Unboring boring beetles

Female White-spotted Sawyer - Monochamus scutellatus

A few days ago I had a great string of beetle sightings within about 24 hours. It began with the beetle shown above. We had a couple of friends from back in Toronto for a visit, dropping in en route to their cottage north of here. We’d set up some chairs on the deck, a nicer place to sit at this time of year than indoors, and were chatting when Dan noticed a large beetle walking along the deck railing. He pointed it out to me and I hurried to get my camera. It didn’t sit still, but I was still able to run off a few passable shots of it before it paused, spread its elytra, and flew away.

I was quite enamoured with it, as it was the first long-horned beetle I can recall seeing (that’s not to say there weren’t perhaps earlier ones, just that I don’t remember them if there were). These beetles are quite large, by beetle standards. This one was probably over an inch long in body, with the antennae being the same length again, perhaps 2.5″ (5cm) long overall. It was interestingly patterned, patchy black and white. I didn’t get a chance to look it up right away, so I didn’t know its name or any of its ecology.

Female White-spotted Sawyer - Monochamus scutellatus

The following day we were out at Rock Ridge. The morning had been rather slow as far as the birds went, and we were feeling tempted to call it a bust. Nine birds captured in six hours wasn’t much to keep one stimulated, particularly when your alarm had gone off at 3:15 am after just 4.5 hours of sleep. On my penultimate net check, an hour left to go in the morning, I discovered this beetle in one of the nets. Another longhorn! Well, considering that was the only thing I’d pulled out of the net over the past couple of hours I was feeling somewhat buoyed by the discovery. I carefully tucked the beetle into a cloth bag and took it back with me to photograph. I wasn’t sure if this one was the same species as the previous day’s or not – on the one hand, it looked more mottled and brownish than the first, but on the other, some species of insect can be quite variable. Only one way to know – photograph it and look it up.

Male White-spotted Sawyer - Monochamus scutellatus

On the final net round, as I was starting to close up the nets, I discovered this third longhorn hanging from the fine mesh. Was I stoked! This one looked like it was a different species, too, all black with a bold white spot at the base of the elytra and a bold red thorax. Its antenna were markedly longer than the first couple I’d found; in this photo the spread from one tip to the other might be as long as my open palm. It was quickly slipped into another cloth bag and secured to my belt while I finished closing. Interestingly, this one squeaked in protest at its confinement, a noise one doesn’t often associate with beetles.

I took photos of them both and brought them home to compare to the previous day’s beetle and look up an identification. It didn’t take long to find it in my Kaufman Insects: White-spotted Sawyer, Monochamus scutellatus. I was a bit disappointed to learn that they weren’t three species after all but just one. However, the difference in appearance, it turned out, was due to the sex of the beetles. The first two I’d got, with the shorter antennae and mottled patterning, were both females. The shiny black one with the extra long antennae was a male. The white spot referred to in the name is actually present on all of them, but blends in with the other mottling on the females.

Male White-spotted Sawyer - Monochamus scutellatus

The red collar on the male turned out not to be part of his colouration at all, but was in fact a thick band of mites. I only saw one other reference to this online, another photo of a White-spotted Sawyer, its thorax thick with mites, posted to BugGuide.net. No explanation of what the mites were doing was present, and it’s possible that it’s not really known.

The species is a borer of dead and dying conifers. Around here it would primarily target White Pine, though spruce and balsam fir are also favourites where they’re present. At Rock Ridge there is plenty of White Pine, it’s one of the dominant tree species, but around our home there’s virtually none except for a bit on the far side of the lake from us. They spend about ten days to two weeks feeding on the soft new growth of conifer twigs before mating. Then the female beetles will search out some appropriate wood (they’re decent fliers, which explains their presence in the nets) and lay her eggs in the crevices of the bark. When the eggs hatch, the larvae bore through the outer layers of the bark into the softer inner layer, the cambium. There they spend the rest of the summer feeding, then gradually tunnel in to the heartwood to spend the winter. The following spring they return to the surface layers where they finish their growing. In warmer climates the larvae may pupate and emerge as adults that summer. In cooler climates, such as ours, they may require a second winter as a larvae, finally emerging as an adult two years after the egg was laid.

(The squeaking, it turns out, is made by the beetles as they rock their head from side to side, rubbing tiny ridges on the inside surfaces of their thorax. I’m not sure if this is a behaviour evolved for the purpose of startling predators, or if its production when captured is a secondary purpose to the evolutionary reason for the noise.)

Dicerca divaricata - metallic woodboring beetle

It was also on that closing round that I ran into this flashy beetle. Or rather, he ran into me. I had paused to disentangle my shoulder bag so that I could set it along our exit trail and avoid carrying it all the way back to our banding site, and then all the way back out. As I was standing there, something flew into my forehead at high speed and pinged off, falling to the ground at my feet. It was easy to spot, as it practically glowed in the sunlight. As soon as it had hit my head it had folded its feet to its body and played dead. It was sitting on the ground on its back, a compact nugget of bronze. It was a metallic wood-boring beetle, another group of beetles that I knew right away but couldn’t remember having ever seen before.

Dicerca divaricata - metallic woodboring beetle

I took photos of both sides and then left it for a bit as I photographed one of the longhorns. As I was doing so, it came back to life, putting its feet out and taking a few tentative steps. It caught the attention of a couple of ants who came over to check it out (or maybe the beetle was sitting in the middle of their pheromone trail; in any case, it gives the beetle some scale). I had photos that better showed the metallic sheen, but I liked the species interaction in this one. (The ant, incidentally, I believe is a species in the genus Formica, but there are 86 species in this genus in North America, many of which look very similar, so I can’t say much beyond that.)

Upon returning home, I found an approximate match in my Kaufman Insects and followed it up on BugGuide.net. Some poking around on the latter suggested that this might be Dicerca divaricata, a fairly common species in the east that targets the heartwood of dead hardwoods such as maples, oaks, and others. Most records for this species on BugGuide.net are from June. However, there are a number of Dicerca species that all look somewhat similar, and I couldn’t definitively rule out the possibility of another species.


Author: Seabrooke

Author of Peterson Field Guide to Moths. #WriteOnCon Mastermind. Writer of action/thriller SF/F YA. Story junkie. Nature nut. Tea addict. Mother. Finding happiness in the little things. Twitter: @SeabrookeN / @SeabrookeLeckie

7 thoughts on “Unboring boring beetles”

  1. When I lived in the NWT on the MacKenzie we saw a great many long horned beetles. They went by two common names up there, Pine Beetles and Hair Eaters. Interestingly, they will bite, and it is a painful bite.

  2. The ant and beetle image is a classic!

    I’m curious about the mites (that’s a lot of mites). Are they along for the ride or are they feeding on/with/via the beetle? Or something else entirely? That would be interesting to find out.

  3. Nice post about some beetles that are anything but boring ;-)

    The mites have a phoretic association with the adult beetles – i.e., they utilize the beetles as a form of transport to aid dispersal to new hosts. The mites attach to the beetle as it emerges from its host and climb off when the beetle lands on new dead trees. I’m not sure in what way the beetle benefits from the association, but the fact that some longhorned beetles have evolved specialized pits on their elytra to house the mites during transport suggests that they do benefit in some way.

    I’ll go along with Dicerca divaricata for the jewel beetle – another species it could be is D. tenebrica, but that species has a rather distinct channel along the pronotal midline.


  4. Your little buddy – the one in the first picture – gave me a nice haircut last summer while taking up shop in my baseball cap. It was a service that I did not ask for and one I didn’t pay for either! Ha ha. That little “bugger”.

  5. neat (while being very grossed out).
    We thought those red mites were their babies. Round here we call em pinchers.

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