Purdue Extension Service

Vanderburgh County, Indiana

Carpenter Bees Hammering Our Structures

By Larry Caplan, Extension Horticulture Educator, Vanderburgh County, IN

For the Evansville Courier and Press, June 20, 2008

Many tri-state residents have been seeing large, black bees hovering around and drilling holes into homes, out-buildings, wooden furniture and decks. These are carpenter bees, which first glance look like bumble bees. The difference is that bumble bees have a hairy and often yellowish abdomen, while the carpenter bee has a shiny black abdomen.

Carpenter bees are troublesome because the females will bore into wood to build tunnels for egg laying and protection from the winter. In nature, the carpenter bee will build her nest in dead trees and logs, but in urban landscapes, they will attack our wooden structures. They prefer bare, unpainted and weathered softwoods including redwood, cedar, cypress and pine. Harder woods and those pressure treated or painted are less attractive to them.

The tunnel entrance is usually a perfectly round hole, about one-half inch wide. The tunnel penetrates into the wood one-half to 1 inch across the grain of the wood and then turns at a right angle to follow the wood grain for 6 to 8 inches. The bee will create individual cells using bits of sawdust along the length of the tunnel. Each cell is provisioned with a pollen ball into which she will lay an individual egg before sealing it off. As the eggs hatch in midsummer, the larvae feed on the pollen. They mature and emerge as adults in the fall. Fall emerged bees can either create new holes or enlarge holes made previously in order to pass the winter within them. Carpenter bees then emerge again in April and May to repeat this cycle.

The male bee will often patrol the area where the female is making her nest. He will fly aggressively at intruders, but it's all for show: male bees and wasps don't have stingers. Female carpenter bees can sting, but they rarely defend their nests, so they are really mostly dangerous if you try to grab one.

The damage done by the tunneling is mostly cosmetic, but many years of bee attack can weaken the wood. Occasionally, woodpeckers may go after the bee larvae, making the damage even worse. If the damage is severe, you may want to try controlling the carpenter bees.

Using hardwoods that are less attractive to carpenter bees or making woods less attractive by painting, pressure treating or varnishing is the best preventative control. Liquid sprays containing carbaryl (Sevin) or synthetic pyrethroids (such as cyfluthrin or permethrin) can be applied to wood surfaces so that bees die upon landing or boring there. Hovering bees can be killed by using aerosol sprays labeled for wasp or bee control. Be sure to read and follow all label directions.

Once holes are drilled, an insecticide powder or dust (carbaryl, boric acid or diatomaceous earth) can be puffed into the tunnel opening. The tunnels must be left open for a few days to allow the returning bees to contact the insecticide. Afterwards, the holes should be plugged using caulking, a short dowel or wood putty and then repainted to discourage future use.

For more information on preventing insect damage around the home, contact the Purdue Extension Service at 812-435-5287.