In mid-June of 2021, I was hiking along a river in Colorado’s San Juan Mountains. Their were Aspens trees lining the trail. The sun shone through the Aspen leaves, making them glow. It was very pretty, and I stopped to look at the leaves. As I looked, I noticed that some of the leaves were much darker than other, nearby leaves.
It turned out that the darker leaves were actually two leaves that were stuck together. They appeared to be darker because the sun was shining through two leaves that were tied together to move as one.
The overlapping Aspen leaves seemed more insect damaged than the other leaves. In some cases, the overlapping leaves looked as though they had been ‘mined’ – eaten partially through. In other cases, their were notches missing from the leaves.
Hmm. That’s the sort of damage that caterpillars would do to leaves. But where were the caterpillars?
I peeled a couple of the leaves apart. They were held together by silken strands, similar to a spider’s web. But instead of finding a spider nestled in the silk, I found a caterpillar.
There are a number of different caterpillar species that use their silk to build shelters from leaves. Most of them fold the leaves, or roll the leaves into tubes, or wad a number of the leaves together. The fact that most of these leaves were tied together in pairs, with the flat of the leaf blades pulled neatly together, was strange.
When I got back to the home, I started trying to identify these creatures. It was tough sledding, since I’m certainly no moth expert.
But I think these are caterpillars of Enargia decolor – the “Aspen Two-leaf Tier Moth”. That’s “tie-er”, as in “one who ties things together”. They are a nuisance pest in Canada and the Rocky Mountains, usually not causing long-term damage to the Aspens.
I thought this idea of caterpillars stitching leaves together was interesting. I’m fascinated by plant galls – domiciles that insects, mites and other creatures ‘tell’ the plant to grow for them. It seemed to me that these webbed-together leaves were sort of like a gall – except the insect had built the house themselves.
A week or so later (late June), I went back to the same spot to see how the caterpillars were doing. This time, many of the ‘tied’ leaves were on the ground beneath the young Aspens. I’d estimate that there were dozens of these paired leaves on the ground.
I didn’t think the tied leaves had been on the ground for very long – they were still green, and only beginning to dry out. I didn’t see ‘normal’ (non-damaged, non-tied) leaves on the ground. The fallen leaf pairs had separated from the tree at the petiole bases, as though they had been dropped by the tree, or plucked, not cut from the tree. There were caterpillars in at least some of these fallen leaf-pairs.
This struck me as yet another odd thing.
The Natural Resources Canada page on the Aspen Twoleaf Tier says “Once the larval stage has been completed, the insects fall to the ground and pupate.” I’m wondering, though, if perhaps these caterpillars the caterpillars didn’t just “…fall to the ground and pupate”.
I wonder if the caterpillars caused the leaf pairs to drop, with the caterpillars still nestled safely between them. It seems odd that both petioles of the tied leaves would release at nearly the same time, before at least one of the leaves had dried. And why were there so many leaf pairs on the ground, seeming as though they had fallen in synchrony?
I wonder – perhaps the caterpillars pupate between the leaves as well, continuing to use their leaf-shelter even when it’s no longer on the tree?
I don’t know. I’m hoping that I see these caterpillars again these Spring, though. I’ll watch more closely next time.
Sources:
Much of the information on this page came from the Natural Resources Canada’s page on the Aspen twoleaf tier.
