Carpenter bees build nests in wood, creating galleries that can weaken structures; however, they rarely cause severe damage. People may be frightened by carpenter bees because of their large size, their similarity to bumble bees, and their annoying noise.
Most carpenter bees, are large and robust insects resembling bumble bees. They are usually about 1 inch long and colored a metallic blue-black with green or purplish reflections. They differ from bumble bees in that their abdomen is shiny with fringes of hairs on some segments. Males of some species are lighter colored, ranging into golden or buff hues.
Carpenter bees cause damage to wooden structures by boring into timbers and siding to prepare nests. The nests weaken structural wood and leave unsightly holes and stains on building surfaces. Second, undecayed wood without paint or bark is usually selected for nests. Carpenter bees also frequently attach to dead wood on trees or lumber from southern yellow pine, white pine, California redwood, cedar, Douglas fir, cypress, mimosa, mulberry, ash and pecan trees. They avoid most hard woods. The presence of carpenter bees around buildings and wood structures can be annoying or even frightening; however males cannot sting and females rarely attack.
Carpenter bees resemble large bumble bees, but have very different nesting behavior. They bore long tunnels into wood, and divide these tunnels into cells where individual larvae will develop. While several females may be nesting in wood of the same structure or other site, each is acting in a solitary fashion, as these are non-social bees.
The common eastern species, X virginica (Linnaeus), resembles many of the bumble bees closely enough that it is often confused on casual observation. This carpenter bee is black in color and marked with areas of yellow hair, but the dorsal sides of the abdominal segments (except for the apparent first segment) have no areas of yellow hair. In bumble bees, at least some of these abdominal segments will have yellow hair on their dorsal surfaces.
Other species of carpenter bees, from other genera, may be black, green or somewhat purplish in color, and they are variously marked with whitish, yellowish or reddish hair. The dorsal surface of the abdomen is also generally bare in these species.
The typical carpenter bee gallery has an entrance hole on the wood surface, and continues inward for a short distance. It then turns sharply upward and runs in the same direction as the grain of the wood. The female provisions the galleries by inserting a ball of pollen on which an egg is laid. Live prey, such as insects or spiders, are not used. The female then closes the cell by placing a mass of wood pulp in the gallery. A series of cells are constructed as the bee works backward, out of the gallery.
Females often enlarge existing galleries or use old ones, so very complex gallery systems can be developed over a number of years. These galleries are often made in the siding or window trim of homes, and in such cases the structural strength of tunneled timbers may be reduced.
Carpenter bee nests are usually not difficult to locate. Some of the more common sites chosen within buildings include siding, eaves, wooden shakes, porch ceilings, window sills and doors. They will also nest in telephone poles, fence railings or posts, and even in lawn furniture.
Many types of wood are selected for nesting, but softer woods are preferred. Unpainted or well-weathered wood is much more susceptible to attack than hardwood or well-painted timbers. Another sign to look for in locating carpenter bee galleries is the yellowish or brownish excrement stains created on the side of the home, below the entrance holes to the galleries.
Carpenter bees complete one generation per year in most areas of the United States. Tunnels and pupae develop in the closed cells in early summer. Adult bees emerge in late summer and return to the same tunnels to hibernate for the winter months. In the spring, the adults mate and the females lay eggs, completing the cycle.
The abandoned nests of carpenter bees are frequently infested by any number of secondary pests, including dermestid beetles, dried fruit moths and other scavengers that will feed on unused pollen and nectar. Certain wasps, ants and bees will also be found in old galleries, using them as shelter and nesting sites.