Trying to Control Leafminers? Don’t Bother.

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.

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, Mass., yard — and not the easy ones, like birds or mammals, either.

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

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

You may not be familiar with leafminers, 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 leafminer — marks that you feared were symptoms of disease, but aren’t.

Mr. 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.

Mr. 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 doesn’t 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 Mr. 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.”

As Mr. 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 brings to mind a William Butler Yeats poem, “The Fascination of What’s Difficult.”

He is ever on the lookout for what’s 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 stands out in my obscenely large collection of field guides because it emphatically reminds the reader that nature isn’t all brightly colored flowers and male birds’ breeding plumage. The mostly unflashy invertebrates are the ones who often perform the most crucial roles — they’re “the little things that run the world,” in the words of the biologist E.O. Wilson.

Mr. 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, Mr. Eiseman wrote an e-book on leafminers and founded a North American leafminer 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 leafminer observations there, representing over 800 species.

Mr. Eiseman is currently working on a guide to the larvae of sawflies — the most primitive group of Hymenoptera, the relatives of ants, bees and wasps. During his 2020 backyard survey, he counted 50 sawfly species in his one-acre yard, which was little more than lawn and an arborvitae hedge when he and his wife, Julia Blyth, the collections manager for a natural science museum, bought it. Now it is a home orchard underplanted with meadow through which paths are mowed. As for the lawn, only a strip around the house remains.

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

He added: “They’re somehow convincing the plant to do all their work for them.”

A lot of these interactions haven’t been studied thoroughly, he said, but “it’s some combination of chemical and mechanical stimulation — and some are actually doing genetic engineering, where they’re either switching on and off genes in the plant or maybe even inserting bits of new material.” Shades of CRISPR!

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.

Mr. 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 to 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.”

All of these little guys are never far from Mr. Eiseman’s mind — or out of sight. Rows of identical glass jars fill his home-office bookshelves. Each contains a bit of soil and a larva he is rearing in hopes of another confirmed identification. Soon they’ll go into what he calls his “dorm fridge” for a simulated winter.

On a lower shelf, bags of maybe 100 small vials contain more insects-to-be, and are checked daily to see if any have emerged and are ready for their photo ops. On the computer, a backlog of recent iNaturalist submissions awaits Mr. Eiseman’s verification. Is there perhaps a new species among them?

The work is slow and methodical. No surprise, probably, that a walk with Mr. Eiseman doesn’t take you very far, very fast — but you’ll definitely go deep.

“The less you move, the more you see,” he said.

Ready to look around in the garden? Maybe a moth has mined the leaf of a native black cherry (Prunus serotina) in dramatic fashion. About now in the Northeast, you may witness the geometrically patterned larvae of the dogwood sawfly chewing on Cornus foliage. No cause for worry, as the plants have done most of their photosynthesizing — besides, this is an ancient relationship with which both species are apparently content.

Another dogwood leaf may be in service to a different insect altogether: the gall midge, Parallelodiplosis subtruncata, a mosquito-like fly that makes blister galls. Each flat-topped compartment contains a midge larva that began as an egg that the adult female inserted into the leaf.

On a sumac (Rhus), bladder-like galls attached to the underside of the foliage in high summer may have colored up from green to mottled red, the work of the Melaphis rhois aphid. Mr. Eiseman doesn’t have to be there to know that if you look beneath the shrub, you will find a patch of moss. This is an essential part of the insect’s life equation — the alternate host plant for the overwintering generation.

Mr. Eiseman understands: This is not the stuff of headlines like the monarch on its milkweed or the pollinators’ plight. But he wants to make the case for these unsung, unseen heroes. And he wants us to make room for them — undisturbed and a bit better understood — in our gardens.

Margaret Roach is the creator of the website and podcast A Way to Garden, and a book of the same name.

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