Carpenter bees (Xylocopa spp.) are large bees (1/2 to 1 inch long) that are often mistaken for bumble bees. In fact, they share with bumble bees the distinction of being the largest of the United States’ native bees. (There is a genus of small carpenter bees, Ceratina, that are more likely to be confused with sweat bees than bumble bees, due to their small size. This article only covers large carpenter bees.)
Carpenter bees differ from bumble bees in both appearance and behavior. Carpenter bees are less hairy than bumble bees, have a shiny black abdomen, a black head, and a yellow fuzzy thorax with a bald patch in the middle of their back. Male carpenter bees have a white spot on the front of their face. Even though carpenter bees forage for pollen like bumble bees, they do not have a corbicula, or pollen basket in which to carry it. Where bumble bees are social insects, forming colonies of hundreds of individuals, carpenter bees are solitary.
Females nest individually in tunnels they excavate in wood. For this reason, they are sometimes called “wood bees” or “wood boring bees”.
This behavior makes them a nuisance pest around homes. They are loud, they make a mess of wood dust and waste matter, they make holes in thick wooden parts of buildings, and the males are aggressive (though harmless). Here, we’ll learn a little bit about the holes carpenter bees drill, whether they eat the wood that they remove, and the potential damage they can inflict on wooden structures.
Carpenter bee holes
Carpenter bees prefer to bore into soft, unpainted woods such as cedar, cypress, fir, pine, and redwood. However, some species also drill into hardwoods such as oak or eucalyptus. They prefer dead, but not decaying, trees, stumps, and logs. Painted and pressure-treated woods are less attractive to carpenter bees and staining can deter them somewhat. Their holes are most often found within eaves, rafters, wooden roof shingles, fascia boards, siding, wooden decks and railings, outdoor furniture, fence posts, and firewood. Carpenter bee holes are about ½-inch in diameter and almost perfectly round. You could be forgiven for mistaking them for holes drilled by a machine. But behind those rather innocuous-looking holes are often networks of branching tunnels.
How do carpenter bees drill holes? A female carpenter bee will chew at least ½-inch into the wood (against the grain), then make a 90o turn and excavate a long tunnel following the grain of the wood. A single tunnel is typically 6 to 7 inches long but can stretch up to 1 foot in length. One entrance may be used by multiple nesting females, each with her own tunnel off the main branch. Inside each individual tunnel are typically between 6 and 10 cells, each containing a ball of mixed pollen and nectar and an egg or a developing bee larva. Each cell is sealed off with a pulpy mixture of saliva and chewed up wood.
In case you’re not sure if you have some random holes or if you’ve been invaded by carpenter bees, there are indications of carpenter bee activity. Telltale visual signs that a hole is not just a hole are dark yellowish-brown or black stains (bee waste) around the hole and piles of wood dust beneath the hole. You may also be able to hear the bees working on the tunnels. They bore about 1.5 cm into the wood per week. This requires a lot of work and you can hear the muffled buzzing within the structure as the bees furiously chew through the wood. In addition, male carpenter bees often stand guard outside holes, hoping to snag a mate. They will dive bomb any other carpenter bee males, other insects, and even larger animals like people. However, male carpenter bees can’t sting and thus pose no danger to you, your family, or your pets. Female carpenter bees can sting but aren’t aggressive and will only sting if provoked.
Do carpenter bees eat wood?
Do carpenter bees eat wood? The short answer is no. They are not like termites.
While female carpenter bees do use their mandibles (mouthparts) to make tunnels, they discard much of the dust and use the rest to seal the cells in their tunnels.
Instead of feasting on wood, carpenter bees forage for nectar and pollen, like other bees. This makes them important pollinators of a variety of ornamental and food plants. If you’re looking for bees that eat wood, only disappointment is in your future. While bees and wasps do feed on plant matter, wood is not their matter of choice.
Can carpenter bees cause structural damage?
Are carpenter bees harmful to your home or other wooden structures? By itself, a single season of carpenter bee tunneling is unlikely to result in damage to your home or other wooden structure. However, carpenter bee tunnels can attract other insects (e.g., carpenter ants) and allow moisture to infiltrate the wood, potentially leading to fungal colonization and/or rot. Furthermore, carpenter bees prefer to use the same nest site as their mothers, returning to overwinter before enlarging and adding to the tunnels the following spring. Tunnels that have been used for multiple years by multiple female carpenter bees can stretch up to ten feet long. This can significantly weaken the wood.
Carpenter bees are much like other bees in their foraging habits. Though they bore into wooden structures, they don’t eat the wood. Rather, they remove it during the excavation process and use it to build walls inside their brooding tunnels. Even though the initial drilling is unlikely to damage your home, years of repeated nesting can introduce structural weaknesses. Despite the potential threats they pose to homes, if carpenter bees are nesting in disused wooden structures or trees around your home, it’s best to leave them be. Like other bees and wasps, they provide an important service in the form of pollination.