Those weird marks on your leaves? Here’s how to decipher them

Leaf miners, the tiny creatures that excavate a leaf’s epidermal layers to feed themselves, are larvae from some 50 families of moths, flies, beetles, and sawflies. Each species targets particular host plants. (Photo: Shutterstock)
During the early days of the coronavirus pandemic, some of us mastered bread-baking (if we could get our hands on flour) or devoted ourselves to nurturing some new mail-order houseplant.اضافة اعلان

Charley Eiseman set the bar a bit higher, as he always does. In 2020, he decided to keep count of certain creatures living within the confines of his Northfield, Massachusetts, yard — and not the easy ones, like birds or mammals, either.

For Eiseman, a freelance naturalist who conducts biodiversity surveys for conservation groups and other clients, it is the little things that matter most.

Before the year was out, he had recorded 212 leaf miner species, among his various tallies.

You may not be familiar with leaf miners, but even if you haven’t seen the miners themselves, which typically go unnoticed, you have most likely witnessed their handiwork: the squiggles or blotches within leaf tissue, known as mines.

The tiny creatures that excavate a leaf’s epidermal layers to feed themselves are larvae from some 50 families of moths, flies, beetles, and sawflies. Each species targets particular host plants in ancient, intimate relationships.

If you grow columbine (Aquilegia), a popular meal in many regions for larvae of flies in the genus Phytomyza, you have probably seen evidence of the presence of a leaf miner — marks that you feared were symptoms of disease, but aren’t.

Eiseman is also a keen observer of the architects behind galls, those abnormal growths that can take the shape of Ping-Pong balls when they form at leaf buds, or blisters or conical knobs when they project from the foliage surface.

We gardeners may view these growths as disfigurements, asking our local nursery staff (or Google) how to “fix it”. But nothing is broken. It’s usually just some insect — often an aphid, midge, or wasp — setting up housekeeping or building a nursery to raise their young.

Eiseman identifies these leaf mines and galls with forensic precision in the name of his own insatiable curiosity — and for another reason: “I would love to put out a major message: If you see evidence of things eating and living on your plants, that is a good thing.”

He is not talking about invasive species like spongy moth caterpillars or Japanese beetles; those he does not celebrate. Otherwise, though, he believes we should try to let go of the reflexive alarm of “something’s harming my plant”.

At issue is biodiversity, which is damaged by our relentless pursuit of garden perfection and unblemished leaves. The more insects and other invertebrates that Eiseman sees engaging with plants, the happier he is.

“I know other people don’t feel the same way about bugs,” he said. “But if you like any living things — like birds or frogs or whatever else — they pretty much all eat insects, or eat something else that eats insects. If you don’t have that base of insects, you’re not going to have those other things.”

The unflashy supporting cast
As Eiseman examines a spider web for clues to which species made it, or peers at a folded leaf to assess which insect may be sheltering inside, he is ever on the lookout for what is described in the title of a field guide he wrote with Noah Charney in 2010: “Tracks & Sign of Insects and Other Invertebrates: A Guide to North American Species”.

That book emphatically reminds the reader that nature is not all brightly colored flowers and male birds’ breeding plumage. The mostly unflashy invertebrates are the ones who often perform the most crucial roles — they are “the little things that run the world”, in the words of the biologist E.O. Wilson.

Eiseman’s motivation for writing the book was purely pragmatic. While doing fieldwork, he would find himself taking pictures of odd things and would jot a note to himself to look them up later and identify them.

“And then I realized that 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 of that stuff.”

To further satisfy his need to know, Eiseman wrote an e-book on leaf miners and founded a North American leaf miner project on iNaturalist, the online community where nature lovers seek plant and animal identifications and contribute sightings. More than 1,500 people have contributed over 50,000 leaf miner observations there, representing over 800 species.

Inspiration in a poison ivy leaf
Leaf miners and gall insects are both parasitic, living inside plant tissue. While miners spend their time excavating, Eiseman said, gall insects are “actually manipulating the growth of the plant to create this shelter that they then live and feed inside”.

Gall insects are everywhere: It is estimated that 1,000 species of gall wasps in North America are responsible for the galls on oaks alone.

Eiseman was familiar with galls long before writing his first book, particularly the common ones referred to as “oak apples”, for their large size and roughly spherical shape. You may have seen them on the ground, either freshly fallen and green (sometimes with dark polka dots) or after they have dried and faded to resemble brown Ping-Pong balls.

The leaf mines — those serpentine tunnels within layers of tissue — revealed themselves to him later, alongside a Vermont trail, courtesy of poison ivy with blotchy patches on its foliage.

Sitting on a leaf were two tiny moths, with the pupal skins they had just emerged from still poking out of the leaf mines. He filed that information away and later came upon a reference in an old book describing a moth, Cameraria guttifinitella, that makes such blotches on poison ivy.

“It suddenly kind of clicked with me that if you know what the plant is and what the pattern is, you can identify these things to species just by looking at that pattern,” he said. “I got fascinated with that idea, and I’ve spent the past decade trying to learn all of those different patterns on leaves and stems.”

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