If you have been around the barn, deck, or any exposed wood, you have probably noticed these large bumble bee-looking insects flying around or boring into the wood. These are actually called carpenter bees. They resemble bumble bees, but the upper side of their abdomen is bare and blue-black with green or purple highlights, whereas bumble bees have a hairy abdomen with yellow markings.
They bore into any dry, seasoned wood but prefer softwoods such as pine. They do not feed on the wood but excavate it to make a nest for their young. They make an almost perfectly round hole about one-half inch in diameter and one to two inches deep and create a tunnel parallel to the surface of the wood to store food and lays eggs.
Painted and treated wood are not a favorite, but they are still vulnerable to damage.
The damage done by one bee in one year is usually not significant. However, if used year after year, the tunnel may extend several feet along the timber and begin to weaken it. Similarly, an overabundance of these bees in one piece of wood can cause a problem.
Carpenter bees overwinter as adults (often in old nesting tunnels) and emerge in April and May. Several bees may use one tunnel, but they do construct different galleries for their young. Each female usually lays six to seven eggs. The young hatch in a few days and the adults emerge in late summer. The adults store food in the existing tunnels and return there to hibernate over the winter ready to reinfest the same site the following year.
Control is difficult, even though they are very sensitive to most insecticides. The residual activity of insecticides available to homeowners is short-lived, so making preventative applications does not work.
The best method is to treat the holes as you see them with an appropriate spray or dust. You will not only kill the adults, but you prevent the future generations. Dusts containing carbaryl (Sevin) or aerosol sprays will do a good job.
Existing holes can also be plugged with a mothball to prevent adult bees from returning or the second generation from emerging. Be sure to follow the labeled directions of any product.
Painting the surface may discourage infestation. In addition, covering with vinyl or aluminum will prevent damage.
If you have other questions, please contact your local Extension office.