Fall Migration

The Monarch migration is now surging, with huge numbers reported from the U.S. heartland and Northeast this week, including “too many to count” at Vermont’s Mt. Philo today. Mt Philo to Mexico: 2,400 miles.

And even a few ambitious dragonfly species, including the Common Green Darners I chased around on Tuesday, are now heading south for winter. My backyard to Everglades National Park: 1,400 miles.

Our caterpillar, hidden in its shelter, which it cut and fashioned from a Sugar Maple leaf.

Yet here in the woods of home, rather than chasing hawks or butterflies, I’m tracking the epic migration of a caterpillar called Maple Leafcutter. Maple sapling to forest floor: 4 feet.

From whales to warblers to wildebeests, animals are now on the move in some of the planet’s most audacious feats of adaptation and survival. Even so, you need not visit the Serengeti or the high seas to witness these mass movements. Instead, visit the woods, recalibrate your horizon, and lower your gaze: the drama of migration unfolds at your feet.

As some of you know, I’ve been obsessed with these little caterpillars for more than two decades, having reported on them as recently as last month. Yet nature is nothing if not an endless opportunity for discovery. Even a prosaic caterpillar continues to inspire and amuse me. Maple Leafcutters (Paraclemensia acerifoliella) are now on the move — from (yeah, you guessed it) maple leaves to the forest floor for winter.

Perhaps you’d prefer to discover more charismatic migrants likes hawks and Monarchs; you can do that at my new feature here on the site called Migration Central. But if you cannot get yourself up a mountain for hawks or out to sea for whales anytime soon, if you’ve never seen tropical leafcutter ants in action, well, here is our own insect transporting bits of leaf through the forest … yet ever so slowly.

I will summarize in more detail, at the end of this essay, the four-month, even four-foot, odyssey of a caterpillar, most of it carried out on a single leaf. But first, I’ll admit that I’ve been harassing Maple Leafcutters into performing some heroic acrobatics.

I’ll admit that I’ve been harassing Maple Leafcutters into performing some heroic acrobatics.

Since May these tiny larvae have been munching on maple, most of the time while shrouded in two or more discs that they cut from their leaf and stitch together like little green “pitas.” Actually, most are now inside their pitas and covered by yet another larger disc. It allows the caterpillar to poke its head out of its pita and feed on the live leaf tissue under cover and without detection. Well, at least until I came along and turned their worlds upside down.

In the video below, I flip a shelter — like turning over a turtle — to expose the smaller pita and the caterpillar inside. I did that a lot this week. In most instances, the caterpillar righted its leafy ship within a few minutes (or I lost patience waiting). This one below takes about a minute.

You can now see all this for yourself in hardwoods here in the Northeastern U.S. I’m finding Maple Leafcutters almost exclusively on Sugar Maples (Acer saccharum), often near forest edges or sunny breaks, although my colleague Shelby Perry recently discovered them rarely on American Beech. (I suspect their occurrence on other maple species may be incidental or accidental.) Many are still on their leaves and feeding waist-high on saplings where you can easily observe them.

But many others have already “migrated” to the forest floor. Rather than walk, they float to earth in their leafy shelters when the autumn winds shake the trees, when the squirrels might disrupt things, or when their host maples leaves themselves fall for the season. Yes, this is a passive migration, to be sure, but animal movement nonetheless.

Once on the ground, the caterpillars have two options. They pupate in their shelters and begin their transformation next spring to a free-flying moth with steely blue wings and an orange head. Or, if they’ve fallen too soon in the season, they’ll sometimes climb back up a tree — like that one in the banner image above — to continue feeding on its maple leaves. My suspicion had been that by mid September they’re mostly done feeding for the year — and therefore have no business climbing back up a maple. But Shelby reports big-time tree climbing in Hubbard Park here in Montpelier on 25 Sep 2018. (This is why naturalists keep field notes.)

The world already spins too fast. Nothing like a caterpillar to slow us down.

In any event, besides flipping them like pancakes, I’ve been on my belly a lot watching bits of maple leaf marching through the forest. Okay, marching isn’t really accurate. These things make snails look like cheetahs. But that’s okay. The world already spins too fast. Nothing like a caterpillar to slow us down.

So take an hour or so and watch a Maple Leafcutter lug around its shelter for a few inches. Let that be your meditation on slowing down in nature, which is good for you.

After all, somewhere on the planet right now an Arctic Tern, the epitome of migration, is flying from its breeding grounds in the high Arctic toward Antarctica — a four-month journey between the poles covering more than 20,000 miles. Meanwhile, over the course of four moths here at home, a tiny forest caterpillar might move no more than four feet. Its singular purpose is metamorphosis — to be released from the bounds of a maple leaf so that it may at long last fly free in our forests on wings of its own — a journey no less remarkable than the far-flung peregrinations of a tern.

References and Resources

Bionomics of the Maple Leaf Cutter, Paraclemensia acerifoliella (Fitch), (Lepidoptera: Incurvariidae) by D.A. Ross. The Canadian Entomologist, Volume 94, Number 10. October 1962.

Paraclemensia acerifoliella (Lepidoptera: Incurvariidae) in western Canada: A newly discovered host, an expanded range, and biogeographical considerations by Gregory R. Pohl, Christi Jaeger, Vazrick Nazari, Chris Schmidt, Danika Richard, Stan Gosche. The Canadian Entomologist. December 2014.

Maple Leafcutter records from iNaturalist — nice examples of discs and adults.

And special thanks to my leafy, inquisitive colleagues: Nona Estrin, Shelby Perry, Susan Sussman (for reports from Mt. Philo today), and especially Trish Hanson, who first helped me discover these insects (and many others) about 20 years ago.