coping with invasive jumping worms, with brad herrick of uw-madison


THE QUESTION “What do I do about the Asian jumping worms that are destroying my soil?” has outpaced what was the most common thing I was asked, year in and year out, for decades as a garden writer—the relatively simple challenge of “How do I prune my hydrangea?”

Now gardeners from an ever-widening area of the country are voicing this far more troubling worry about an invasive species that seems to be on a mission of Manifest Destiny.

Today’s guest, ecologist Brad Herrick from the University of Wisconsin-Madison Arboretum, has been studying jumping worms for a decade and is here to share the latest insights. Brad is the ecologist and research program manager at the UW-Madison Arboretum, where the staff first noticed the destructive handiwork of Asian jumping worms in 2013.

He explained what tactics and products have been explored by researchers to try to limit the spread, and whether we should keep mulching and improving our soil as we always have despite their presence, and more.

Read along as you listen to the Sept. 11, 2023 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).

coping with jumping worms, with brad herrick


Margaret Roach: I’m always glad to talk to you. Sort of. Welcome back, Brad. I wish we could talk about happier topics someday together [laughter].

Brad Herrick: I know. Someday we will.

Margaret: I know. Can we talk about a plant or something someday? Oh, my goodness. Remember when we were just gardeners, not warriors, right?

And I learned about the worms, from—I think it was called Great Lakes Worm Watch organization—I learned online about the jumping worms in around 2013, and I contacted you. I don’t even remember how the trail led there.

I think we first talked in around January 2018, and we’ve discussed these invasive pests a few times before on the show and so forth. But just so that if people don’t know about them, just the really brief version of what are they, what’s the main problem that we’re talking about?

Brad: Yeah, so jumping worms, or snake worms as they’re commonly referred to, are kind of a new group of segmented earthworms that have made their way over to North America from parts of Asia, primarily Japan and the Korean Peninsula and parts of China. And they’re spreading quite quickly and over wide ranges. So they’re found right now in about 38 states …

Margaret: Oof!

Brad: …and several Canadian provinces.

And the main concern is that they consume organic matter, which a lot of organisms do, all earthworms do, but they do so in a way that really depletes the organic matter quickly. And so whether it’s a forest or your garden, they consume it very rapidly.

And then through that process, they really change the soil structure and the nutrient and carbon content of those soils. And those are impacts that have cascading effects to our plant health, to other organisms that live in the soil, to carbon that might have been in the soil that now is released into the atmosphere just through their burrowing activities and soil mixing.

And it is also important to note that where we’re seeing some of the biggest changes, especially in forests, are in areas that never had native earthworms to begin with. So areas that were in the last glaciation, those glaciers exterminated any native earthworms. So this is the last 10,000 years or so. And so forests, for example, in the Upper Midwest and New England are evolved without the presence of earthworms. And so these are novel critters coming in, and really fundamentally changing the ecosystem.

Margaret: Yes. Forest, you say, and that’s a place where leaf litter [laughter] is right?

Brad: Exactly.

Margaret: It’s the recycling, the natural slow recycling, and all the detritivore organisms, the helper creatures that do that, turning the organic material that falls down back into feeding the soil and the cycle repeats itself. And it’s the basis of all of our, really, of the environment.

So the soil layer is destroyed. And the thing is that when you have these worms, as you just said, they do such a quick job of processing the soil and they turn it to what looks like sort of a cross between coffee grounds and hamburger meat. It doesn’t have the same tilth as it used to have, and it almost feels like the plants are loose in the soil, things aren’t holding.

And I think in the past, you’ve told me that in places like the forests of the Great Lakes and in the Smoky Mountain areas where they are been present a long time, that the saplings can’t even get a root hold in because the soil is disturbed in areas where they’re very concentrated, or something like that.

Brad: That’s right.

Margaret: That sound about right?

Brad: That’s right, yeah. So you have, for example, sugar maple seedlings, any kind of seedlings really, woody species, have a hard time getting established in that upper layer of the soil, which is exactly the area that those jumping worms are continuously turning over, incorporating nutrients and making, which you think might be a good thing, incorporating nutrients. But what they’re doing is they change the soil, like you suggest into this coffee ground-like, really porous soil, and there’s just nothing for the roots to hold onto. And what we find often is that soil, it’s so erodable because there’s nothing holding it in place. It just gets sucked away. And you’re almost left with just mineral soil, which is very inhospitable for a lot of them.

Margaret: So I had a reader write to me last week actually, and it was a very emotional note, she honestly said, “I want to sell my house. I want to get away from what I’m seeing.” And very, again, very sincerely said, “How are you coping?” She said, “It’s very emotional and it’s terrifying.”

And how do people cope with watching this decimation and feeling powerless? That increasing number, you said 38 states, the last time we spoke a couple of years ago was fewer than that. And I remember when they first came to your, you noted them on the campus, they moved acres within a short period of time. Correct? They spread exponentially.

So it’s heartbreaking. It’s terrifying. And I know there’s research going on in a lot of different places, but what are you guys, you’re a decade in, what are you doing; are you doing anything? Are you going around picking up worms and destroying them? What are you doing? I know there’s different materials that are being suggested to apply and so on and so forth. What are you doing?

Brad: Good question. It really depends, in terms of control or management of these critters, it really depends on specific and local sort of situation. So whether you’re talking about a forest, or someone’s urban garden, or maybe a larger organic garden, it really depends on the situation.

For us at the arboretum here, where we’re talking about acres and acres of sugar maple-basswood forest being invaded, we’re not talking about control anymore. That’s not a feasible thing, and we have to accept that. And what’s interesting is we don’t really see, just walking through the forest, a change in the vegetation.

Now, I have seven years of data that I’m hopefully analyzing this fall, which may sort of pop out some effects specifically in terms of what’s happening at the sapling-seedling layer.

But the plants are just one part of the ecosystem. And what happens with earthworms, I think, is that their impact can often be less obvious in the short term than something like spongy moth or Emerald Ash Borer, because they’re really changing nutrient cycling and affecting the microbial community and the fungi relationships, all these things that eventually will impact a lot of different things.

It’s hard to kind of talk about the impacts because while we see some impacts in certain situations, it may be that it’s a longer-term situation where we’ll see impacts later on.

Now that’s in our specific urban sugar maple forests. In gardens, where we’ve done some work as well with folks, we’re seeing much more of an impact more quickly. But it does, again, depend on the situation. So it seems like for folks that have had jumping worms for four or five years, where at first it didn’t see much of an impact, now they’re seeing all sorts of impacts to even plants like hostas, which are the poster child of hardiness and ability to transplant them. And again, transplanting anything into a really porous soil medium, it’s not feasible. And I feel for the person.

Margaret: My reader, sure.

Brad: It’s very much an emotional response.

Margaret: And, again, research has gone in terms of… And we should say that these are annual creatures; the adults die in the winter, but they leave behind these cocoons of eggs that are so small, they’re almost invisible. So that’s how they perpetuate year to year. And that different scientists, researchers such as yourself, people have experimented with extremes of heat or cold to kill the cocoons, the overwintering cocoons or blah, blah, blah, other things.

To bring the worms to the surface, everything from mustard powder, and then collect all the worms and kill them in plastic bags in the sun to fertilizers, natural fertilizers that are not labeled for the purpose, like tea seed meal, I believe. And then sharp, sort of irritating to the skin of these animals. Soil amendments like diatomaceous earth and bio-char sort of ground-up charcoal, all these things that you read about.

But is there any thought that, again, in the garden setting, that any of this has any is worth doing? Because none of it’s cheap and none of it’s easy. So you said “we’re not talking about control anymore.” So anything to say about any of those?

Brad: We’re not thinking about control in some of the forests, but the gardens, it does depend on how invaded your garden is. You can still do a lot of good if you catch an invasion early. And so again, I always kind of harp on education, and understanding what these critters are, identifying changes in your yard. Because there are things, so there’s a couple of things that have been researched, relatively new.

One is a product that’s been on the market for a while called BotaniGard, and this is an entomopathogenic fungus, which is the fungus called Beauveria bassiana; it’s a naturally occurring fungus in the soil. And there’s a research paper out now that showed that this product as well, as just the fungus isolate itself, is pretty effective at killing the actual jumping worms.

It’s as like a granular application that you irrigate in, or you spray in. And the benefit of that is that it’s already labeled for this use. It’s been in the market for a while. It does affect all soft-bodied pest organisms; so there’s always that to know about. But that will work. That’s an approach with some science behind it.

And like you mentioned, we also know that heat works, and heat is probably the best solution we have right now. It addresses both the live earthworm and its cocoons, which you mentioned earlier.

Margaret: So we could solarize in spring into summer, we could solarize a raised bed, for instance, that we grow vegetables in; let it be fallow, but solarize it, something like that. But we can’t do that in a planted ornamental bed that’s densely planted. Do you know what I mean? We can’t solarize it. So where there are no plants, and agriculturally, the same thing would be true. Bare soil, I guess?

Brad: Exactly.

Margaret: Can be there for sure. Or if you want to take everything out of an area, you could do it. But what I find is that they—and I’m probably in the five-year mark as well—some years like last year was very dry here, and it was almost as if they had gone away. And this year, which has been very, very wet, it’s as if they’ve quintupled in a minute and they’re prospering.

So it’s not the same year to year, even in the same place. And the population explosion though, seems pretty impressive. They really move.

Brad: And that’s sort of what’s sort of disheartening, is we’re in our second year in the Midwest here of drought, and our numbers are way down.

Margaret: That’s exactly what happened here last year.

Brad: Exactly. Yeah. And then you mentioned you guys have had a lot of precipitation this year, and that just shows that what’s happening is those cocoons are forming a cocoon bank where they’re not going to hatch until the conditions are ideal. And so one year might look great, it’s like, “Oh, I’ve taken care of them. This drought took care of them.” And the next year, like you’re saying, you see a quintuple of earthworms coming out, just because those cocoons are able to rehydrate and hatch again.

So yeah, it’s really frustrating in that sense. And that’s the kind of thing that solarization or some other heat treatment that can get hotter than just the ambient air temperature, where it not only dries them out, but it actually kills them. Even folks are using things like torches, [laughter] or I’m thinking about how do you steam the soil sterilize?

Margaret: Well, people with greenhouses do that. If they get a fungal pathogen in a greenhouse, like my flower-farming friends nearby, they’re organic farmers; they can’t use most fungicides, so they have to rent a steamer. Yeah, exactly. But for our basic garden, so the BotaniGard is one possibility, but some of these other, either these sort of irritants or the tea seed meal or whatever, is that just going to bring them to the surface? Is that the idea of that as well? I don’t really know what it even does or if it has any efficacy or not?

Brad: So the tea seed meal, similarly with people who have been using a mixing of vinegar and water, or soap and water.

Margaret: I’ve seen mustard powder and water being recommended, too [laughter].

Brad: Yeah, mustard powder and water. So the mustard powder and water and the tea seed meal will bring them to the surface. Mustard powder will not kill them, but the tea seed meal will, as will the vinegar and water and the soap and water.

We don’t think that the tea seed meal impacts the plants at all, which is a good thing. Same with the mustard powder. But the vinegar and soap and water could very well, and that’s the other. There’s always a trade-off.

Margaret: Well, and it depends on where you apply it. So again, if it’s along your walkway and it’s among the cracks in the pavers, okay, you could probably do the vinegar, etc., but yeah.

Brad: You want to stay away from the plant root zone.

Margaret: Right. It’s a pretty gruesome experience also, I will say, the collecting of worm bodies [laughter]. It’s fragrant. It’s also extremely fragrant in summer. And the other thing is, and this is really crazy to think about, but I do think about it, is so, again, those adults are going to die anyway in the winter. They’re annual organisms. So at a certain point, those cocoons have already been laid, or whatever the word is.

So you could put lots of this stuff on and gather up all these hideous, smelly dead worms that are decaying and so forth. But they’ve left their mark, haven’t they? So I guess this would be a multi-year thing to do, until you diminished and diminished and diminished the adult population to the point where you didn’t have as many cocoons. It would be a multi-year… It wouldn’t be a one-and-done at all?

Brad: That’s right. That’s an important message, Margaret, that it’s not, and this is the case with any invasive species, frankly.

Margaret: I agree. Yes.

Brad: It’s hardly ever a one-and-done thing. It’s a multi-year process, and what you’re basically wanting to do is deplete that cocoon bank over a number of years. And I think that’s the way to look at it. And I could see that in theory being effective, depending on how large of a scale you’re working on. At some point, it just becomes too labor-intensive maybe. But yeah, I think that’s a good way to look at it.

There’s things that are really labor-intensive, like physical barriers. Driving in—there’s experiments that have experimented with jumping worms in forests by putting in metal flashing little quadrats to experiment with. They don’t want jumping worms going into these, testing the soil without them. And so putting that kind of physical barrier in down to six inches, so they’re not—these are only surface-dwelling earthworms, and they’re not usually going to go very deep. And so if you’re like, “My neighbor has them, I don’t want them,” you could spend some time and put some kind of physical barrier around your raised bed.

Margaret: A worm fence [laughter]!

Brad: A worm fence.

Margaret: Underground though.

Brad: It’s probably extreme, but it’s something that could be done.

Margaret: I want to make sure to talk about the other side of the equation of what to do in the powerlessness and the worry and so forth. And I’ve read different things about this. Do I add mulch at the times when I normally do? In other words, do I “feed them” more organic material, or do I just leave them there in their degraded soil? Or do I add more soil on top of where the soil has receded? What about that we have our usual methods of caring for our soil and garden beds, but what do you think about that? Feed them, don’t feed them?

Brad: That is the question. I would say mulch is a food for these critters, but really at the end of the day, we want to be able to enjoy our gardens. We want to garden, and by enjoying our gardens, we want to make sure that the plants are as healthy as they can be. And mulch is a really important part of that.

Margaret: Yes. Serves other purposes, yes.

Brad: Absolutely. And one thing to try is experimenting with different types of mulch. So all mulch, all organic matter is not created equal. So leaf mulch has a very low, depending on the type of leaves, has a lower carbon-to-nitrogen ratio than something like grass mulch or hay mulch. So the leaves break down faster, and the earthworms are able to consume it more readily. Whereas native grass mulch or hay or straw mulch, even pine mulch, pine needles, aren’t going to break down as fast and aren’t as palatable to jumping worms or any earthworms.

And so trying some different types of mulch, I think, is a good first step to see, “Hey, maybe I found something that sticks around for a while,” and the earthworm don’t do as well.

Otherwise, that’s one option. The other option is just stick with, if you already have them, stick with your mulch system that will… at least maybe the benefit of the mulch will outweigh the negative part of the earthworms, and those plants will still be able to do O.K. and persist. So it’s like too much of a good thing will overcome a negative thing.

Margaret: That’s the direction I’ve gone, because I can’t watch the… You may have a beautiful established tree and suddenly it’s got surface roots because the soil’s receded. Do you know what I mean? I’m like, well, not that I’m going to bury the tree and suffocate it. I don’t mean that, but I want to keep it at the level it was at.

Brad: One thing that I’ve just started thinking a little bit about is as we learn more about which plants, which plant families, which plant traits, are more susceptible to jumping worms, we might think about where these jumping worms come from, which are parts of Asia, and are there ornamentals from that part of the world that might be just fine coexisting with jumping worms, and that maybe we think about shifting some of our garden targets to plants that do just fine in the presence of all sorts of disturbances [laughter], and maybe not the ones that we traditionally wanted to garden, but maybe there’s an opportunity for some new gardening ideas and practices.

At some point, we have to probably accept some level of this jumping-worm pressure, at least in some of our gardens, and see how we can adapt to that, which is not always what people want to hear.

Margaret: No, but it’s realistic, because again, the first few years we were talking more about like, “O.K., don’t spread them. Don’t share plants, don’t do this, don’t do that.” But in my region, in New England, I’m hearing from people everywhere who have them. It’s super-widespread. And similarly, I think, in the area adjacent to where you are, there’s a lot of…

Well, Brad Herrick from University of Wisconsin-Madison Arboretum, like I said, I want you to tell me, you should email me and tell me what your favorite plant is, and we’ll do an episode on that, instead of talking about these damn worms all the time [laughter].

Brad: Sounds like a deal.

Margaret: O.K. And thank you so much for making time. I know you’re swamped, and thank you so much.


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 14th year in March 2023. 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 Sept. 11, 2023 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).


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