Carpenter Bees: The (Harmless!) Garden Pollinators

Carpenter Bees, Both Large and Small

Most gardeners are probably familiar with large carpenter bees (Xylocopa), which are quite distinctive in appearance. Most appear to be black, bald bumble bees, quite shiny in appearance. Much less familiar are the small carpenter bees (Ceratina) that appear nothing at all like their giant cousins. Currently carpenter bees are placed as the most primitive members of the family, though their exact position relative to other bees is not yet writ in stone.

Beginning at the less familiar and smaller end of the duo, there are more than 300 small carpenter bees in the world, with 71 North American species (about 20 species in the United States and Canada). They range in size from 1/8 to 1/2 inch (3 to 12 mm), appearing black with a metallic blue or green sheen. Females of some species have a yellow stripe in the middle of the lower half of the face. All nest in pithy stems of woody plants, which must be broken so that the ends are exposed. The female bee chews into the stem’s pithy interior, extruding bits of plant tissue (sawdust) as she progresses. The cells are placed in a linear fashion, one atop the other, and are unlined, containing no gathered materials such as leaves (as in leafcutter bees) or secreted substances (as in plasterer bees). The partitions between cells are made from tightly packed, chewed pith.

More than 400 species of large carpenter bees are known throughout much of the warmer regions of the world, but the biology of most is unknown. An Asian carpenter bee is credited with laying the largest egg of any insect, some 5/8 inch (15 mm) in length and nearly 1/8 inch (3 mm) in diameter (Vicidomini 2005). In North America there are 39 species, only seven of which occur in the United States, and so far none have been reported from Canada. Most occur in Mexico. Large carpenter bees are about 1 inch (25 mm) in length, the females being entirely black or black with metallic blue or green tinges. In some species the male has a yellow face, and in one species from the American Southwest males are entirely covered with straw-yellow hairs, but the body color is actually dark as in the female. In another species from the southern United States, the males have pale reddish orange hairs on the front half of the body.

In our gardens, these bees are often noticed because of the males, which are territorial and appear to be attacking us defenseless humans face to face. In fact they can do no harm and are basically bluffing their way through life when it comes to confronting large, blundering animals. Unlike small carpenter bees that nest in relatively narrow twigs, most large carpenter bees need a substantial piece of wood in which to bore. Many species nest in solid wood, stumps, logs, or dead trees, but a few adopt the habits of their smaller cousins and nest in plant stems. These have to be stems of a sizeable nature, such as old agave stalks. Unfortunately for humans, they do not object to using lumber that might be associated with our houses, car ports, sheds, or barns. Apparently carpenter bees won’t burrow into painted wood, but unpainted wood is just fine. They will even burrow into chemically treated lumber, though this is not likely their first choice. Carpenter bees do not eat the wood, but chew it, gradually pushing the sawdust-like material out the nest entrance or using it to form partitions between cells.

In the United States, the common eastern carpenter bee (Xylocopa virginica) is perhaps the best studied of the carpenter bees. Females bore into wood, hollowing out a channel about 1/2 inch (12 mm) in diameter and 12 inches (30 cm) or more in length. They burrow with the wood’s grain, but to gain access to the grain they may first have to burrow perpendicular to it. For example in a 4- by 4-inch (10- by 10-cm) pressure-treated post, I have seen them bore through the side, then make a right-angled turn to proceed with the grain. I’ve also seen them bore into 2- by 12-inch (5- by 30-cm) wood from the side. Obviously the female must make a right-angled turn before she bores through or she would pop out the other side of the board. She likely uses some change in sound to know when to initiate the turn. At the end of the burrow the female places a pollen ball and regurgitated nectar, upon which an egg is laid. A space is left for development of the egg to adulthood, and a partition cap made of chewed wood is placed over the cell. The cap then acts as the base for the next load of pollen and an egg. This is repeated until 6 to 10 end-to-end cells are completed. Females complete but a single nest in the north, but where it’s warmer, several separate nests may be built. Carpenter bees overwinter as pupae or adults (accounts vary) within their nests, emerging in the spring to mate. Nests are reused from year to year by different generations, and new nests are built by an ever-increasing population of bees. Eventually an aggregation of many nests may be found. A friend of mine has a perennial aggregation nesting site in the post of a wooden railroad wigwag that he’s set up in his yard.

Some carpenter bees in other regions of the world are reported to be somewhat social in that several females will use the same nest, one or more females will act as guard bees at the entrance, and there is some overlap of generations. This behavior is not yet known in North American species.

What’s the Buzz? Read more about bee families in All About Bees: The Great Garden Pollinators.

Reprinted with permission from Bees, Wasps, and Ants by Eric Grissell and published by Timber Press, Inc, 2010. Buy this book from our store:Bees, Wasps, and Ants