Large Bees Active Around Buildings

Timothy J. Gibb, Insect Diagnostician, Department of Entomology, Purdue University

Many complaints are currently being forwarded to the laboratory concerning “large bumble bee look-a-likes that bore into wooden structures.”

These are carpenter bees and are very active during late April and throughout May in our area.  They can be distinguished from bumble bees by their nearly all black and a much more naked (hairless) abdomen. Carpenter bees are often noticed working around a nearly perfectly round entry hole in wood (pine, cedar, cypress) that is about 1/2 inch in diameter. The hole goes straight into the wood for about 1-2 inches, then makes a 90 degree turn and runs with the wood grain for some 4-6 inches. The female bee fills 6-8 cells (separated from one another by partitions of wood pulp) with pollen and nectar and an egg.   She then seals the tunnel and soon dies.  Larvae inside the cells hatch, feed on the store of pollen and mature by late August.  New adults emerge by early September. These adults forage for nectar but eventually reenter the tunnel (or a nearby one), clean it, and then pass the winter within it.

The cycle starts anew the following spring. Note that carpenter bees may refurbish an existing tunnel instead of boring a new one but new tunnels are often constructed near old ones.  This means that that an infestation may persist for several years or more in the same general location.

Individually, the carpenter bee actually causes little serious damage, however, continuous tunneling in wood over several years or by many bees may weaken the structure. Perhaps the biggest problem is the annoyance and fear associated with the large carpenter bee. Males patrol the area and often fly about the faces of people; however, they cannot sting. The females do not defend their nest, but will sting if mishandled.

Carpenter bees are less attracted to stained, varnished painted or pressure treated wood, than they are to seasoned bare wood.  Control of existing infestations can be safely and effectively accomplished by dusting into the tunnels with 5% Sevin, leaving them open for a day or so, then plugging the opening with a dowel or wood putty to prevent future use.  Pyrethroids applied to the outside surface of the structure, where the bees may land, serves the same purpose and may offer slightly longer residual control.

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Photo courtesy of John Obermeyer