Carpenter bees get their common name due to the females' habit of excavating galleries in wood to create nest sites for their young. These bees do not consume wood; they feed on pollen and nectar and are important plant pollinators.
Large carpenter bees belong to the genus Xylocopa. Two native species, Xylocopa virginica and Xylocopa micans, occur in the eastern United States. A number of native carpenter bees also occur in the western United States. This fact sheet primarily pertains to X. virginica, with the common name of carpenter bee.
|Figure 1. Carpenter bee, Xylocopa virginica. An arrow points to the abdomen. (Photo courtesy of D.J. Shetlar, OSU Entomology.)|
Carpenter bees are large (~three-quarters- to 1-inch long) and robust. The upper surface of their abdomen is largely bare and appears shiny black (Figure 1). The thorax is covered with yellow, orange, or white hairs. The head is almost as wide as the thorax. The female has an entirely black head, whereas the male has yellow or white markings. Carpenter bees have a dense brush of hairs on their hind legs.
Carpenter bees resemble bumble bees as they are similar in size except for their head, with the bumble bee’s head being much narrower than the width of the thorax. Unlike carpenter bees, bumble bees have a very hairy abdomen with yellow markings and they also have large pollen baskets on their hind legs. Bumble bees are social insects that live in colonies with nests typically located in the ground.
Biology and Life History
Carpenter bees are solitary insects that do not form colonies. Each female bee has its own separate nest gallery inside the wood where its offspring develop. Numerous carpenter bees often occupy the same piece of wood, with nest galleries occurring close to each other (Figure 2), but each carpenter bee behaves independently of the other bees.
|Figure 2. Numerous nest galleries in wood, with arrow pointing to nest entrance hole. Galleries contain the remnants of wood pulp separating each brood cell. (Photo by S.C. Jones.)|
Male and female carpenter bees emerge in the spring (April and early May) and mate. Territorial males hover nearby as mated females begin nesting activities. Because gallery construction is a time- and energy-consuming process, the female preferentially refurbishes an old nest rather than creating a new one. She may reuse an existing gallery, lengthen an existing gallery, or excavate a new gallery from an existing entrance hole.
In order to create an entirely new nest, the female uses her strong jaws (mandibles) to excavate a clean-cut, round nest entrance hole that is slightly less than a half-inch wide, approximately the diameter of her body. She bores into the wood perpendicular to the grain then turns at about a right angle (about 90 degrees) and excavates along the wood grain for 4 to 6 inches to create a gallery (tunnel). She excavates the gallery at the rate of about 1 inch in six days.
Each female bee creates a series of provisioned brood cells in a gallery. The larval provision consists of a mixture of pollen and regurgitated nectar formed into a ball. The female forms a food ball at the far end of an excavated gallery, lays an egg on top of the mass, and then walls off the brood cell with a plug of chewed wood pulp. A female often creates six to 10 partitioned brood cells in a linear row in one gallery, and she dies soon thereafter. Males likewise are short lived. Larvae feed on the pollen/nectar food mass, which is sufficient food for them to develop to the pupal stage then the adult stage.
The life cycle (egg, larva, pupa, adult) is completed in approximately seven weeks, although developmental time varies depending on temperature. All of the new adult bees typically remain in their gallery for several weeks then chew through the cell partitions and venture outside in late August to feed. They collect and store pollen in the existing galleries but also spend much of their time just huddled together inside the same gallery where they developed. These new adults require shelter during the winter and they hibernate within their old nest gallery and then emerge the following spring. There is one generation per year in the northern states, but sometimes two generations in the southern United States.
Wood Damage and Telltale Signs
Eastern species of carpenter bees prefer softwoods such as cedar, redwood, cypress, pine, and fir. The bees can more easily tunnel through woods that are soft and with a straight grain. Western species often nest in oak, eucalyptus, and redwood.
Carpenter bees avoid wood that is well painted or covered with bark, but they can nest in a wide variety of wood products, including fence posts, utility poles, firewood, arbors, and patio furniture. In buildings, carpenter bees nest in fascia boards, roof eaves and gables, porch ceilings, decks, railings, siding, shingles, shutters, and other weathered wood.
Carpenter bee damage to wood initially is minor, and carpenter bees seldom cause consequential structural damage. However, extensive wood damage can result over time as many generations of carpenter bees enlarge existing galleries in wood. A gallery can extend for 10 feet if used by many carpenter bees over the years. As carpenter bees construct new tunnels near old ones, their complex system of tunnels can result in extensive damage to wood. Wood replacement is necessary when the strength of structural members, posts, poles, and other wood products is reduced due to carpenter bee damage.
Carpenter bees also may be indirectly responsible for extensive, unsightly wood damage when woodpeckers and flickers riddle the wood with holes searching for food (immature and adult carpenter bees). These birds seem to particularly target overwintering adult bees.
Carpenter bee nest entrance holes used to be commonly situated in exposed sites, but those bees and their offspring were the most easily targeted by management efforts; now the carpenter bee population is dominated by those bees that hide their entrance holes. The inner surface of roof fascia boards is a common site of attack. Nail holes, exposed saw cuts, and unpainted wood are attractive sites for the bees to start their excavations.
To locate nest sites, one needs to look for the bees’ yellowish to brownish fecal staining/streaking and dislodged pollen beneath hidden entrance holes (Figure 3). One also may look for coarse sawdust from the bees’ borings (Figure 4).
|Figure 3. Carpenter bee staining on brick beneath hidden nest entrance holes. (Photo by S.C. Jones.)||Figure 4. Coarse sawdust from carpenter bees’ excavations. (Photo courtesy of D.J. Shetlar, OSU Entomology.)|
Other Nuisance Aspects
Carpenter bees often are noisy around structures. Male carpenter bees often cause alarm when they dive-bomb and fly erratically around humans that approach nesting sites, but in actuality, these bees are bluffing as they lack a stinger and are harmless. Only female wasps and bees have a stinger, which is a modified egg-laying device (ovipositor). Female carpenter bees are docile and are reported to sting only if handled. A female carpenter bee can sting more than once.
Integrated Pest Management
It is preferable to locate tunnel entrances during the daytime, but conduct any management efforts at nest sites after dark on a cool evening when carpenter bees are less active. Wear protective clothing to avoid stings.
Keep all exposed wood surfaces well painted with a polyurethane or oil-based paint to deter attack by carpenter bees. Periodically inspect painted surfaces because the coatings will begin to deteriorate due to weathering, thereby exposing wood that the bees can easily attack. Wood stains will not prevent damage from carpenter bees. These bees do not damage non-wood materials including aluminum, asbestos, asphalt, and vinyl siding, and these can be used as alternatives to wood.
A non-insecticidal management approach is to deny carpenter bees access to their galleries by sealing each entrance hole. Thoroughly plug the hole with plastic wood, steel wool, or copper gauze and seal it with wood putty or a wooden dowel affixed with wood glue. If possible, first fill the entire gallery system with a sealant. Carpenter bee galleries are a critical resource, since the bees spend much of their time inside a gallery and they require its protective conditions to survive the winter. Bees that are trapped inside a sealed gallery typically will not chew out due to behavioral constraints. This barrier approach has promise for reducing future carpenter bee infestations in an area.
The female often can be swatted and killed, or she can be captured and crushed or otherwise destroyed near her nest site. Larvae and pupae can be killed by inserting a sturdy wire into the entrance hole and probing into the gallery as deeply as possible.
A number of pyrethroid insecticides (cyfluthrin, bifenthrin, deltamethrin, permethrin, etc.) are labeled for use against carpenter bees. Treatment of nest galleries with a dust formulation provides long-lasting activity (residual) and typically is the most effective approach. Precisely inject the dust directly into each nest entrance hole and as deeply into the tunnel as possible. It also should be applied to the surface of adjacent wood. Wait for a few days before plugging any treated entrance holes so that the adult bees have enough time to distribute the dust within the galleries and to accumulate a lethal insecticide dose.
For use as a short-term preventive, an appropriately labeled insecticide can be applied to the surface of wood, preferably in early spring before female carpenter bees begin excavating nests. The insecticide kills the bees that contact the treated wood. Microencapsulated formulations often are used. Note that this type of preventive approach has limitations because of the difficulty in applying a chemical to all exposed wood on the house where bees can nest. Furthermore, the insecticides usually degrade in a matter of weeks or months so repeated applications are needed to maintain a lethal insecticide dose.
Insecticides that act as stomach poisons, such as borates, typically are ineffective against carpenter bees, which do not ingest the wood that they excavate.