Carpenter bees (genus Xylocopa) are large insects that cause concern due to the damage they inflict on wooden structures like decks, eaves, and fascia boards. While they are beneficial pollinators, their nesting habits create conflict for homeowners. Understanding their behavior is key to management, as they are often mistaken for bumblebees, leading to misdirected control efforts.
Distinguishing Carpenter Bees from Other Pollinators
The most reliable way to differentiate a carpenter bee from a bumblebee is by examining the abdomen, which is the rear section of the body. Carpenter bees have a shiny, black, and hairless abdomen, giving it a polished appearance. In contrast, the bumblebee’s entire body is covered in dense, fuzzy, yellow and black hair. Both bees are similar in size, typically measuring between one-half to one inch long.
Carpenter bees are solitary insects; each female drills her own nest hole and raises her young alone, unlike social bumblebees that live in colonies. The male carpenter bee has a distinctive white or yellow patch on its face and often aggressively patrols the nest area. Female carpenter bees, with their entirely dark faces, are the ones that bore into wood. This difference in nesting behavior is the core reason one species becomes a structural concern while the other does not.
Structural Damage Caused by Nesting
The female excavates tunnels in wood to create nesting galleries for her offspring. She chews a perfectly circular entrance hole, about a half-inch in diameter, straight into the wood for an inch or two. Once inside, she turns at a right angle and tunnels along the grain of the wood, creating a gallery that can extend six to twelve inches.
Visible signs of this activity include coarse sawdust, known as frass, which accumulates in a small pile directly below the entrance hole. Over time, repeated use of these galleries by successive generations can significantly weaken structural wood members. Females often reuse and lengthen old tunnels, and a gallery system can eventually reach up to two feet long. The resulting damage is compounded by woodpeckers, which often chisel into the wood to feed on the bee larvae inside, further compromising the structure.
Ecological Impact and Threat Level
Despite the property damage they cause, carpenter bees play an important role as effective pollinators for many flowering plants, including certain fruits and vegetables. Their large size allows them to perform “buzz pollination,” where they vibrate their flight muscles to dislodge pollen, a technique required by crops like tomatoes. They can sometimes engage in “nectar robbing,” but they still contribute substantially to plant reproduction.
The threat level they pose to humans and pets is minimal compared to social insects like wasps. The male carpenter bee is highly territorial and will hover aggressively near the nest to ward off perceived threats. However, the male lacks a stinger and is completely incapable of stinging. The female does possess a stinger but is generally docile and will only sting if she is physically handled or severely provoked.
Strategies for Prevention and Control
Managing carpenter bees involves a combination of proactive prevention and reactive control methods. Prevention focuses on making wooden surfaces unattractive to the solitary females seeking nesting sites. Carpenter bees strongly prefer bare, unpainted, or weathered wood, especially softwoods like cedar and pine.
Painting, sealing, or varnishing exposed wood surfaces is a highly recommended preventative measure, as the hard finish deters the bees from chewing into the material. Sealing existing cracks, nail holes, and crevices with caulk or putty also removes potential starting points. Homeowners can consider using harder woods for construction, which are more difficult for the bees to penetrate.
For existing, active nests, reactive control involves treating the tunnels before sealing them. The best time to treat is in the spring or early summer when the bees are actively drilling. Applying an insecticidal dust directly into the entrance hole is the most efficient method, as the bee will carry the dust further into the gallery system. The hole should be left open for a few days after dusting to ensure the female and any emerging offspring contact the insecticide.
After activity has ceased, the holes must be sealed to prevent water infiltration and to discourage other bees from reusing the existing gallery. Sealing materials like wood putty, caulk, or wooden dowels are suitable for closing the entrance. Sealing the holes too early, while a bee is still inside, can trap the bee, potentially causing her to drill a new exit hole nearby and expand the damage.