Birch Munchers, Large and Small

The woods behind our house were logged not long before we moved here, and as a result there are lots of black birch (Betula lenta) saplings around. Yesterday on my morning walk I spotted this amazing caterpillar eagerly devouring the leaves on one of these saplings:

Caterpillars of this species get to be well over 10 cm long. In the spring they emerge as cecropia moths (Saturniidae: Hyalophora cecropia), which some sources say are the largest moths in North America. Others say the black witch (Erebidae: Ascalapha odorata) is larger, which may be true for adults, but the caterpillars of that species are only up to 7 cm long. Here’s the only adult cecropia I’ve photographed, from over a decade ago:

Just a little farther down the trail, I noticed some black birch leaves containing larvae of Nepticulidae, the family that includes the smallest moths in the world. This one leaf has 14 larvae in it:

If you look closely at the above photo, you can see the bright yellow larvae at the ends of the mines. Here is a close-up of the largest mine:

I believe these are Stigmella corylifoliella or something else in the S. betulicola species group. Here is a moth I reared last year from a similar larva:

This two-millimeter moth would fit on one of the red knobs on the cecropia caterpillar’s back.

Looking at some other mined black birch leaves, I saw that not all of the Stigmella larvae were yellow. Note the nearly colorless one at the bottom of this photo:

Here’s a leaf with only the pale larvae, some of their mines nearly complete:

The pale larvae pretty consistently fill the first part of their mines with frass particles carefully placed in closely spaced, zigzagging arcs, then transition to making a narrow central frass line toward the end of the mine.

Erik van Nieukerken has determined that there are at least six different Stigmella species forming linear mines on birches in North America, and I believe this is one that doesn’t have a name yet. I reared some adults this spring, but I haven’t gotten that far in my photo sorting yet.

I saw some other leafminers and leaftiers on black birch as I continued on my walk, but I didn’t feel moved to photograph these. But then I saw this leaf, and couldn’t resist taking a closer look:

Flipping the leaf over to look at the underside, I saw what was responsible for the little whitish patches. See the elongate object near the tip of the leaf?

Here’s a closer look:

It’s the portable case of a casebearer moth (Coleophoridae: Coleophora), made from a piece cut out from a leaf. In this backlit shot, you can see the larva inside, its head right at the mouth of the case:

A casebearer feeds by attaching the mouth of its case to the underside of a leaf, chewing a hole in the lower epidermis, and mining into the leaf by extending the front of its body out from the case. Well, that’s how older larvae feed; the youngest larvae feed as regular leafminers, their bodies entirely within the mine. The mine is then cut out from the leaf to form the portable case. In the case above, you can see that the larva left the very tips of the leaf serrations unmined. The presence of leaf serrations tells us that the case was cut from the edge of a leaf—in fact, it came from the base of this very leaf.

The photo above shows the underside of the leaf, and you can see that there is something projecting from the left edge of the missing leaf piece. This is a smaller case that the larva abandoned when it mined into the leaf to create the case that it is currently wearing. And where did that case come from? I surveyed the various holes in the leaf (most of which were not made by this larva) in search of one the right size and shape. I knew I had found the right one when I saw that it, too, had a smaller abandoned case attached to it!

This one was just a millimeter long. To the right of it, you can see a tiny mine the larva made while living in this case; the entrance hole is in the lower right corner. The origin of this original case appears to be a nearby hole that is visible in this zoomed-out view (which shows several more mines made by the young larva before cutting its third case):

Five Coleophora species are known to mine birch leaves, but taking this one’s habits into account I can rule some of these out. I suspect it is C. lentella, which happens to be the only one of the five that has been reported from black birch (and its name is a reference to this host, Betula lenta). It might turn out to be C. comptoniella, though; if so, it will make yet another, much larger case after overwintering. Of the five birch feeders, I have so far only reared C. serratella:

I found another larva of the same type on another leaf of the same sapling. This one was wandering in search of a new feeding site:

I’m going to attempt to rear these, but I’ll be more likely to succeed if I can find more in the spring.