2 June 2008
Virginia Cooperative Extension, Charlottesville/Albemarle County Office
460 Stagecoach Road, Charlottesville, Virginia 22902
phone: 434.872.4580 fax: 434.872.4578
If you are seeing large, black and yellow bees hovering around the eaves, decks, and wood siding of your home you may be hosting some of our beneficial pollinators. Many of these are carpenter bees setting up housekeeping. Although they can pollinate our flowers, carpenter bees can cause cosmetic and structural damage to wood as well. They can also be quite intimidating to homeowners and have the potential to inflict painful stings.
Carpenter bees are similar in appearance to bumblebees but differ in that they have a black, shiny tail end or abdomen while the bumble bee abdomen is covered with hair. You may remember the difference by thinking carpenters keep their tools clean and shiny. These bees also differ in their nesting habits. Bumblebees nest in the ground, whereas carpenter bees tunnel into wood to lay their eggs. Common nesting sites include eaves, fascia boards, siding, wooden shake roofs, decks, and outdoor furniture. The nest entrance holes are usually found on the underside of a board. Bare, unpainted, or weathered softwoods are preferred, especially redwood, cedar, cypress and pine. Painted and treated woods are less preferred, but they are not immune to attack.
The entrance hole and tunnels are perfectly round and about the diameter of a dime. Coarse sawdust, the color of fresh cut wood, is often seen beneath the entry hole, and burrowing sounds may be heard within the wood. Female carpenter bees may excavate new tunnels or enlarge and reuse old ones. Serious damage can result when the same piece of wood is utilized for nesting year after year until it looks like a Swiss cheese. The holes start upward (or inward) about 1/2 inch, then turn horizontally with the wood grain and may run six to seven inches or more. Occasionally, several bees may use the same entrance hole and have individual branches off the main tunnel. If the same entrance hole is used for several years, tunnels may extend several feet in the wood.
Carpenter bees overwinter as adults in old nest tunnels and emerge in the spring, usually in April or May. After mating, the fertilized females excavate tunnels in wood, laying their eggs within a series of small cells. The cells are provisioned with a ball of pollen on which the larvae feed. Additional cells are similarly provisioned until the tunnel is completely filled, usually with six to seven cells. Males do not drill tunnels, but they are territorial and will harass other bees and people who venture near their protected areas. The males, which can be distinguished from females by a whitish spot on the front of the face, do not sting. Females are capable of stinging, but rarely do so unless confined in your hand or are highly agitated. These adult bees die in a matter of weeks. The eggs hatch in a few days and the offspring complete their development in about 5 to 7 weeks. Adults begin to emerge in late summer. Although the bees remain active, feeding on pollen the general area, they do not construct new tunnels. With the onset of cold weather, the bees seek overwintering shelter.
The best time to control carpenter bees is before the tunnels are fully constructed. Treating the entrance holes with an insecticidal spray or dust may substantially reduce nesting activity. Be sure to read the pesticide label before buying or applying to make sure you have the right product and to understand how to apply it safely. Leave the holes open for a few days after treatment to allow the bees to contact and distribute the insecticide throughout the nest galleries. Then plug the entrance hole with a piece of wooden dowel coated with wood glue, wood putty, or other suitable sealant. This will protect against future use of the old nesting tunnels, as well as moisture intrusion and wood decay. The insecticide treatment is intended to kill both the adult bee and its offspring as they emerge later. Plugging untreated tunnels with wire mesh or similar material might trap bees inside, but more resourceful bees will simply chew another exit hole. Aerosol sprays sold for wasp or bee management are also effective and are often more convenient than dusts. Although carpenter bees are less aggressive than wasps, female bees provisioning their nests will sting. Treatment is best performed at night or while wearing protective clothing.
The initial appearance of carpenter bees in both the spring and summer is difficult to predict and their activity continues for several weeks. Preventive sprays applied to wood surfaces are effective only for a short period, meaning that you would have to repeat the application about every 2-3 weeks. Since virtually any exposed wood on the house is subject to attack, it is difficult (and usually not practical or safe) to try to protect all of the possible sites where the bees might tunnel. Spraying bees seen hovering around is not a sensible use of pesticides either. Since these bees are not aggressive and they do contribute to the pollination of our plants it would be best to learn to live with them as much as possible.
For more information about this and other landscape topics contact your local Virginia Cooperative Extension office. The local Virginia Cooperative Extension office numbers are Albemarle 872-4580, Fluvanna 591-1950, Greene 985-5236, Louisa 540-967-3422, and Nelson 263-4035.