Carpenter bees are large, yellow and black (or blue-black) bees that become active in early spring. This bee is commonly 2/3 to 1 inch long, usually with a shiny abdomen and a yellow thorax. Its look-alike cousins (bumble bees) have a fuzzy abdomen. Although it is rare to be stung by one, their sheer size is scary and people generally stay clear of them. Bumble bees do not bore into wood, as do carpenter bees. Male carpenter bees spend most of their time flying around the nest playing guard. This is ironic as nature has left him with no stinger. The female can sting, but she is normally very docile.
Carpenter bees get their name from their ability to drill through wood and nest in a single hole. They do not eat the wood. A single pair (male and female) occupies each nest. Female carpenter bees use their strong jaws (mandibles) creating a near-perfect hole, approximately 1/2 inch in diameter, around the diameter of her body. She excavates the gallery at the rate of about one inch in six days. Gallery construction is a time- and energy-consuming process, and the female will preferentially refurbish an old nest rather than excavate a new one. Although the hole appears to be only an inch or two deep, it rarely ends there. The female carpenter bee will turn 90 degrees and bore a channel from 6 inches to as long as 4 feet. (Note: For this reason simply spraying inside the hole doesn’t always do the trick.)
This channel serves as a main corridor from which she will drill small chambers a few inches deep. These chambers become egg holders. She will deposit an egg, bring in a portion of “bee bread”, a mixture of pollen and regurgitated nectar, which serves as food for newly hatched larvae to feed on. She will then seal it all off to ensure its development before she repeats the process for the next egg. The larvae that hatch from the eggs complete their development and pupate. Newly developed adult carpenter bees emerge in August, feed on nectar and return to the tunnels over winter. They mate in the spring and set about to clean out and enlarge the old tunnels or to excavate new ones as brood chambers for their young.
Here in Port Huron, Michigan we find these holes located underside wood surfaces; including siding, soffits, decks, overhangs, fence posts and window frames. Carpenter bees attack bare, unpainted or weathered soft woods. Common woods we find them in are redwood, cedar, and pine. They prefer to nest in woods greater than two inches thick. It is not uncommon to find several pairs of carpenter bees nesting in one structure. They frequently nest near each other and often in the same area year after year, causing extensive damage. You may find old holes near newer ones.
After proper treatment you want to seal the hole for a couple reasons:
You don’t want other carpenter bee females to continue to reuse the hole year after year.
When you have one hole, it attracts other carpenter bees to use the same location.
We’ve found simply mudding over the hole isn’t enough at times. Female carpenter bees will drill easily through most mud and wood puddy.