In late July, I spent some time watching maple trumpet skeletonizers under a microscope. At that time they were tiny, just an eighth of an inch long, wispy, and translucent. The “trumpet” part of the name of this insect comes from the very slender, tapering tube that each caterpillar larva constructs from its own excrement (called frass) and into which it retreats for safety and resting. Each trumpet was tucked into a pucker at the base of a maple leaf, and the puckering was caused by a tent of silken threads that the caterpillar had constructed by stitching back and forth across the leaf. As the silk threads dry, they shrink and pull the sides of the leaf into a pleat.
Watching a house being built of excrement may not sound very appealing, but the sight of these animals retrieving their pellets of frass, one by one, and then sewing them to the outer margin of the trumpet with yet more silk was so fascinating that I recruited everyone else in the office to observe the process. The long trumpet increases ever so slowly in diameter to accommodate the growing insect. Between building sessions, the brave little larva would crawl out of its haven to fatten on the tender parts of its leaf, gradually enlarging a triangular patch of brown veins. Housed with frass and wrapped in a silk-reinforced leaf, the little builder is quite safe from most predators.
The basic trumpet building blocks increase in size as the caterpillar grows. Pellet size, insect size, and trumpet size are nicely synchronized. At first, the damaged leaf area is small, but as the caterpillar, its trumpet, and its leaf fold all grow, so does the brown blotch. By August, the caterpillar has folded the leaf right up and you need to pull it open to see it and the trumpet. By the time the caterpillar has matured, it is about a half-inch long and the trumpet may be two inches long, often crooked or curved.
In September a heavily infested maple is most unsightly, especially when, as sometimes happens, two or more maple trumpet skeletonizers inhabit the same leaf. Although a tree might look terrible, its recovery is almost assured – for several reasons. These insects tend to be scattered throughout a tree, thus the damage is well distributed. The skeletonizing affects only part of each leaf, and some live, photosynthesizing tissue remains. Most important, by the time the brown blotches attain significant size, the leaves are within a few weeks of dropping from the tree. They have been shipping carbohydrates back to other tree tissues for most of the growing season and have more than repaid the tree for its initial investment.
Sometime before leaf-fall, the caterpillar drops to the ground, constructs a cocoon of silk between two fallen leaves, and pupates. The white-and-black mottled moth that emerges the following June has about a 5/8ths-inch wingspan and is dusted with gray or brown. After mating, the females search out the undersides of sugar maple leaves on which to lay their eggs. The eggs are laid singly, and when the larvae hatch, they begin feeding near the base of the leaf. Sometimes a red maple (and less frequently another tree species) is chosen.
I brought inside a couple handfuls of infested leaves to look at under the microscope and found that some of the trumpets were empty, showing that some predator had figured out how to thwart the maple trumpet skeletonizer’s elaborate defenses. Indeed, different insect species are adapted to feeding on the eggs, larvae, pupae, and adults.
Watching these industrious creatures under the microscope for just a couple of hours convinced me that overall they add more to the forest than they take away. The browned leaves of a September maple are a tribute to the insect world – and to the maple, for that matter. It can host these fascinating little beasts and, at the same time, carry on in good health.