Daily Archives: July 20, 2008

Construction workers

Carpenter Bee sawdust

My parents’ house is an old century farmhouse, onto which a three-room addition was built. Two rooms are on the same level, but the one at the back is slightly raised, and the roof is at a shallower pitch. Where the two meet there’s a slight overhang, which has been edged with the same trim that runs along the front of the house. For the last few weeks, there have been small piles of sawdust collecting on the shingles under the eaves of the house here. In rainstorms they all get washed away, but in short time they have returned.

There are actually termites in Ontario, and they have been reported from Halton County where my parents currently reside. However, termites tend to be subterranean creatures, building tunnels on the surface of objects (such as walls) when they want to move someplace. While termites might be the creature that immediately jumps to mind at seeing such evidence, sawdust observed so high off the ground and without any apparent tunnels most likely belong to another culprit.

I climbed up onto the roof while I was painting that bit of the exterior wall, and got a second clue. From the bit of trim above the sawdust piles, I could hear very distinct, very loud chewing. Because of the way the roof slopes I could look up and under the eaves and see if I could spot whatever it was, expecting a vertebrate of some sort, but nothing was there. Annoyingly, while I was trying to check things out, and then later as I was painting, a bumblebee kept coming up and hovering around me. I’d back off and let him disappear and then continue on.

Carpenter Bee female

I overlooked the most important clue: the bumblebee. Which, it turned out, was not a bumblebee at all. I came home and looked up bumblebees. They are all generally solitary insects, nesting underground. I had this vague notion of “carpenter bee”; I knew carpenter ants, carpenter beetles… I googled Carpenter Bee anyway. The very first webpage that came up, an information page at Penn State, started out by saying, “People who complain about bumblebees flying about under the eaves of their homes are probably being annoyed by carpenter bees.”

Well. Mystery solved. Here in eastern North America, the species we have are Eastern Carpenter Bees, Xylocopa virginica. They are large, chunky, bumblebee-sized bees that are also marked in black and yellow fuzz. The primary visual difference is that bumblebees always have fuzzy abdomens with a bit of yellow, while carpenter bees have completely black, fuzz-less abdomens. Of course, I didn’t know this at the time. I also think I had at some point learned that bumblebees nested underground, but was forgetting that at the time. Bumblebees will generally have no interest whatsoever with your eaves.

Carpenter Bee female

Carpenter bees, on the other hand, are very fond of eaves. Or really, dead, unpainted wood in general. Carpenter bees, as the name implies, will construct their nest burrows in dead wood. In the wild they would ordinarily do this in snags, dead branches or fallen logs, on the underside or the lee side of the wood. However, they’re not especially picky about the wood they use, and the processed wood we humans use to build our homes and outbuildings, as well as fence posts and boards and other outdoor constructions, will suit them just fine. If you want to make sure the bees don’t target your buildings, make sure they’re all varnished or painted, which generally discourages them.

Females do all the nest-building. The individual in the photo above is a female; she can be told by her longer, pointy abdomen and her all-black face (males have a splash of yellow across their forehead). Females also have larger heads with more widely-spaced compound eyes (those of a male come close to touching at the top). Females use their powerful mandibles to carve out the tunnels. She looks like she might be holding a wood chip from her tunnel in the photo above.

Carpenter Bee holes

They start out with a hole approximately 1 cm (less than half an inch) wide, building straight in to the wood for an inch or so, usually against the grain, and then make a right turn and build the rest out perpendicular to the entry tunnel, this time with the grain. They generally prefer wood that’s at least two inches thick, so the 1×8 boards that formed the trim on the eaves didn’t actually present a very thick tunneling substrate.

The tunnel will end up being 4 to 6 inches (10-15 cm) long when completed, and it takes a female on average about 6 days to complete an inch of tunnel, which means anywhere from 30 to 42 days to do the whole thing. She pretty much spends her whole life focused on that activity, and when she’s done and has laid her eggs, she’ll die.

Once the tunnel is complete, she’ll systematically start filling it with future bees. She’ll begin by forming a ball of pollen and regurgitated nectar and placing it at the end of the tunnel. She’ll lay an egg on the ball, then seal it off into its own compartment using chewed wood pulp to form the barrier (perhaps that’s what she’s carrying that woodchip for in the photo). She’ll form 6 to 8 compartments within the tunnel, each with a single egg. This is a lot of work for just 8 offspring, not all of which will survive to adulthood and reproduction age.

Carpenter Bee nest holes

Carpenter bees are not truly social bees, the way honeybees are; every female has the potential to reproduce, and there’s no queen organizing the group. Each female will build her own tunnel. However, they will often congregate in the same general area. This is partially because a female will tend to reuse the nest she hatched from, or build her new nest nearby. A single board can be targeted and eventually become riddled with nests while other boards may be left untouched. In cases where these holes are excavated in a structural beam they may cause concern, but generally speaking carpenter bee damage is mostly cosmetic.

From the laying of the egg to the emerging of the adult takes about seven weeks, though the timeframe may be longer or shorter depending on local temperatures through that period. The adult will emerge from the chamber where it developed by chewing its way out through the partitions along the long tunnel to the entrance. Adults will generally emerge in mid- to late-August, and will spend the rest of the summer and fall feeding on nectar from flowers (an interesting habit they have is to not always go through the mouth of the flower, but instead sometimes pierce the base of the flower to get at the nectar without pollinating it).

In the fall they’ll return to the tunnel where they hatched from, and they’ll spend the winter there. In the spring they’ll emerge, and the females will start building their tunnels. The males, while the female is hard at work, will patrol the area. Males are harmless, as they have no stinger and cannot sting, but they are curious and may come over to check you out if you wander into their area, which can be very disconcerting. Females do have a stinger and are capable of stinging, but are generally fairly docile, ignoring you if you’re just simply in the area, and will only sting if handled.

Carpenter Bee larvae

When I went up to paint the walls on other side of the house, I discovered this scene underneath the eaves. It looked like the half-developed contents of one of the tunnels had been pulled out by something. Scattered on the shingles were a few chunky larvae, one of which that looked like it was halfway into turning into a pupa, as well as some thick orange chunks. I’m not sure what the orange chunks are, but I suppose them to be the pollen balls that the larval grubs are provisioned with.

I have no idea why these were all on the shingles. If it had been a predator one would think that the grubs would have been eaten, not left to roast on the hot roof. It takes the adult female about a day to provision and seal off each cell, so I was wondering if perhaps the egg that had been laid first had already pupated and emerged as an adult, and as it chewed its way through to exit the tunnel it ended up knocking all the others out. But that seems a bit of a stretch. Maybe a rival female went in and pulled everything out so she could use the tunnel? Also seems a bit of a stretch. I couldn’t find anything online to suggest the reason for it.

Carpenter Bee female

The book Insects, by Stephen Marshall, indicates that Eastern Carpenter Bees used to be more restricted to the United States, but in recent decades have been moving north into southern Ontario. Given their strong site fidelity I’m not sure how quickly they spread, or why they would be moving north. However, this may explain why this is the first year we’ve noticed their presence there. They sure seem to like the spot now that they’ve found it!

Carpenter bees are, like most bees, important pollinators in an ecosystem, and many people will tolerate their presence in their buildings despite the damage because they provide a valuable services to their flowers and trees. Unfortunately, pollinators the world over are in decline due to habitat loss and widespread pesticide use, as well as other pressures. A group called Pollination Canada runs a citizen science project where you can make observations on your local pollinators that will help scientists to better understand what’s going on with this very important group of insects.