["Stinging" Insects] The .45 Caliber Bee

True story. Several years ago at a famous Washington, D.C., landmark, Secret Service agents discovered evidence that somebody had fired a .45 caliber weapon into the wooden framework around a window. How could this have happened undetected at one of the world’s most secure buildings? Was a silencer involved? But before the CSI guys arrived to dig out the bullet, a pest-savvy building manager informed officers they were actually looking at a fresh nest hole made by the common eastern carpenter bee, Xylocopa virginica.
I know what you’re thinking. How could a bee hole, although undeniably circular, fool experienced weapons experts? So I did exactly what you would do. I went out and carefully measured a sample of 10 Xylocopa entrance holes to see how close to .45 caliber (.45 inch diameter) they really were. And what do you know, they ranged from .40 to .45 (four examples) with a mean of .43, barely distinguishable from one another. Impressive, no? The technical evolution of man and the biological evolution of bee are inextricably linked.
Over the years I’ve been called upon to answer a variety of similarly intriguing questions — perhaps not as earth-shaking as the one above — about our bold and burly friend the carpenter bee, excavator extraordinaire and dive-bombing terror of suburban decks throughout the eastern and central United States. Here are a few of my favorites.

MAKE MINE MILLED. The first question is another one of those “inextricably linked” issues, and might be phrased: How is the eastern carpenter bee like the beloved purple martin, specifically flocks of these birds east of the Rockies? The answer is that both are almost totally dependent on human beings for their nests. The case of the purple martins is slightly more extreme. Originally nesting in tree cavities but gradually shifting over to man-made structures first provided by Native Americans, the birds no longer use natural hollows. They won’t even use artificial nest houses that are too far from human habitations. This is one of the best examples of animal synanthropism (“living with man”) that I know of. The martins’ dinosaur ancestors must be spinning in their graves.
There’s a lot more carpenter bees out there than martins and a lot more potential nesting sites for them, but how many times have you seen X. virginica using natural (non-milled) wood? I run across an instance now and again, the most memorable of which was the bald eagle perch (a huge dead snag) in the Indianapolis zoo. The eagle was clearly finding those pesky, territorial male bees to be just as much of a pain as everybody else does. But even this borderline example (which, after all, was still in an urban area) is very much the exception. When we finally wise up and build our houses out of something else besides wood — the ultimate IPM solution for termites, by the way — I believe the eastern carpenter bee will revert back to the much less common species it must have been before Europeans arrived in North America.
A “STINGING” INSECT? Most pest management professionals I know routinely give the same message to Xylocopa clients that they do for their customers with cicada killer wasps — yes, they look tough, but almost every one you see is a male. Male bee and wasps don’t have a sting. You’ve been fooled by a humble bug. See that carpenter bee now, hovering in front of your nose, with the yellow face and big, goggle-eyes?
Male. Harmless. Yes, I take a Visa card.
I’d like to go even further, and suggest that entirely too many references in the literature treat the sting-packing female with just a little too much wary respect. You know, the ones that say they “may,” “seldom” or “rarely” sting unless disturbed. I believe it’s more accurate to say they “will not sting, unless handled.” A number of entomologists have apparently not been able to resist doing just that, my favorite being the indomitable Phil Rau (The Jungle Bees and Wasps of Barro Colorado Island, 1933), who dutifully reported that “they can administer several stings in succession and the last is as intense as the first.” Noted, sir. Nevertheless, I don’t believe anybody in the history of the world has ever been attacked by a female Xylocopa under any circumstances. If you have a tale to the contrary, I’d love to hear from you.

THE SINGLE ROW PROBLEM. The female carpenter bee bores into wood perpendicular to the grain for about the length of her body, then makes a right angle turn and industriously mines the shaft of her nest parallel to the grain. When she’s decided her mandibles have had just about enough, she provisions the furthest point from the entrance hole with a dough-like lump of “bee bread” made out of nectar and pollen, and lays an egg on it. She then seals off this chamber with a thin wall made from wood pulp, and repeats the process. Eventually a tunnel is partitioned into as many as eight adjacent cells.
Now think about it. You’re the first-born grub at the very end of the tunnel, a week to 10 days older than your youngest sibling. You finish your bee bread first, pupate first, and presumably turn into an adult first. But then what? Your path to freedom is blocked by all of your brothers and sisters. Are you going to just sit there twiddling your tarsi while the rest of the brood takes their sweet time to mature? The paradox of the oldest bee, apparently trapped in the most remote spot of a single row of cells, took more than 200 years for entomologists to solve. As it turns out, even though there is room in the tunnel to scoot by each other, emergence of adult bees from pupae is nearly synchronized in the nest, regardless of age. The actual mechanism, which causes the younger bees to develop faster, is still unknown. In addition, the new adults typically don’t leave their tunnel anyway for the first two to three weeks after emergence.

GIANT RESIN BEE. A technician recently asked me whether there were two types of bees that bored into solid wood in our area. He was seeing what appeared to be “baby carpenter bees” hovering around and flying into what looked like normal Xylocopa holes.
In fact, if you had the time and inclination to spend your days observing carpenter bee colonies, you’d see a lot of interesting stuff using their burrows, either as parasites or simply taking advantage of vacant tunnels to rear their own young. In the past few years, one of the most conspicuous of these opportunists is a species accidentally introduced from Asia called the “giant resin bee” (Megachile sculpturalis). Sporting a similar color pattern to X. virginica, the largest female resin bees come fairly close to them in size; the males are always much smaller (see Figure 1 on page 84). These solitary bees do not excavate their own tunnels, and like the carpenter bee, are not a stinging hazard unless handled. Their common name refers to their habit of collecting tree sap and resin to seal the cells in which the brood are reared. At this point, their status seems to be mainly that of a beneficial pollinator.

IS PREVENTION POSSIBLE? Or, stated another way, what’s the IPM approach for carpenter bees, assuming replacement of wood with something non-edible isn’t an option? Call me cynical, but from my observations I’ll bet the most common type of carpenter bee job consists of nothing more than hoisting up a can of whatever pyrethroid is available on an extendable pole and hosing down the surfaces near the holes. This is repeated as necessary, the technician returns every year, and the bees mysteriously keep on making headway. Would you call this a best practice?
On the other hand, Xylocopa has proved amazingly adept at munching through a wide variety of wood treatments and preservatives. The standard recommendation has been to cover all exposed wood with a thick coating of paint, which is generally effective. But paint just isn’t going to happen with a lot of wood, and it surely doesn’t happen on the inner lip of most fascia boards, a ubiquitous architectural element specifically designed to promote healthy carpenter bee populations. The old type of pressure treated lumber (soaked with chromated copper arsenate) was protected, but that’s now been phased out for new residential construction. And in case you haven’t noticed, no current borate product makes the claim that carpenter bees are affected by it.
I don’t have a problem with surgically injecting a pesticide into the tunnels. That’s part of what I do when I treat for them. But the longest lasting solution involves a barrier, not a poison. There is no more devastating blow that you can strike against a X. virginica colony than to deny them access to their most critical resource — their burrows. It is no exaggeration to say that this species literally spends almost all of its life in a tunnel, either the one in which it developed or one nearby. Even after maturing into adults, the bees venture outside only rarely throughout the rest of the summer and fall, spending most of their time huddled in the dark with their siblings. And those tunnels are the only place where they can survive the winter.
Seal up their mines and the bees are toast. Even for this highly specialized insect, digging into solid wood to create new shafts is tough work, requiring lots of time and energy. Many nest starts are failures. So like numerous other burrowing animals, female Xylocopa prefer to refurbish old nests rather than start new ones or disperse to new sites. The biggest, most sustainable carpenter bee colonies, sometimes persisting for decades, are always where there is a pre-existing burrow system. In these circumstances, the tunnels become precious resources that are guarded and fought over by nesting females (see Figure 2 on page 86). Even when the surrounding wood becomes riddled by woodpeckers searching for the developing bees (see Figure 3 at left) — always a treat for the homeowner — the females often will continue to slip in through the ragged gaps to access the tunnels that remain intact.
I’ll grant that finding and neatly injecting caulk into Xylocopa holes way up in the eaves and soffit is painstaking, labor-intensive and not for anyone with a fear of heights. Bill accordingly. By the way, contrary to what you may have heard, this can be done at any time of year — constrained by their behavior, adult bees trapped inside will not chew their way out. It’s annoying that even this solution isn’t permanent, since females randomly starting new tunnels from scratch will eventually strike it rich and break into the “abandoned mines.” In this regard, I’ve long wished for some clever entrepreneur to come up with a way to shoot in some pressurized sealant that will totally fill the burrow system. Now that would be slick. Anybody out there up to the challenge?

All photos are © Al Greene/Nancy L. Breisch

The author gratefully acknowledges Dr. Nancy Breisch for working with him on more carpenter bee jobs than he cares to remember. The opinions expressed herein are the views of the author and do not necessarily reflect the official policy or position of the U.S. General Services Administration.

The author is regional entomologist for the U.S. GSA, Washington, D.C. He can be reached via e-mail at agreene@giemedia.com.