Landscape Confidential: Tracking insects

This witch hazel has been visited by multiple species of insects -- those that skeletonize and those whose feeding forms large holes. Photo by Audrey Clark

This witch hazel has been visited by multiple species of insects — those that skeletonize and those whose feeding forms large holes. Photo by Audrey Clark

 

Witch hazel is a tall, many-trunked shrub with branches that arch overhead and wavy-edged, oblong leaves. This one, in Sunny Hollow Natural Area in Colchester, looks like it had a run-in with a summer hailstorm — or something. Its leaves are riddled with holes. Big, round holes. Irregular, rough-edged holes. Tiny holes between the tiniest veins. Even half-hearted holes that don’t make it all the way through the leaf and instead form odd-shaped brown patches.

Peer closer, and you see that some leaves are folded over. Some of the folded bits are shriveled and brown, some still green and fresh.

Look still closer and you might find odd bumps or growths on the leaves, some of which look quite disgusting: green bumps blushing red, brown warts, and skinny or flattened nipples on the leaf surfaces.

The witch hazel is not alone. Nearly every other tree and shrub in the forest is likewise marked.

The brown cone on this witch hazel leaf is a gall caused by the cleverly named witch hazel cone gall aphid. Photo by Audrey Clark

The brown cone on this witch hazel leaf is a gall caused by the cleverly named witch hazel cone gall aphid. Photo by Audrey Clark

While it is the time of year to celebrate Vermont’s foliar beauty — oh, the colors! — it is also the time of year to note the many ways insects make a living off of leaves. They’ve had all season to chew upon, cocoon into, and lay eggs inside leaves and the accumulated damage is remarkable. Tune your eyes to it, and you will see it everywhere.

Charley Eiseman, a graduate of the University of Vermont’s Field Naturalist Program and author of the book “Tracks and Sign of Insects and Other Invertebrates,” is one of the few people in the country who can look at a leaf and deduce what ate it or nested in it.

Tracking insects is “taking animal tracking to its most absurd extreme,” said Eiseman.

But Eiseman’s perspective on insect tracking is not absurd; it’s quite clearheaded. “Each species of insect has a very particular way of interacting with its environment and a series of things that it does in its life cycle,” he said. “Each one of those things leaves an observable sign, so you can really see an insect’s whole life spelled out on a little tiny piece of the landscape in the same way that you can with larger animals looking at their tracks and sign.”

A feeding sign on leaves is often in the form of holes. Big holes with rough, serrated edges are typically made by grasshoppers. Holes with scalloped edges are generally made by caterpillars. Beetles make ragged-edged holes.

When tiny holes appear between tiny leaf veins, that’s called skeletonizing. A variety of creatures skeletonize leaves, including slug sawflies (common on oak leaves, and members of the rose family). Slug sawflies are not slugs at all, but rather are related to ants, bees and wasps. These translucent, slimy, caterpillar-like creatures perch on one side of the leaf and gnaw at the surface. That’s one reason you find irregular brown patches on leaves. Most skeletonizing can be blamed on moth larvae and beetles, but Eiseman says there are “scattered examples in other groups.”

Another part of the insect life cycle you can easily find on leaves are leaf shelters. Leaf shelters are made when an insect wants to protect itself from predators or inclement weather. Birds are really, really good at finding insects to nibble, which is why so many insects have evolved camouflage — think green caterpillars that blend in with leaves, or brown-spotted moths that match the underlying tree bark.

Other insects have evolved to tell birds they are toxic — many of the insects that feed on milkweed, for example, have orange or yellow and black coloration. Some insects are liars, though: they have warning coloration but aren’t toxic. Birds avoid them just in case, and the insects survive.

If you aren’t carefully colored or you are in a particularly vulnerable stage of life, it’s handy to take shelter somewhere birds won’t find you. Many insects construct leaf shelters by folding leaves over and cocooning themselves inside while they undergo metamorphosis or hibernate through the winter.

Some shelters are simple: the side of the leaf is folded over and sealed shut with silk. Many moth species cocoon themselves inside these simple leaf folds — in fact, Eiseman says most leaf shelters are made by moth caterpillars, which eat the leaf from inside the shelter’s safety. These shelters are often brown and shriveled from having been munched. Other insects make more complicated shelters, stitching two leaves together and nestling between them, or folding several leaves together into a sort of ball — you can find the latter kind of shelters on sumac trees.

This simple leaf shelter housed a spider. Photo by Audrey Clark

This simple leaf shelter housed a spider. Photo by Audrey Clark

Many leaf shelters aren’t made by insects at all. Spiders make leaf shelters, too. The witch hazel in Sunny Hollow hosted a simple folded leaf shelter that, when pried open, revealed a pale spider skittering away from the light behind a wall of silk webbing. Eiseman says spiders will make leaf shelters to hide themselves or their eggs.

Then there are the galls. According to Eiseman, galls are deformities on plants that are caused by anything from fungi to bacteria. Among the gall-making insects, there are several hundred species of wasps and midges, as well as some aphids, beetles and moths. Gall makers typically lay eggs under the leaf or stem surface, stimulating an immune reaction in the plant that isolates the infection in one area while also sending nutrients to that area, feeding the insects.

Hackberry, a tree with rough, raised bark and asymmetrical, toothed leaves, often hosts what are officially termed “nipple galls.” These galls look like tiny green nipples on the underside of the leaf and little puckers on the upper side. They are made by plant lice, which are related to aphids.

Though it’s near impossible to generalize about insects, Eiseman did have a useful rule of thumb for identifying the sources of galls: “If it’s on an oak, chances are a gall wasp made it. If it’s on anything else, it’s most likely a gall midge [a kind of fly].”

Eiseman isn’t daunted by the monumental diversity of insects and the multifarious ways they interact with plants. He is currently working on a book about leaf-mining insects — those that lay their eggs inside leaves, which then hatch larvae that chew a circuitous path through the innards of the leaf itself.

“There’s a lot more to learn,” he said. “I’ve pretty exhaustively covered the literature on North American leaf-miners and because I’ve done that I can go for a walk anywhere and pretty reliably find one if not a few things that are new to science.”

Eiseman collects these new species and sends them to specialists, who then describe them, adding to the ever-growing list of the remarkable diversity of life on Earth. It’s not unusual for Eiseman to find new ecological relationships, too — ways that insects interact with their environment that no one knew about, even in well-studied Massachusetts or Vermont.

So head out into the Vermont countryside to admire the foliage, sure — but lean in, look close, and give a whole new meaning to the term leaf-peeping.

Audrey Clark

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