Walking through the woods in November is surely a pleasure – with the brilliant sunshine, the leaves swirling through the air in a gust of wind, and all the piles and piles of brown leaves in which my feet disappear. And every leaf tells a bit of the story of the forest that summer.
How an insect alighted on a leaf, inserted its ovipositor, laid an egg – perhaps many times that season. And then how the tree responded – perhaps forming a gall around the egg or sending chemical warning messages to surrounding trees of possible defoliators in their midst. And then how that insect hatched, ate, grew, changed form through metamorphosis and flew or crawled away – or how it was eaten by a predator and so contributed to the food web, and carbon cycle, of the forest that summer.
Pick up a leaf and see if you can find signs of the story of that summer. What kind of creature laid its egg on the leaf? Was it a moth, fly, beetle, caterpillar, or mite? Caterpillars cause the most defoliation – but they are also a critical food for warblers feeding their young in the spring. If you see the edges of the leaves look ragged, perhaps it was the chewing mouthparts of a caterpillar – though it could also be a variety of beetles or sawfly larvae.
Leafcutter bees remove circular sections of leaf to construct nests for their larvae. This defoliation is often seen on roses. “Shot holes” on a leaf can be the early stages of a variety of caterpillars including gypsy moths, or beetles. “Leaf notching” results when insects feed on the margins of leaf blades. Among the most damaging of leaf notchers are beetles known as weevils, long-snouted beetles whose nostrums outdo Cyrano de Bergerac.
As leaves age, they become hardened, and this toughening serves as an impediment to chewing insects, as the insects are forced to contend with a matrix of hard leaf veins interspersed with the tender, nutritious leaf tissues. Many large caterpillars have jaws powerful enough to cut the tough leaf veins. Smaller caterpillars lack this capacity are forced to “feed between the lines,” removing tender leaf tissue and leaving veins left behind. This type of chewing injury is called skeletonization. Hold a skeletonized leaf up to the sunlight above your head and it will look almost like stained glass, with the leaf ‘skeleton’ outlining the light behind it.
All summer long, much happens high in the forest canopy where we cannot see. But come November, all the signs are laid at our feet if we can read them.
