Preservation by Prevention Are Carpenter Bees Harmless?

Carpenter bee begins a gallery tunnel; note the black, shiny abdomen.

Carpenter bee begins a gallery tunnel; note the black, shiny abdomen.

Over a recent weekend while working outside, I noticed a very large bee buzzing around near the back door of our house. Being somewhat afraid of all things that sting, I pointed it out to my fiancé, but he assured me that it was just a bumblebee and perfectly harmless. Though he may have been right about the risk of stinging, he was proved wrong a few days later on the “harmless” claim. I found the same bee in the exact same location and noticed two concave depressions in the wood trim over the door. Then, I knew that it was a carpenter bee.

Carpenter bees are loud and large, usually ¾ to 1 inch in length; they do closely resemble bumblebees and are sometimes mistaken for them. Both sexes can be differentiated from bumblebees by their glossy, black, and bare abdomens, which differ from the bumblebee’s banded abdomens covered in black and yellow hairs. Females, which may sting when agitated or handled, have dark faces while males have white faces. Males exhibit aggressive and territorial behavior, buzzing and hovering near humans and other animals, but they are without stingers and harmless.

Carpenter bees emerge from their winter nests in the spring, usually during April and early May, to mate and lay eggs. They are solitary insects and so do not live in hives or colonies. The behavior I witnessed on our wood trim – tunneling – is how females create new nests in which to lay their eggs. The eggs develop into adult bees during the summer, and usually emerge from their nests in August or September. The new adult bees reuse the nests in which they were born for winter hibernation.

A female bee begins to form a new nest by cutting a round entrance hole, with a diameter approximately the same as her body, into a piece of wood. Although carpenter bees do not eat wood, they do use their strong jaws to cut through wood. The surface of the wood may be exposed, as it is in the case of our door trim, or it may be hidden underneath a piece of siding or otherwise unexposed.

After she digs into the wood an inch or so, the female bee turns at a right angle and tunnels a round gallery (i.e. tunnel) through the wood in the direction of the grain. New galleries are usually 4-6 inches long, although carpenter bees have a tendency to reuse old galleries when possible. When a gallery is reused by bees over many years, it can extend for several feet and branch into many separate tunnels, affecting the wood’s structural integrity. After the new gallery is complete, or old gallery is cleaned, the bee lays her eggs in separate chambers in the gallery. Each egg chamber also includes a source of pollen to nourish the developing bee. The adult females die soon after finishing this process.

Damage from carpenter bees is usually cosmetic, as the entrance holes mar the appearance of decorative trim and railings. Bee feces often stains areas near the entry holes because carpenter bees usually evacuate waste before entering the galleries. The noise and activity of the bees can, however, attract woodpeckers, which feed on the bee larvae. In the case of clapboards or other thin pieces of wood, woodpecker damage may be so severe as to necessitate replacement.

Carpenter bees are generally attracted to unpainted wood, so one of the best methods of preventing an infestation is to keep any exposed wood surfaces covered with a good coat of paint. Stains are not as effective, though they do offer some protection. At my house, the piece of trim being attacked is unpainted – a classic case. Until we can fill and paint the piece of trim properly, we have stuffed the holes and covered the entire trim piece with aluminum foil, sealing all of the edges with strong tape. Thus far, it has deterred our carpenter bee from returning.

If you already have a carpenter bee infestation, the best thing to do is spray an insecticide dust into each entrance hole to kill any bees or larvae within. Most entomologists recommend insecticides using carbaryl (e.g. Sevin, cyfluthrin or resmethrin). A few days after application – allowing time for the adult bees to pass in and out, distributing the insecticide – you can plug the holes with a dowel or wood filler, sand the plugs flush, and repaint. If you seal live or larval bees in a gallery without an insecticide they will chew their way out of the wood, causing further damage and providing an open gallery for the next generation of bees to use.

Disclaimer: This newsletter is intended to provide a non-expert reader with basic information. For professional advice, please consult architects, contractors, and/or engineers.

For More Information

Bambara, Stephen B. and Michael Waldvogel. Carpenter Bees. Insect Note-ENT/rsc-4. North Carolina State Cooperative Extension, 2008. http://www.ces.ncsu.edu/depts/ent/notes/Urban/carpenterbees.htm (accessed May 13, 2009).

Barnes, Jeffrey K. Carpenter Bee. Arthropod Museum Notes, No. 34. University of Arkansas Arthropod Museum, 2005. http://entomology.uark.edu/museum/carpbee.html (accessed May 13, 2009).

Ellis, Tom. Carpenter Bee. Home Maintenance And Repair 01500558. Michigan State
University Extension, 2008.http://www.msue.msu.edu/objects/content_revision/download.cfm/revision_id.498900/workspace_id.-4/01500558.html/(accessed May 13, 2009).

Gibb, Timothy J. and Ralph E. Williams. Solitary Bees and Wasps: Carpenter Bee, Cicada Killer, and Mud Daubers. Household and Structural E-63-W. Purdue University Extension, 2007.http://extension.entm.purdue.edu/publications/E-63.pdf (accessed May 13, 2009).

Jones, Susan C. Carpenter Bees. HYG-2074-06. The Ohio State University Extension, 2006.http://ohioline.osu.edu/hyg-fact/2000/pdf/2074.pdf (accessed May 13, 2009).

Potter, Michael F. Carpenter Bees. Entfact-611. University of Kentucky College of
Agriculture, 1994. http://www.ca.uky.edu/entomology/entfacts/ef611.asp (accessed May 13, 2009).

Powell, Peggy K. Carpenter Bees. Household Pest Management 6005. West Virginia
University Extension Service. http://www.wvu.edu/~agexten/ipm/insects/hpm6005.pdf (accessed May 13, 2009).

University of California Agriculture and Natural Resources. Carpenter Bees: Integrated Pest Management in and around the Home. Pest Notes Publication 7417. University of California, 2004.http://anrcatalog.ucdavis.edu/InsectMiteMolluscPests/7417.aspx (accessed May 13, 2009).

Weaver, Martin E.  Conserving Buildings:  A Manual of Techniques and Materials.  New York: John Wiley & Sons, 1997.

Young, Robert A.  Historic Preservation Technology.  New York:  John Wiley & Sons, 2008.

, , , , ,