Weird marks on your leaves? How to decipher them…

In the early days of the coronavirus pandemic, some of us mastered baking (if we could get our hands on flour) or devoted ourselves to growing a new mail-order houseplant.Add announcement

Charley Eiseman, as always, has set the bar a little higher. In 2020, he decided to keep a count of some of the creatures living in his Northfield, Massachusetts, backyard—not the easy ones, like birds and mammals.

It’s the little things that matter most to Eiseman, a freelance naturalist who conducts biodiversity research for conservation groups and other clients.

Before the year was out, he had listed 212 species of leaf miners among his various numbers.

You may not be familiar with leaf miners, but even if you haven’t seen the usually overlooked miners themselves, you’ve likely witnessed their handiwork: curls or spots known as enamel in leaf tissue.

The tiny creatures that dig into the epidermal layers of a leaf to feed themselves are the larvae of about 50 moths, flies, beetles and sawflies. Each species targets specific host plants in ancient, intimate relationships.

If you grow columbine (Aquilegia), a popular food in many regions for fly larvae of the genus Phytomyza, you’ve probably seen evidence of the leaf miner—signs you feared were symptoms of the disease, but they’re not.

Eiseman is also a keen observer of the architects behind abnormal growths that can develop into Ping-Pong balls that form in leaf buds, blisters or cone-like nodules as they emerge from the leaf surface.

We gardeners can treat these growths as unsightly, asking our local nursery staff (or Google) how to “fix” it. But nothing is broken. It’s usually an insect—often an aphid, midge, or wasp—that sets up its housework or builds a nursery to raise its young.

Eiseman identifies these leaf mines and galls with forensic precision because of his own insatiable curiosity—and for another reason: “I want to send a big message: If you see evidence of things eating and living in your plants, that’s a good thing.”

He’s not talking about invasive species like sponge moth caterpillars or Japanese beetles; that they did not mention. Otherwise, he believes, we should try to let go of the reflexive alarm that “something is hurting my plant.”

It’s about biodiversity being damaged by garden perfection and our relentless pursuit of blemish-free leaves. The more insects and other invertebrates Eiseman sees interacting with the plants, the happier he is.

“I know other people don’t feel the same way about mistakes,” he said. “But if you like any living thing—birds, frogs, or whatever—they almost all eat bugs or something that eats insects. If you don’t have that insect base, you won’t have those other things.”

Extraordinary support staff
When Eiseman examines a spider’s web for clues about what species is making it, or looks at a twisted leaf to assess what insect might be hiding inside, he looks for what is described in the title of the field guide he co-wrote with Noah. Charney, 2010: “Tracks and Marks of Insects and Other Invertebrates: A Guide to North American Species”.

This book strongly reminds the reader that nature is not all brightly colored flowers and breeding male birds. Invertebrates, which are mostly non-bright, are often the ones that fill the most important roles – they are, in the words of biologist EO Wilson, “the little things that rule the world”.

Eiseman’s motivation for writing the book was purely pragmatic. While doing fieldwork, he would find himself taking pictures of strange things and then making notes to himself to look at and identify them.

“And then I realized there was no book to tell me what these things were,” he said. “So I ended up writing the book so I could learn all this.”

To further satisfy the need to know, Eiseman wrote an e-book about leafminers and founded the North American Leafminer Project on iNaturalist, an online community where nature lovers search for and contribute to the identities of plants and animals. More than 1,500 people have observed more than 50,000 leaf miners there, representing more than 800 species.

Inspiration on a poison ivy leaf
Leaf miners and gall bugs are both parasites and live in plant tissue. While the miners spend their time digging, the gall bugs are “actually manipulating the growth of the plant to create this shelter where they live and feed,” Eiseman said.

Gall bugs are everywhere: In North America, an estimated 1,000 species of gall bees are responsible for galls on oak trees alone.

Eiseman was familiar with galls long before he wrote his first book, especially common apples, called “oak apples” because of their large size and roughly spherical shape. You’ve seen them either freshly fallen and green (sometimes with dark polka dots) on the ground, or after they’ve dried and faded to look like brown ping-pong balls.

Leaf mines—snake tunnels within layers of tissue—poison ivy with mottled patches on its leaves politely revealed themselves to him near a Vermont trail.

Sitting on a leaf were two small moths, still emerging from their leaf mines with their newly emerged pupal skins. He took this information and later came across a reference in an old book describing a moth, Cameraria guttifinitella, that causes such spots on poison ivy.

“All of a sudden it occurred to me that if you know what a plant is and what its pattern is, you can sort these things into species just by looking at that pattern,” he said. “I was fascinated by this idea and spent the last ten years trying to study all these different patterns on leaves and stems.”

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