Carpenter bees resemble bumble bees but have a shiny black, smooth abdomen; the abdomens of bumble bees are “hairy” and often marked with yellow. Carpenter bees are useful pollinators but their tunneling in soft structural wood can cause significant damage. Also, their intimidating appearance and buzzing flight can be unsettling to be around.
Carpenter bees are solitary insects, living in individual tunnels. However, large numbers can build up in suitable nesting material – weathered soft wood. Females use their strong mandibles to chew 1/2-inch diameter entry holes into wood; the tunnel follows the wood grain. The length increases about 1 inch per week. Ultimately, it can be 6 to 10 inches long and can contain 6 or 7 individual larval cells. Each is provisioned with a ball of nectar and pollen as food for the grub-like larva. Over the years, galleries may become several feet long.
Carpenter bee control is not easy so prevention is the best long term strategy. Use of hardwoods, when practical, or covering softwoods with flashing or screen will prevent injury to areas that are chronically attacked. General maintenance helps because carpenter bees exploit rough surfaces to begin a nest. Filling cracks and crevices and painting or varnishing exposed wood will make wood less attractive. Stains will not deter the bees.
There are some insecticide options but accessibility and dimensions of infested surfaces can make treatment impractical or limit its success. The use of dust formulations of insecticides, applied directly into tunnel openings, has been the favored option. In this approach, bees are exposed to the dust as they enter and leave. Ultimately, they should receive a lethal dose.
Insecticide sprays can be applied into tunnels but pick up of the dried residue may not be as rapid as with dusts. Insecticide applications to wood may provide some preventive effect but bees are not ingesting the wood, only gouging it away and can work quickly though the treated surface. After treatment, tunnel entries should be filled and sealed so they are not attractive to bees next season.
Carpenter bee traps are an option to explore. To be effective, they should be out early when nests are being developed.