Pretty much everyone, I expect, has at least a nodding acquaintance with our large carpenter bees. Our sole species, Xylocopa virginica, the eastern carpenter bee, is a conspicuous beast, resembling a very large bumblebee. Females, famously, chew extensive nest burrows in soft, straight-grained wood, including trim pieces, siding, and the like on human structures. Male Xylocopa, marked by a white patch on the face, are less industrious but are well known for strong territorial instincts that often lead these bees to fly right up into your face.
Don’t worry: The males, like the males of all bees and wasps, are stingless and utterly harmless. And the females, despite their daunting size, are docile and unlikely to sting. Both sexes can be distinguished from bumblebees by their shiny, hairless faces and largely hairless abdomens.
Much less well known are the smaller relatives of Xylocopa, the aptly named small carpenter bees of the genus Ceratina. On a casual glance, you’d never imagine a close relationship between these two genera. Dull metallic bluish-green in color and never more than about three-eighths of an inch in length, the Vineyard’s four Ceratina species could hardly be more different from their massive relatives. But taxonomists put them in the same subfamily, making them cousins if not siblings.
Recognizing Ceratina becomes easy with practice but poses some initial challenges. We have several other groups of bees that resemble small carpenter bees. One group, the so-called metallic sweat bees, run a bit larger than Ceratina and are truly bright, metallic green in color, quite different from the muted green of Ceratina. At the other extreme, generally somewhat more blackish in color than Ceratina, the many species of the large, difficult subgenus Dialictus can be a source of confusion.
Small carpenter bees, though, have a distinctive shape, with blunt, club-shaped abdomens. Distinctive, if difficult to see under field conditions, the end of a Ceratina abdomen sports a tiny projecting point or nipple — a distinctive feature when visible. Ceratina is also much less hairy than other genera it can be confused with. I feel there is even a distinctive Ceratina posture, with the head held low and extended forward.
It’s under magnification that Ceratina really shines, and I mean that literally: These bees are exquisite under a dissecting microscope, their exoskeletons highly polished and ornately sculpted with tiny pits. These pits, a feature that adds strength and stiffness to the exoskeleton, are universal among bees (and many other types of insects); the size, spacing, and distribution of these pits are often critical traits for identification. The pitting rarely shows as more than a general impression of roughness or frostiness to the naked eye, which is one reason why either a specimen or a series of very sharp close-up photos is usually necessary to identify a bee to species level.
True to their carpenter bee nature, female Ceratina chew nest burrows. But being smaller and less muscular than their larger relatives, Ceratina bees typically tunnel in very soft substrate, such as the pithy core of thick-stemmed herbaceous plants. They never damage human structures. Seen under magnification, the mandibles, or external jaws, of all our female Ceratinas, are broadened into a three-pronged tip. It looks like a chisel a carpenter might invent for quickly removing very soft material — and indeed, that’s exactly what it is.
When not excavating nests or pursuing mating opportunities, small carpenter bees of both sexes visit a wide range of flowers for nectar or pollen. At this time of year, dandelion flowers are a good place to look for Ceratina. Being less hairy than many bees, Ceratina is probably not especially efficient at pollination. But females do have elongated, pollen-carrying hair (called “scopa”) on their hind tibia, and a certain amount of pollen invariably sticks to the surface of both sexes as they forage.
As a genus, Ceratina is on the wing from early April until about mid-October. The genus as a whole is common and widespread on the Vineyard. At the species level, though, things may be more complex. Based on a 2010-2011 study of Vineyard bees, we have four species of Ceratina. Two — C. strenua and C. calcarata — appear to be quite common; the other two, C. dupla and C. mikmaqi, appear to be scarce or even, in the case of mikmaqi, downright rare.
Given their role as pollinators, their harmless nature, and the possibility that one or more of our species may be of at least local conservation concern, it’s not a bad plan to look for ways to encourage our Ceratina bees. Deliberately growing flowers or shrubs with thick, pithy stems will provide potential nest sites for these bees; leave the stems standing or at least intact over the winter, since the immature bees are lying inside, waiting for spring. Planting a season-long array of native wildflowers, or at least exotic ornamentals that attract pollinators, will help feed adult Ceratina and provide females with the pollen they need to provision their nests and ensure the survival of their offspring.