Beetle mining for mining beetles

If I have a defect (I said, "if"), it is that I don't let go of a topic until it is thoroughly dealt with. (Well, that's one. There may be a few others.)

So, after we brought home those photos of the engraved branch, last Sunday, I wondered aloud what critter had done that work, how, and why. When I mentioned that I had dug the branch out from the rocks and logs, and propped it up well above the tide line, and that, "maybe it will still be there next time we go down that trail," Laurie proposed going the very next day, with a saw.

(Laurie is incredibly patient with me.)

So he carried his saw down that steep trail, and sawed the branch in half. It was just right for a tall, sturdy walking stick for me, and a cudgel for him. To ward off any dangerous falling leaves, I presume.

At home we examined our haul. It is birch, from the remains of bark on the "cudgel" end, still flexible, but dry. I peeled off the bark; the tunnels underneath were full of wood paste, still dampish. They made quite a pile.

Another "carving"; some species of fantastic centipede?

I remembered that Snail had recently discovered larvae and adults of leaf miners by following their tunnels: I would try the same thing here. With a sharp hook, I carefully dug out the sawdusty filling, checking it under a lens as it accumulated.

Soon I found something; something tiny, shiny and black. It looked, under the microscope, like the pronotum and part of a head of a small beetle. I was on the right track! I dug some more, and found an elytra, then two.

I needed that encouragement, because it was over an hour before I found an entire beetle, all 3 mm. of it. And then I couldn't believe that I really had it all.

Here's beetle # 1:


It looks as if it were missing the end of the abdomen. And the legs are too short, almost not there, as if something had eaten them away.


I couldn't make any sense out of the mouthparts, even with the microscope.

I found a couple more, in the same state. One was a paler brown:


See how it looks almost broken off at the end?

Twisting and turning them, I realized that the stubs of legs had the normal claws at the end; maybe I had the whole beetle, after all.


Oh, Google Images! Trying "birch bark beetle". And I hit it first thing. Should have Googled earlier.

The master engravers are probably Birch bark beetles, Scolytus ratzeburgi Janson. Or possibly Elm beetles, which are more or less identical; a page from Virginia Tech says that,

Species identification is difficult because the adult beetles of the various species are very similar, cylindrical and hard-shelled.

And VT has an explanation of the intricate patterns the beetles create:

Adult bark beetles bore through the bark to the cambium layer of suitable host trees. The female excavates a tunnel between the bark and wood along which she lays her eggs. Upon hatching, each grub burrows away from the egg tunnel and feeds on the live bark tissue (phloem) and outer cell layers of wood (xylem). The resulting network of egg and larval tunnels beneath the bark is called a gallery.

If you look at the photo of a gallery above, you can see a tiny hole at one end, where the female entered. They bore nicely rounded holes; here are two, from the outside:


I followed one of the tunnels along from the entry hole, and found a beetle trapped at the end. Here's the hole and channel, after I had cleaned out the sawdust:


And turning it around, here's the beetle:


Unfortunately, when I pried this one out, it came apart.

I would imagine that this was a female; she came in from outside, didn't get around to laying eggs, and died. There are no side tunnels, and she'd barely travelled a little more than half an inch.

Again, from VT;

The "shot hole" appearance of the bark in infested trees indicates that numerous beetles have matured, chewed exit holes, and flown off to find new breeding sites.

This branch, at least in the parts that still had bark, didn't really have too many exit holes. It is possible that the branch fell in a windstorm, and landed in salt water, killing the larvae before they matured.

I also found, under the bark, a microscopic shell of a marine snail.
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