While out snowshoeing the 100-acre woods yesterday, I passed through a grove of hemlocks I don’t often hike by because the trail runs through a wet section with about a foot of standing water. With the water all frozen and covered in snow I decided to loop around that side of the woods. It turned out to be a productive decision. Besides finding the woodpecker mentioned yesterday, there were also these trees.
Hemlocks aren’t particularly unusual in our woods here; they’re perhaps the most common evergreen, behind the ubiquitous white cedar. They look lovely in snow, but otherwise I may not have ordinarily paid them much attention. However, I happened to noticed that the trunks of these ones were covered in small round holes. My first thought was Yellow-bellied Sapsucker, which drill their sap-producing holes in straight lines.
But they weren’t really sapsucker-ish enough. Aside from the straight-line thing, sapsucker holes are usually oblong to rectangular in shape, not generally round, and they’re quite often clustered together or even stacked, rather than randomly scattered such as these ones were. The sapsucker holes above were ones I found in the forest at our previous house last April. You can see three sets of freshly-drilled rectangular holes, stacked in vertical lines. This is stereotypical sapsucker to me.
These were in straight lines alright, but the holes were small, round, and packed together, the lines widely spaced, and man, were there a lot of them. This didn’t look like the work of sapsuckers. I’m fairly sure that these are exit holes of wood-boring beetles, and more specifically, I suspect the Hemlock Borer, Melanophila fulvoguttata. It seems to be a fairly widespread species, never widely abundant but reasonably common. As its name suggests, its primary host is Eastern Hemlock, though it’ll also take advantage of White Pine, Tamarack, Balsam Fir and a few spruces.
The Hemlock Borer is a secondary pest of hemlock trees – that is, although the tree I found the holes in was still living, the beetles only attack stressed or weakened trees, never healthy ones. The stressor might have been environmental, such as drought, or another pest, such as Hemlock Looper (a type of moth, which is fairly common and which I caught several of this fall). The adult female lays her eggs in the crevices of the bark, whereupon the larvae, when they hatch, tunnel in to the soft, rich cambium layer underneath. They can spend one to two years there before they pupate and emerge as adults over the spring and summer. Once you discover holes in the tree’s trunk the beetles have already left and the damage has been done to the tree.
I did a lot of searching of the web but could find no photos of the emergence holes of this beetle. Any photos I found of tree trunks were illustrating woodpecker foraging activity that exposed the reddish inner bark of the tree, not so helpful to me. But the descriptions I found seemed to match: small round holes about 3mm across. On the other hand, “small round holes about 3mm across” could probably describe a lot of different wood-borer holes! I’m looking forward to that book on insect tracks and sign that Eric Eaton mentioned on his blog a little while ago. It should be out in March, just a bit too late for Christmas, but still in lots of time for my birthday. :)