perfect imperfections: galls and leaf mines, with charley eiseman

SINCE I TOOK a walk with today’s interview guest about 10 years ago, I’ve adopted a whole different way of looking at what I might have once seen as imperfections in plants. Now when I spy a squiggle in a columbine leaf or what looks like a green ping-pong ball on an oak, instead of thinking, “What’s wrong with my plant?” I think, “I wonder who made that, and why?”

Curiosity has replaced panic, thanks to naturalist Charley Eiseman, co-author of the field guide “Tracks & Sign of Insects and Other Invertebrates” (affiliate link), who’s decoded some of nature’s beautiful little mysteries for us.

Charley is a freelance naturalist, and when he’s not conducting biodiversity surveys for clients, he devotes himself to learning more about the natural world through an e-book he’s created on leafminers, and a North American Leafminer Project on iNaturalist that he started that has some 50,000 sightings submitted. (Above, a mine created by larvae of a moth, Stigmella macrocarpae, in an oak leaf; Charley Eiseman photo)


Plus: Enter to win a copy of Charley’s “Tracks & Sign” field guide by commenting in the box near the bottom of the page.

Read along as you listen to the Oct. 3, 2022 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on Apple Podcasts (iTunes) or Spotify or Stitcher (and browse my archive of podcasts here).

Naturalist and author Charley Eisemanleaf mines and galls, with charley eiseman



Margaret Roach: Hi, Charley, and thanks for coming indoors long enough to talk today [laughter].

Charley Eiseman: Hi, thanks for having me on.

Margaret: So you’re across Massachusetts from me away, as I’m on the New York-Massachusetts border. So we’re both Northeasterners, I should say, for perspective.

I’ve confessed to you and to others over the years, I have this obsession with field guides, and I have a big cabinet filled with them, but this one really stands out. It’s just so different. And as I said in “The New York Times” garden column that we just collaborated on, I love it because it doesn’t call out the obvious stuff. It doesn’t call out beautiful bird feathers and flowers, wildflowers and stuff, and butterflies. So tell us a little bit about how you kind of got into this crazy… like the tiny micro [laughter], the tiniest world of all, the invisible stuff, almost.

Charley: Well, it started with just being a generalist naturalist, and being into mammal tracking, and having jobs where my job was to identify all the plants I encountered in different plots. And so I was spending a lot of time staring at plants and noticing these signs of tiny animals. And I discovered there weren’t any books to tell me what these squiggles and patterns and objects on leaves were. So I started doing the research to put together a guide, so I could learn what they all were.

Margaret: So necessity was the mother of invention, literally, with this book [laughter] and with your subsequent projects too, I guess, right?

Charley: Yeah. Soon after that “Tracks & Sign of Insects” book was published, I did a survey of the galls and leaf mines of Nantucket just for a few days. And when I got home and was trying to identify everything I found, I discovered that the leaf mines, I really couldn’t get them all to species without doing this huge project to create a guide to all the leafminers of North America, which I’ve kind of been doing ever since.

Margaret: And you’ll probably be doing it for a lot more years [laughter], because I bet there’s a lot more of them than we have any idea of. Must be a lot.

So you said galls and leaf mines, leafminers. So tell us what those are. Because, again, like I think I said in the introduction, even I used to see them as imperfections: “Ooh, what’s wrong with my plant? Who did that? Why is that that way? Fix it.” That kind of reaction and panic almost for a gardener who wants pretty, pretty, right? So what is a gall? What’s a leaf mine? What’s going on? [Leaf mine in rose foliage created by the larvae of the moth Stigmella rosaefoliella; Charley Eiseman photo.]

Charley: So these are both types of insects that are basically plant parasites. They live inside the plants. But it’s a relationship that’s been developing over a long time. And so it’s not mutualistic, but it’s symbiotic to the point that it’s minimizing the damage to the plant. And with galls, the insects are actually modifying the growth of the plant. So the plant does all the work making this shelter around the developing insect larva or nymph. And that’s caused by some combination of mechanical and chemical inputs from the insect. And in some cases, something like genetic engineering is going on, where they’re either switching on and off genes or maybe even inserting some more DNA.

Margaret: So you mean humans didn’t invent CRISPR, is that what you’re telling me?

Charley: [Laughter.] Right. Humans didn’t really invent anything, anything. No.

Margaret: Anything. They didn’t invent art. They didn’t invent geometry. They didn’t invent intricate patterning and things that we think of as art. Right.

Charley: Right. And the leafminers are definitely… some of them are real artists. And in the case of leafminers, the larvae are actively excavating the plant tissue. So everything you see in a leaf mine is the work of the insect as opposed to the insect making the plant do something for it. So a leafminer is an insect larva that’s feeding between the two epidermal layers of a leaf.

Margaret: So those squiggles… And I think the columbines…When we’ve talked before, you’ve told me around the country there’s columbine leafminer insects that will mine—like you said, excavate in between the epidermal layers, make these sort of tunnels and squiggles and mazes that we see and from where they’ve tunneled inside the leaf. It’s quite amazing actually when you think about how tight a space that is and what they’re doing in there and yeah, it’s amazing.

Charley: Yeah. They have to be incredibly skinny to be able to do that.

Margaret: So what kind of insects do this? I mean what kind of groups of insects do these fall into? Because you said larvae, right?

Charley: Yes.

Margaret: I mean larval stage.

Charley: Right. So on columbine (above), they are all flies, and many of the leafminers we see are flies. But there are also moths in 40 different families that have at least some species that are leafminers. And then there are some beetles, about 200 species of leafmining beetles in North America, and sawflies, which are in the order Hymenoptera—they’re like primitive wasps basically. And there are a few dozen leafmining species of sawflies also.

Margaret: So do we call these then host plants? Is that these… Host plants, right? Is that what we… The relationship?

Charley: Yes.

Margaret: O.K. Yeah. I was out this morning just early, early just outside the kitchen door, and a bunch of asters are growing, and there were some beautiful caterpillars. And I always think of you when I see something special because as I said in the introduction, when you took me on a walk a decade ago or whatever and…

I tease you all the time about this, we didn’t get very far on the damn walk [laughter]. We’d go about 2 feet and you’d see something else and you’d say, “Wait a minute, we need to look at this.” And it was like, it wasn’t a lot of progress, but it was a lot of progress. Because I started to see completely differently.

Charley: It’s really true, though. The less you move, the more you see. A couple of weeks ago, my wife and I drove to a place a couple of hours away to find leaf mines on dogwood that we had seen there a few years ago. And some species that’s unknown to science. And we went and we spent hours looking and we didn’t find any there. But then just on Monday I noticed them in my neighbor’s yard [laughter].

Margaret: Wow.

Charley: So really didn’t have to look far.

Margaret: Isn’t that crazy? That’s really funny. Speaking of crazy things: You told me about how you spent the early pandemic lockdown months… We all had to stay closer to home; what were we going to do? Some of us took up knitting or whatever [laughter] and got in more into crossword puzzles or whatever it was going to be. But you set a goal to do surveys, in a sense, on your property, like an acre of space, and to count who was there. Yeah?

Charley: So I wanted to see just how many different species of leafminers I could find in my yard. And I ended up finding 212 was the ultimate tally. And I was also looking for sawfly larvae, which are not just leaf mining sawflies but the free-living caterpillar like ones, too. I’m also starting to work on a guide to those. And I ended up finding about 50 species of those, although, because I’m not done with that guide, I couldn’t put names to all of them.

But there were 50 different types, and I was collecting them and raising them to adults, which is a process that takes a year. You have to let them burrow into soil and refrigerate them over the winter and then the adults emerge the following spring. So I’m slowly getting names on all the sawflies that live around me.

Margaret: So some people in their bookcases in their office, in their home office, have books…and some people have little jars with bits of soil and larva of a sawfly, and that’s you [laughter], that you’re rearing. [Above, one shelf at Charley’s.]

Charley: Exactly.

Margaret: Oh my goodness. Yeah. Well, and so sawflies we should say… Because again, I didn’t really know what they were until I met you. And now I get kind of get excited when I see… Right around this time of year in the Northeast for me. I have a lot of dogwood on the property of various kinds; a lot of the twiggy shrub dogwoods. And I’ll see the different larval stages [above, Charley Eiseman photo], and they’re quite beautifully patterned on some of these dogwood sawflies at different stages. And I’ll see them on the leaves of one of the shrubs or the other.

And I always go up and look up in that part of the property, and look and see who’s doing what, and they’re not hurting anything because why? I mean, this is like you said before: Even though these are parasites, some of this behavior, it’s not harming anything. And it’s not super-harmful. Right?

Charley: And so with externally feeding insects like sawflies, they’re definitely causing more damage than the leafminers or the galls are. But it’s still… Because these are native insects and the plants have evolved together with them that they’re just adapted to be able to withstand that kind of thing.

So I just noticed this birch sapling in my yard that was completely defoliated by sawflies. Not a single leaf left on it. And there’s a willow in my backyard like that, too. And we have spicebush plants [Lindera benzoin] that we planted that the spicebush swallowtail caterpillars have completely defoliated. But all of these plants the next year will just be totally fine.

And most of this defoliation happens at the end of the year when plants have done most of their photosynthesizing, anyway. And even with the swallowtails… It happened earlier in the year, but the plants… I forget now; I think they were able to put out another flush of leaves. But whether they did or not, they came back bigger than ever the next year.

Margaret: And really even just the fact that the spicebush swallowtail larvae… That the animals found those plants, because it’s not like you live in a place that’s 100 acres of Lindera, of spicebush [laughter]. Same thing for me. Years ago I planted those in my garden, and I’ve always had that butterfly and their caterpillars. And it’s like they find it. It’s just that in it itself is miraculous. Let alone the level of understanding that you seek to achieve. It’s pretty amazing.

Charley: Yeah. It always blows me away but they’re little antennae of the adults are just attuned to molecules of the host plant floating through the air. And they somehow triangulate and just zoom right in on isolated plants.

Margaret: It’s just so much of a bigger scheme, so much more complex than we could even grasp. I mean, it’s amazing. And so in gardening, especially lately, we hear about pollinators, pollinators, pollinators, and the flowers, flowers, flowers. But that’s not the whole story, is what I’m hearing from you then. And that’s not exactly what you’re looking at, either.

Charley: And even those pollinators, I mean, if you just focus on flowers, I mean, those insects have a whole life cycle that requires more than just flowers. I mean each bee species has a particular microhabitat it needs. Whether it’s bare soil to burrow into or hollow stems that they go into to nest. And a lot of pollinators have herbivorous larvae that need a particular host plant to eat.

But even setting pollinators aside, all kinds of amphibians and reptiles and birds and mammals are insectivores, and they need insects to eat. And most herbivorous insects are very host specific and need particular plants to feed on. So you need plant diversity to have good diversity and quantity of insects to support the whole rest of the ecosystem.

Margaret: Right. All the way up. All the way up. Yeah, absolutely. And these are ancient relationships. And as you point out, they’re often very intimate and specific, but they’re ancient. I mean, these have been going on a very long time. And the spicebush is still surviving [laughter].

Charley: Right, exactly.

Margaret: Right? So some of the galls are pretty amazing looking. Some of the leaf mines are so beautiful I can’t even… And I’ll just say again about doing the “New York Times “column. I’m always worried when I suggest these kind of geeky offbeat stories that aren’t, “How to plant petunias.” Or, “How to grow tomatoes.” I always worry that my editor will say, “That’s too crazy, Margaret.” But she didn’t, and she wrote me a note to say how beautiful the pictures were. How miraculous it was. And she’s not… this isn’t her thing, but she was transfixed by them.

So that’s the great thing is that when people do take the time to slow down and key in, like when you help me with that walk years ago, it opens up a whole other world of seeing, I think, too, to people, doesn’t it?

Charley: Yeah. Oh that’s great to hear that you got that response.

Margaret: Yeah, yeah. No. It’s really wonderful. So even though when people see galls, especially: panic is… So you see a leaf and it’s got these weird blisters, these raised compartments. And there are different colors and they’re screaming like, “Disease, disease. Or, “Panic. panic.” [Laughter.] So tell us about some of the other crazy gall things. I love all the “oak apples,” too. I’m picking up some as I rake things. I always find them at this time of year. Some of the faded ones, too.

Charley: Yeah, well there are some really neat… Both some wasps and some midges make these complex detachable galls that will actually drop off the leaf when the larva is mature inside. So most of the wasp ones are on oaks, but then there are midge galls like this that drop off of grape leaves. There are about 50 different species on hickories that come in all these fancy… Some are kind of thorn-like shapes and some are bullet shaped and spherical, fuzzy ones and sticky ones. And then there are on hackberry, a similar diversity. A few dozen species of midge galls on those.

Margaret: And midges are tiny. Almost looks like a mosquito. Like a tiny little fly thing. Is that what the midge is?

Charley: Yeah.

Margaret: O.K. And again, like the “oak apples” as they… I think, to kind of loosely call them because there’s different ones. They are quite hilarious looking. I mean you think, “Who made this? How could this be? This sphere, this ping-pong ball?”

Charley: Yeah. And some of those green ping-pong balls with purple polka dots you see, those come out of leaf buds. I mean, sometimes the whole leaf is consumed by the gall, and sometimes it’s just kind of stuck on the leaf. But they’re similar-sized ones that are hard and have an apple-like flesh inside. And those actually grow on the sides of acorn caps . We usually don’t get to see them in situ because they’re up in the canopy. But if you happen to see a shrub, like a scrub oak, sometimes you get to see those at eye level. And there will be three big like inch-wide galls all stuck onto a single acorns cap.

Margaret: Oh, see, I haven’t seen it on an acorn except in pictures that you’ve shown me. I haven’t seen it in person on acorn. Yeah. Cool. So I know you teach, you do lectures, you do workshops, and I think often they’re for professionals, but I think you also sometimes do it for laypeople, yes? Gardeners or just people who love the outdoors?

Charley: Yeah. Most of the workshops I’ve done have been more for laypeople or sort of enthusiastic naturalists. I’ve done a few insect tracks and sign week-long seminars at the Eagle Hill Institute in Maine, open to whoever wants to spend a week staring at tiny esoteric things.

Margaret: Right. And not walking very far, very fast. [Laughter.] And so not good for exercise. This is not. So how do you tell people to get started? I just wanted to quickly say, this is a great time of year to look around. Like I said, I just went 3 feet outside the back door and there were these great caterpillars this morning, these brown-hooded owlets [below, at Margaret’s]. There’s lots to see. So how do you tell people to get started? Do they… Camera phone? Hand lens? Is there anything we need to bring, or?

Charley: Yeah, I mean, if you have a camera that does a decent job with… I mean, a lot of phones have decent macro capabilities. Just start taking pictures, and if you have no clue what you’re looking at, iNaturalist or bugguide.net are great places to just start posting pictures, and people will add comments and help you narrow in on an I.D. And then once, if you get excited about a particular group, there are all kinds of great books out there.

Margaret: And is, I mean, some people will probably be more attracted to the leaf mines and some people to the galls. I mean, it’s all very interesting. I remember taking that walk with you and we saw a perfect little circular leaf cut in the edge of… well, actually a hole cut in the edge of a leaf and it was a particular type. You right away keyed in on it. And you said, “Oh, that’s from a particular type of bee,” I think. Do you remember?

Charley: Yeah. Leafcutter bee.

Margaret: Leafcutter bee, right. Yeah. I mean, you even look at spider webs and have insight to who made them [laughter]. How did that happen? That’s crazy. I love spiders so that would fascinate me. I guess I better start learning about their webs.

Charley: Yeah, when I was working on my first book, I just started going through the insect section in different university libraries or the invertebrate sections, I guess. And I found… There was a great old book by Comstock. I think it was published in 1910 or so, just called “The Spider Book.” And I just went through each kind of spider and described and illustrated, in many cases, that the egg sacs and the webs.

And I went through that in other similar books and just kind of jotted down all the descriptions of the webs and egg sac and then tried to rearrange it. So if you found this thing and didn’t know what it was, how could you… I made categories to try to guide people to identify these mysteries.

Margaret: Well, and they are mysteries, but there is a master plan. I mean, clearly a very complex master plan going on out there [laughter]. At this time of year in your work practice and your personal other projects, what are you keying in on? And again, we’re both in the Northeast, so it is sometimes regional, but what are some of the things right now that people should look for?

Charley: Well there are certain leaf miners that are only active at the end of the season, just before the leaves drop. So a fun one to look for, which actually will be a few more weeks before they become evident, but there’s a tiny moth that mines… Or a couple of related tiny moths that mine in aspen leaves. And as the leaves are starting to turn color, they have inhibited the plant from withdrawing its chlorophyll from a patch around where the leaf mine is.

And so when the leaves fall, you’ll see this what’s called a green island surrounding the leaf mine, and the rest of the leaf is yellow. So if you look closely, you’ll see the mine in there, and the larva actually does most of its feeding after the leaf has dropped to the ground. And you’ll find all kinds of other leaves with a little patch of green still on them. And if you look in that patch of green, more often than not you’ll see a leafminer, or sometimes a gall. A gall can cause a similar effect. So that’s a great thing to look for at this time of year.

Margaret: Huh. That’s crazy. And are you doing any particular project right now? Anything new, or is it mostly will work on the iNaturalist project? I mean that is… Maybe we should just take the last minute or two to just talk about your North American Leafminer Project on iNaturalist. I mean, it’s got tens of thousands of submissions. And I mean, it’s got a lot of activity going on.

Charley: Yeah, I’m working on catching up or I have been kind of all summer. Been playing catch-up, but I’m about three weeks behind right now, and there are over 2000 observations to go through. So it definitely helps me a lot when people put some effort into identifying things, so I can just click “agree” if they have it right. But I don’t want to discourage people from posting unknown things [laughter], but that’s my current situation with that. But as we get into winter, I’ll easily get caught up on that.

Margaret: And as you said, a lot of people go to iNaturalist because you can submit a picture and it can often be identified pretty quickly. But that can’t happen unless there is this database of similar pictures that make something recognizable I guess, right?[Above, blister galls on dogwood from a gall midge, Parallelodiplosis subtruncata. Charley Eiseman photo.]

Charley: Yeah.

Margaret: It has to… So that’s why this is so important to compile all these pictures of let’s say, leaf mines and leafminers. I mean, until there’s this critical mass of information to compare to, the more automatic almost identification isn’t going to be possible, because there’s no reference. Just like when you haven’t done the book yet. Your original book.

Charley: Yeah, it’s been neat to watch the computer getting better at… It seems like more often than not, if you post a picture of a leaf mine, it’s going to suggest a leafminer now as the species, rather than the plant that you posted, which that’s amazing progress already. And it is getting better and better at the more common species. I definitely wouldn’t trust it, but it’s pointing you in the right direction.

Margaret: Yeah. Well, Charley, that’s because you started this project, which is great. And I so appreciate, as I said, you’re coming indoors and talking to me this morning. And we’ll have the giveaway of “Tracks & Sign of Insects And Other Invertebrates,” which is definitely the standout field guide in my overstuffed cabinet of field guides. So thank you so much. And I hope I’ll talk to you again soon.

enter to win a copy of ‘tracks and sign of insects

I’LL BUY A COPY of “Tracks & Sign of Insects and Other Invertebrates” by Charley Eiseman for one lucky reader. All you have to do to enter is answer this question in the comments box below:

Have you noticed any tracks or sign of insects, like galls or leaf mines, on a plant in your garden? Do tell.

No answer, or feeling shy? Just say something like “count me in” and I will, but a reply is even better. I’ll select a random winner after entries close Tuesday October 11, 2022 at midnight. Good luck to all.

(Disclosure: As an Amazon Associate I earn from qualifying purchases.)

prefer the podcast version of the show?

MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 13th year in March 2022. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the Oct. 3, 2022 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes/Apple Podcasts or Spotify or Stitcher (and browse my archive of podcasts here).

  1. Randee Pieper says:

    I have seen leaf mines this summer, but it was on a plant I couldn’t identify in a public garden…and didn’t have my iPhone with me! This was a very interesting conversation!

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