carpenter bee can be identified by having bright yellow, orange or white hairs on the thorax and a black shiny abdomen. The bumble bee has a hairy abdomen, black or yellow in color. The male carpenter bee can be identified by having white markings on the head.
Adult female and male carpenter bees overwinter (survive the winter) in abandoned nest tunnels in which they have stored small amounts of pollen. The adults emerge in the spring (April and May) when the temperatures reach 70º F, mate, and search for nest sites. Females may use an old abandoned nest tunnel or excavate a new gallery to lay her eggs in a series of six to eight cells. Females supply each cell with “bee bread” (a mixture of pollen and nectar), lay an egg on the food mass and seal off each cell with chewed wood pulp. The egg hatches and the larvae develops from May through September and emerges by chewing through the wall of the cell as an adult bee sometime in the late summer.
Carpenter bees are nuisance pests in most cases, but they can cause considerable structural damage from repeated colonization of the same area year after year. Fine sawdust caused by the adult bee’s excavating activities during the spring of the year will normally be found lying on the ground beneath the gallery entrances. Repeated boring activities may result in unsightly stains caused by falling bee waste around the entrance hole. Homeowners often notice a buzzing or burrowing sound coming from within the wood this time of year. The excavating bee will bore directly into the wood with her mouthparts for about 1 inch, then turn sharply and bore at a 90° angle usually along the grain of the wood. Normally, the gallery will extend about 4 to 6 inches, but with repeated use galleries have measured up to 10 feet long. Nest sites made by a single bee result in slight damage. Repeated colonization over several years, however, may result in considerable damage. Once carpenter bees have laid eggs at a site, unless the site is altered and made unfavorable, they will return year after year to excavate and expand the tunnel and lay more eggs.
Carpenter bee larvae are noisy and tend to attract woodpeckers who will drill holes along the tunnels feeding on the larvae. This activity results in further damage as they peck through the surface of the wood to reach the immature bee.
Unpainted, exposed, and weather worn wood is especially attractive carpenter bees. The most effective deterrent to carpenter bee activities is a painted (oil base or polyurethane) surface. Wood stains provide little repelling action. Nail holes or exposed saw cuts should be filled in with wood putty or dowels and painted. If practical, remove severely damaged wood and replace with chemical pressure treated wood to deter nest construction. To further discourage carpenter bees looking for potential nesting sites, a homeowner should inspect the exterior to make sure that all wood surfaces are painted, sealed, or covered with vinyl or aluminum. Non-wood surfaces such as vinyl siding are not damaged by carpenter bees.
Established bee galleries should be treated with dusts or liquid sprays labeled for wasp or bee control and sealed after all activity has subsided. The insecticide dust or spray should be applied directly into the passage entrance hole to ensure bee contact. With dusts this is best accomplished by using a duster that will puff the dust up into the tunnel and coat the sides of the entrance hole. For do-it-yourselfers these control efforts should be conducted in late evening or at night when the bees are inside the wood tunnels. Allow three days for activity at each entrance hole to subside. Afterwards, all tunnel entrances should be plugged with wood putty, caulking compound, or wooden dowels to prevent re-colonization. Note: female carpenter bees prefer to reuse or expand existing nests rather than begin new nests by chewing into wood. Therefore, it is critical that entrance holes and gallery system be patched and sealed to prevent annual re-infestation.
Liquid insecticide sprays containing synthetic pyrethroids can be sprayed on adjacent wood surfaces to temporarily reduce carpenter bee activity. However, merely spraying the wood surface will not kill carpenter bees that are nesting in the tunnels, nor will it deter them from chewing new holes. The residual effectiveness of these insecticides on exterior surfaces is short lived and often no more than a week or two. Direct sunlight, heat, humidity, wind, and rain are environmental factors that reduce the residual effectiveness of the materials on exposed surfaces. Residual effectiveness may continue for 30 to 60 days or longer when the material is applied inside the galleries and protected from environmental conditions.
Adapted from published information from Clemson University Home & Garden Information Center, Ohio State University Extension Service, Penn State Cooperative Extension, University of Kentucky College of Entomology, and Cornell Lab of Ornithology.