Variances in Appearance
Carpenter bees, species in the genus Xylocopa, and bumblebees, members of the genus Bombus, are both large, fuzzy bees, and they are often mistaken for one another in a garden. The easiest way to tell the difference between a carpenter bee and a bumblebee is to look at the bee's abdomen. The carpenter bee's abdomen is shiny, black and devoid of hairs on its upper surface. A bumblebee's abdomen is hairy and usually has yellow markings.
Carpenter bees get their name from their nesting behavior; they lay their eggs in tunnels that they bore in wood by using their strong jaws. They are attracted to unpainted soft woods, and they often nest in building eaves, fascia boards, soffits, siding and decks. Carpenter bees aren't social and don't form colonies, although several bees may share the same nest entrance hole.
Bumblebees usually nest in the ground, often in abandoned rodent burrows and bird nests. Unlike carpenter bees, bumblebees form colonies consisting of an egg-laying queen, infertile female workers and male bees.
Aggressiveness and Stinging
Male carpenter bees can be quite aggressive toward humans who get too close to their nests; they will fly toward people and hover nearby. Male bees, however, lack stingers. So although they attempt to guard nests, they are unable to cause any real harm. Female carpenter bees, however, have stingers, but the females are not aggressive and usually do not sting unless handled or otherwise overtly threatened.
Bumblebees also are not usually aggressive when they are away from their nest, but female worker bumblebees will attack and sting when their nest is threatened.
Value in the Garden
Bumblebees are among the most active and effective pollinators of flowering plants. So they are a beneficial addition to a garden. Given their limited aggressiveness, which is generally outweighed by their usefulness, bumblebees usually are allowed to nest in a garden unless their activity becomes a serious threat to humans.
Carpenter bees also function as pollinators, but their destructive behavior makes them less of an obvious benefit, on the whole, in a garden. They rarely cause serious structural damage, but when they nest in the same spot year after year, the cumulative damage may become significant, and their nesting should be discouraged. Because carpenter bees prefer to nest in bare wood, the simplest way to discourage them from nesting is to paint all wood surfaces.