Each fall I spend quiet time in the woods getting reacquainted with – yes, you guessed it – Paraclemensia acerifoliella. OK, I admit you may not know Paraclemensia acerifoliella. But if you are anywhere near sugar maples, you are probably near Paraclemensia acerifoliella.
Paraclemensia acerifoliella is a tiny caterpillar. We also call it maple leafcutter. Here’s why: It cuts maple leaves. And in so doing, its behavior is a vivid example of the unusual ways that some animals use their host plants not just for food, but for shelter, in summer and through the winter.
The maple leafcutter spends most of its life on or surrounded by a maple leaf. From spring through summer, when it feeds and grows, a leafcutter chews two discs of maple – each no bigger than the diameter of a pea – and fastens them together with silk so they will serve as a protective casing. It’s a bit like a pita bread with a caterpillar inside, or, better yet, a turtle with two shells. The casing is formally known as a habitaculum.
A habitaculum is not a bad hideout from warblers and other songbirds hunting the trees for a squishy meal. It allows the caterpillar to feed on the remaining maple leaf without being so obvious to predators. In fact, as the caterpillar grows, it adds larger discs of leaf to its habitaculum. These discs aren’t particularly big, because, fully grown, a leafcutter caterpillar is only about one-fifth of an inch long.
In autumn, when they have finished feeding, leafcutters descend in their protective casings from the trees to the forest floor. They can crawl down the tree trunk in their shelter or flutter within it to the ground. Within the casing, a leafcutter will spin a loose silken cocoon, pupate and spend the winter in a suspended state of development. And in May, just as the sugar maple leaves break out of their buds, the maple leafcutter emerges to fly free as a tiny moth with steel-blue wings and an orange head. The moths will mate, and the females will lay eggs on fresh maple leaves to complete the cycle.
During a hike several years ago on Vermont’s famed Long Trail, my hiking companion and I found ourselves in rich hardwoods south of Clarendon Gorge. Suddenly, we discovered thousands of tiny discs of maple leaf sprinkled across the trail and through the woods. It was as if someone had been busy with one of those hole punchers used for sheets of paper. The maple leaves overhead, still on the trees, resembled Swiss cheese. They were also scarred with the telltale sign of leafcutter feeding: a gray ring of dead tissue where the caterpillar had been dining. We dropped our backpacks and began to investigate, carefully prying open the small casings to find caterpillars inside.
This discovery later launched desktop experiments with Paraclemensia acerifoliella. From the comfort of my office, I once watched a leafcutter chew a near-perfect disc from a maple leaf – one-half of its shell. I flipped over another, and with only half of its shell in place, I exposed it to the world, wondering if it could turn itself, and its disc, back over. It did, and soon it began to cut out what would become the other half of its shelter from a fresh section of maple leaf. I’ve even watched the little critters crawl across my desk like tiny turtles.
I had always figured that Paraclemensia acerifoliella had a relatively safe life tucked in their leafy covers. That was until one fall day, when a black-capped chickadee popped into my binocular view. In its bill was the casing of a Paraclemensia. The chickadee pecked a few times at the shelter and gobbled the caterpillar as the empty discs fluttered to a balsam fir bough below.
Black-capped chickadees, those feisty, delightful songbirds that entertain us during the long winter, eat everything from bugs to buds, seeds to spiders. But in three decades of watching them, I had never seen one munch one of my favorite caterpillars.
But that should come as no surprise. I had walked for years among Paraclemensia without knowing they existed. I’ve yet to see the adult of this moth. Perhaps I will next May. But for now, this autumn, as I hike in the forest, I will be content to look up for migrating warblers but also down for those maple discs.
I’ll slow my pace to walk with the leafcutters.