Carpenter bees are easy to confuse with bumblebees. Behavior and appearance are two characteristics that can be used to tell them apart.
Carpenter bees have a shiny, iridescent, hairless abdomen that may reflect blue, blue-black, purple, green, or bronze colors. Bumblebees are not shiny at all and may, in fact, look furry because they are covered all over with hair.
Male carpenter bees have a dramatic, zigzag flight and aggressively defend their territory. They are very noisy when they fly, wings humming or hovering and they may even seem to threaten humans. It’s like the dog that is all bark and no bite: the male carpenter bee cannot sting, only intimidate! Female carpenter bees usually concentrate on stocking the nest with food but can sting quite badly if they are handled. In contrast, bumblebees do not hover for long periods, they seem relatively cautious around humans, and keep busy foraging for pollen and nectar.
In nature carpenter bees nest in dead, but sound parts of living trees. However, many homes and decks are built of rough, unfinished wood that serves the same purpose as a tree. I have seen carpenter bees nesting in split rail fences, and roof overhangs seem to be a favorite spot, too. The female carpenter bee bores a perfectly round 3/8 – ½” diameter entrance hole, then turns at a right angle and makes a tunnel several inches long. They do not feed on the wood, they only use the tunnel to raise their young, so you may see the sawdust that is pushed out of the hole. The female lays her eggs in the tunnel and then places pollen and nectar in the nest for the grubs to eat after they hatch. One female can make several nest tunnels in one season.
The damage done by carpenter bees is usually only cosmetic in nature. Woodpeckers can cause serious damage by destroying sections of wood with a nest in it looking for grubs. Smooth, painted, finished structures are usually not attractive to carpenter bees so you can prevent damage by applying heavy coats of paint or varnish to wood. If you find tunnels made by carpenter bees you can plug the entrance with a mothball to kill the grubs and then use caulk or wood putty to smooth the surface. Do this after sunset when the adult bees have returned to the tunnel and you will kill them as well.