Carpenter bees are large, wood-boring insects that burrow into sound wood such as rafters, deck joists, fascia boards and other exposed bare wood. Carpenter bees are beneficial as pollinators. However, the tunnels made in the wood of structures is annoying and potentially damaging. Wood that has been used for nesting year after year may be structurally damaged.
Carpenter bees look like bumble bees. Both are three-fourths to 1 inch long. Carpenter bees have a bare, shiny-black abdomen, while bumble bees have a hairy abdomen with at least some yellow markings. The behavior of the two bees is much different. Bumble bees are social (large colonies living together) and live in the ground or in existing cavities. Carpenter bees are solitary and nest by drilling holes into exposed, bare wood.
Female carpenter bees tunnel into the surface of bare wood through a perfectly round hole measuring about ½ inch in diameter. The tunnel goes straight into the wood a short distance and then makes a 90-degree turn and follows the grain of the wood for several inches. Bees prefer lumber that is more than 2 inches thick. Old tunnels may be enlarged and reused and it is not unusual for the bee to excavate several tunnels from a single opening.
Carpenter bees are often discovered by the large quantity of coarse sawdust that falls from the nest openings, or homeowners may hear burrowing sounds within the wood. The bees may be noticed hovering around the outside of homes, barns and sheds in late-spring to early summer as the bees search for mates and favorable nesting. Male carpenter bees appear aggressive as they hover around the nests and make a show of attempting to defend their "territory." However, males do not have stingers and are harmless. Female carpenter bees can inflict a painful sting but seldom will unless they are handled.
Inside the tunnel the female places a mixture of pollen and nectar. She lays an egg on this "bee bread" and builds a partition in the tunnel with cemented wood chips. She continues this process until 6 to 10 cells are constructed. The larvae feed on the nectar and pollen and develop into adults by late summer. After a brief period of feeding in August or September, the bees return to the tunnels to spend the winter. Activity resumes in April or May when the process of producing the next generation is repeated.
Bare, unpainted or weathered softwoods are preferred, especially redwood, cedar, cypress and pine. Painted or pressure-treated wood is much less susceptible to attack and a coat of paint or a heavy stain will protect wood from further attack. Individual carpenter bee nests can be treated with insecticide dust or with "wasp and hornet" aerosol spray. Treat at night to avoid possible stings. You may choose to hire a certified pest management professional to provide this service. Do not plug the nest entrance after treatment if the bees are active. Holes can be filled with wood putty or wooden dowels in the fall and the entire wood surface painted or varnished.
Carpenter bees were considered to be a "southern" insect. However, incidence and distribution of carpenter bees in Iowa has steadily increased over the past 30 years. The map above shows the reported distribution of carpenter bees in Iowa as of June, 2020. If you see or hear of carpenter bees in your county, please drop us a line (email address: insects @iastate.edu)
For more information on carpenter bees see University of Kentucky Extension EntFact # 611.