invasive asian jumping worms: a 2019 research update, with brad herrick of uw-madison
SO-CALLED CRAZY WORMS or Asian jumping worms, several invasive earthworm species that are spreading alarmingly in many areas and degrading soil and natural habitats, are probably the most common pest question I get from readers and listeners in recent years. Many of you have asked specifically, “How can I stop them?”
To find out what scientists know so far, I called researcher Brad Herrick of University of Wisconsin-Madison, who gave us a 101 last year on these destructive worms, and has since published some new insights–specifically about the impact of heat on the embryo-filled cocoons that contain next year’s worms-to-be–that may in time help lead to answers in some situations.
Brad is Arboretum Ecologist and Research Program Manager at the University of Wisconsin-Madison Arboretum, where the staff first noticed the destructive effects of Asian jumping worms in 2013. He’s been studying them ever since.
Learn how to diagnose the presence of Asian jumping worms; what corrective tactics you can try to reduce the population if your infestation if limited to a small area of a garden; and what directions are next in research about these serious soil-wasting pests in hopes of finding larger-scale controls.
Read along as you listen to the August 12, 2019 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on iTunes or Spotify or Stitcher (and browse my archive of podcasts here).
invasive ‘crazy worms,’ with brad herrick
Margaret Roach: Welcome back to the show, Brad. I wish we had a more optimistic topic to discuss.
Brad Herrick: Yes, I know. Well thank you for having me anyway.
Margaret: We can talk about plants someday. Right? Cause I know you-
Brad: Yes, that’d be great.
Margaret: I like plants and you like plants.
Brad: That’s right.
Margaret: I don’t like Asian jumping worms, Brad. Oh my goodness.
Brad: Yes, I know it’s one of those things that is another invasive species, but from the research end of it actually it’s very interesting to study and try and get some information out to folks.
Margaret: Yes. So in case people don’t know the basic issue with these worms, because you know a lot of gardeners as we talked about last time we spoke think, “Oh, earthworms are good, blah blah blah.” Can you explain why these species are so worrisome? And they’re not something that’s new, it’s just something that’s reached sort of a tipping point, I guess, where we know more about them now.
Brad: Yes. Exactly, yes, yes. So definitely in terms of the gardening community, we’ve been told for a long time that earthworms are actually a really beneficial component of the soil environment, right? They create—through their burrowing, their feeding behavior, they’re creating castings—they produce a lot of nutrient and they work with nutrients throughout the soil column, they mix soil. And they provide … their burrow provide areas for air to reach the roots, and water to reach the roots of plants. So generally they have been thought of as doing a kind of beneficial ecosystem service, if you will, for our vegetables and our garden plants.
And that’s still the case, except that now we have kind of a new twist on it where we have these newer invasive earthworms from parts of Asia that don’t behave quite the same way as the earthworms that we’re used to.
And what I like to tell folks is that even earthworms that are in your garden, if you live in glaciated areas of like New England area or upper Midwest, where I’m from, there are really no native earthworms.
Brad: Because there are very few. It’s thought that the last glaciation kind of extirpated them, and so our systems aren’t really familiar with earthworms and don’t … especially our high-quality areas don’t need earthworms to function healthily.
Brad: So anyways, so these … even earthworms in your gardens are also non-native, but they do perform a benefit. These new earthworms, they don’t do the same kind of burrowing. They don’t burrow vertically into the soil. They’re not really good at mixing nutrients or mixing soil. They do all their work in the upper few centimeters of soil, or even above the soil within whatever organic layer you might have in your garden. And so, and they turn over, they eat the organic layer, whether it’s leaves or mulch, decomposing wood, they eat it really quickly, especially when they get to high numbers later this summer. And they turn it over so fast that they release all those nutrients really quickly, like a quick-release fertilizer.
And it’s often two things happen. One is they release it so fast and they’re not mixing that nutrients back into the soil. And so it’s often lost through heavy rain events or just not available to the roots, which are lower in the soil. And they’re often doing most of their work later in the summer, when really plant growth isn’t kind of the main thing that plants are doing. They’re thinking more of storing nutrients for the winter if they’re perennials. And so it’s sort of a … there’s a bit of a mismatch in terms of timing as well.
Margaret: Out of sync. Really out of sync.
Margaret: And they kind of reduce the soil to almost like … I don’t know, it’s like a … as you say, they take out all the organic matter. So I’ve read these reports in Great Lake Forest and in the Smokey Mountains and places, natural areas, where they talk about “tree root gingivitis,” where the roots of the trees can’t even get ahold and stay-
Margaret: You know, the soil is being so degraded. [Above, a chart from Wisconsin Department of Natural Resources on the impact on forest soils and ecosystems.]
Brad: Yup. I was going to say in highly degraded areas you’ll see tree roots that are above the soil. You can actually see the roots.
Brad: They’re not actually … because all that organic layer has been removed, so you’re down to mineral soil. It’s problematic.
Margaret: So I’ll link with the transcript of this conversation that we’re having today to our previous one that has more in-depth, sort of the biology and so forth of these worms. But I saw a map on, I think it was like maybe Oregon State University’s website, I’m not sure, that showed what states are affected. And I was curious, it didn’t say what date it was or whatever. I wondered how wide-ranging is the impact of these worms now? And there’s two genera, are they from two different genus of worms, is that what’s going on?
Brad: Yes. There’s I think we have confirmed around 17 species of what are called pheretimoid earthworms, which are these Asian earthworms. And there’s many different genera. We have primarily two, I believe. So Amynthas is kind of the large genus with many species that we have in the U.S., or actually North America. And then there’s one called Metaphire, which as far as I know, we only have one species of Metaphire here in North America.
Brad: And there’s kind of this invasion happening, it’s a co-invasion with three main species. And the three species are Amynthas agrestis, Amynthas tokioensis, and Metaphire hilgendorfi. And that’s getting kind of technical, but they all live in the same area of soil. They’re all functionally kind of the same.
They do the same kind of things in the soil. They’re just different sizes and they can often exist in different abundances. But we’re learning, we’re trying to learn more about if there are differences in terms of how they affect the environment based on their size or feeding behavior.
Margaret: Yes. So in this map that I saw, it had these states colored in a particular color where they are present, and again, I don’t know that … it didn’t have a date and I just couldn’t trace the origin of it, and it seemed like there was the Eastern part of the United States a lot, the Midwest a concentration. And Nebraska I read about, and in Nebraska … it seemed like it was in a lot of different places. So I don’t know how widespread we think it is at this point.
Brad: That’s kind of … yes that’s … what you described there I think is what we know. I mean, definitely the Southeast-
Brad: Up the Eastern Seaboard of states, and now throughout the Midwest into like the central part of the country. I know Oregon has them.
Margaret: I saw it, I saw Oregon as the one [Western] outpost on this, again, undated map, I saw Oregon was, and that’s why Oregon State University had this document, so it’s not just localized to where you are or where I am or whatever. And more and more people are, I think, reporting it. So probably we’ll find out that there’s even more than we know.
Margaret: Yes. So I watched a training webinar the other day with the organization called iMap Invasives, do you know that?
Margaret: And iMap Invasives and we have a … in New York where I am, we have a particular group related to that and it’s like a training online about certain invasive species and one of the ones they were covering was these worms. So I watched, and they gave some sort of layperson tips for ID’ing them, for sort of what do you want to look for? What … now that you’ve been setting them since 2013, if people ask you, other than doing DNA analysis or whatever-
Brad: Yes, right.
(Note: The jumping worm portion of the webinar above begins at about 13:23.)
Margaret: Like what do you say the telltale signs of these worms versus the other earthworms that we talked about?
Brad: Yes, so a couple of things, and it somewhat depends on what time of year you’re looking for them.
Brad: But the first thing that you can see any time of year is this specific soil signature. And that is these Asian earthworms produce a very coffee-like, or coffee ground-like that signature in the soil, really loose, granular soil that’s actually made up of their castings, their excrement [below]. And so they create this layer of really loose soil, really granular soil, which all earthworms produce castings, but most earthworms produce little … kind of little casting hills or little bumps, kind of sporadic around the landscape. Where jumping worms just create kind of a homogenized, uniform look to it. That’s one thing that you can see in the winter, if you get a snow melt, you can … As far as we know that’s a permanent change to the soil. So that’s one thing.
And then as they mature, so in Wisconsin they’re almost … we’re almost seeing mature adults [in July]. So Amynthas, or jumping worms, generally if you look … if you have one and you look towards the head, even if you can’t figure out which end is up, there’s a white ring around one of the ends.
Brad: That goes all the way around the body, and that’s called the clitellum, and that’s where they produce cocoons that are the new offspring. It’s kind of the reproductive center of the earthworm. And all earthworms have one, but most earthworms, their clitellum is kind of the same color as the rest of the body and it’s raised, and it doesn’t go all the way around. If you turn the earthworm over, it’s kind of like a saddle where it doesn’t connect underneath.
Margaret: Right, not a band.
Brad: Not a band, right.
Margaret: Right, right. [Below, from Wisconsin Department of Natural Resources, photo of common nightcrawler, top, and Asian jumping worm beneath it.]
Brad: The jumping worms all have a band and it’s kind of a milky-white band when they’re fully mature adults. That’s a telltale sign. There’s no other earthworm that isn’t a jumping worm that has that kind of structure.
And then lastly, just their behavior. They’re called snake worm or jumping worm for a reason. They can be very erratic, very … They’re not aggressive, they just don’t like being handled, and they will flop around and they’ll wiggle away. They’ll even try and drop part of their tail, the last several segments, to escape being handled roughly, where other earthworms are kind of wiggly, but they’re not actively flopping around and trying to get away from you.
So … and I guess maybe more thing to mention is that early in the spring, April, May, if you’re seeing fully formed earthworms, large earthworms, those won’t be likely, won’t be jumping worms because jumping worms are an annual species, and so under normal climate conditions they’re going to be hatching from cocoons. They’ll be really tiny in the spring, and won’t be full size until the middle of the summer.
Brad: Or end of the summer even. So any large earthworm you’re seeing in April, May is some other species that is not a jumping worm.
Margaret: Yes. On this training, this iMap Invasives training webinar, one of the things that they showed was the movement that you were just talking about, and they had … I don’t know whether it was a night crawler or some other kind of earthworm, and an Asian worm, and they compared. And it’s as if the older earthworms, they kind of move forward like an inchworm, and not exactly, they don’t curl up their backs, but they move forward in a forward-movement direction as opposed to sort of this sidewinding, snakelike wriggling thing, this-
Margaret: Do you know what I mean? I mean, that was very dramatic.
Margaret: As you were saying, so here we are, a lot of us have them, more and more counties each year as I read about them. I’ve been reading about them for a number of years, and again, I’m seeing more places on these maps and more reports from different cooperative extensions, and university researchers, and so forth in more places.
So you are exploring with your colleagues there at Wisconsin-Madison, you’re exploring some directions of “What do we do?” Not expecting an instant answer but, and I saw you recently published a paper about the heat tolerance of the cocoons, the sort of reproductive … the little egg sac-y kind of thing, isn’t it? [Laughter.] The cocoons of these worms; you’re exploring that. I wonder if you could tell us a little bit about what you’ve been doing and what have you learned so far?
Brad: Sure. So we’ve been working closely with our Department of Natural Resources, and they’re obviously very interested in statewide spread, and if there are ways of controlling the spread. And so we’ve gotten some funding from them the last few years to explore that question of: Is there a way to control them? And if so: What’s the most effective way of approaching that question?
And so we think that given that these are annual species, like I mentioned, and they overwinter as cocoons, which are just little 2 millimeter size brown, like you said, kind of sacks that hold the embryo. We think that they’re being spread primarily through people moving cocoons inadvertently.
You can see the earthworms and you can avoid spreading them. The earthworms themselves won’t stick to your shoe or the shovel that you were working in, but it’s going to be the cocoon that gets stuck easily in soil and then can be easily moved, or even during rain events they can move over land. They can just come with the soil if you have erosion. So and they’re so hard … you can’t see them in the soil. They’re so hard to find. So we thought is there a way that we can sort of throw a wrench into that part of their lifecycle, and focus on those cocoons?
And what we looked at with this most recent paper is that many states, including Wisconsin, regulate compost. So large commercial composters have to follow statewide standards that are pretty standard across the U.S. in terms of state policy. But that is that depending on the type of compost pile that has … the pile has to be heated up to 131 degrees Fahrenheit, which is 55 degrees Celsius, for a period of three or 10 days, depending on what type of pile. And that includes mixing it periodically.
And that standard is there to minimize the potential of having harmful bacteria, or fungi, or disease in the compost that may hurt your soil. And so our thought was, well, at that temperature, can these cocoons still remain viable?
We know from previous research that earthworms themselves cannot withstand that kind of temperature. And so we basically … and in the lab collected hundreds of cocoons and subjected them to different temperature regimes from 20 degrees Celsius to 55 degrees Celsius for three or 10 days. We ran that for several months. And then we dissected each cocoon to see if there was an embryo, or an actual worm, or what was happening within the embryonic tissue.
And what we found was that at 40 degrees Celsius, which is a 104 degrees Fahrenheit, anything at that temperature or above contained no viable cocoons. There was not one cocoon that had a viable, from what we could see, a viable embryo or an earthworm. And so that’s … I would say 104 Fahrenheit is much less than 131 degrees Fahrenheit. And so the temperature that’s being reached in these large compost facilities would kill the cocoons.
Now it’s always … with research there’s always a caveat that that’s the temperature … It was a lab experiment. But 131 degrees is 131 degrees, and so if it’s reaching that much in the compost, then we’re pretty sure that if it’s being mixed correctly and all of the compost is getting to that temperature for at least three days, then that should be free of cocoons.
Now what happens after that, that’s where we could get another sort of an avenue for new invasion, if the equipment that they’re using to move the compost to the packaging facility is not clean, that could be a source of contamination or … you know what I mean?
Brad: At other places along that line of producing the compost, that you could always have an accidental introduction. But the temperature itself, it won’t be looked at, and at least from our study, we found that 104 degrees Fahrenheit is enough to kill cocoons. [The abstract of the paper is here; an article on the University website explains it.]
Margaret: So do you feel, or hope, or hope, we all hope [laughter] but do you feel that dot, dot, dot down the road, this exploration of heat tolerance of cocoons, that there may also be some way to apply this to other than a compost setting or what are you thinking about that?
Brad: Yes, that’s the question.
Margaret: Because you have them in the Arboretum. I mean, you have these worms in the Arboretum, right?
Brad: Right. And that’s the question that I’ve been getting a lot, too, is how as a homeowner and gardener, “How can I apply this to my yard?” And that’s sort of the next step. What is … how do we apply this research now to on-the-ground management. And some of it may not be applicable. It all … it’s very, I think, case-specific. So one of the things that we’ve sort of talked about is this idea of solarization of soil.
Margaret: Yes. Yes.
Brad: So if you have a yard or a garden that has the right aspect, it’s not shaded, and you have areas that you can put down a clear plastic over an infested area of soil, it’s possible that that could heat up to a high enough temperature where we think the cocoons are going to be right at the surface of the soil or just below the soil surface. So they should be … you should be able to reach 104 degrees … at that soil level.
Margaret: Yes. For three days, right.
Brad: Yes. That’s definitely one way to do it. And actually there’s been research looking at prairie systems and using prescribed fire, and what they found was that when they put a fire through an area that had cocoons, that the fire didn’t actually impact the earthworms, because the earthworms could actually move below the surface far enough-
Brad: ...where they are buffered. But the cocoons were significantly impacted.
Brad: And so we know that heat can work on the cocoon. And then obviously if you do that enough times you’re going to deplete that cocoon bank. And sort of like a seed bank, right?
Brad: If you deplete the cocoon bank, you’re going to reduce the population over time.
Margaret: Yes. What do you think about … I mean, so here are those of us who have them, we might consider solarizing probably can’t hurt. Some cooperative extension websites I see, they say when you identify the adult worms at this time of year [July] or a little later, put them in a plastic bag and speaking of heat, cook them to death, so to speak by putting them in the sun. But I don’t know, I mean how many are there? There must be 8 billion.
Brad: Right, yes.
Margaret: But are there any other tactics that you think that you think that we should be under … sort of doing at this point?
Brad: Yes. So, and again, like this is a case where the need for information is outpacing the research knowledge. [Laughter.]
Margaret: Definitely, definitely, yes.
Brad: Yes, right. So a lot of it depends on your scale of infestation. If it’s a really small scale, and this is the thing about education, is knowing what to look for and identifying areas that look like they might have the jumping worms.
Brad: If it’s a really small area, you can use just a simple mustard pour. A third of a cup of dry mustard powder with a gallon of water, and pour that over an area that you think might be infected. [Above, graphic from Karen Ceballos, NY Master Naturalist Program Assistant, Cornell Department of Natural Resources.]
Brad: And just literally with the adults, it irritates the adult skin and they will come … their responses to go vertically up the soil and out. And you can pick them out.
Brad: And you can put them in a plastic bag or whatever. Over time, you’ll deplete their ability to produce cocoons.
Brad: Again, it’s not a one-shot deal-
Margaret: Oh no, no, O.K., good.
Brad: …but that’s a small-scale remedy.
Margaret: Good, good, good.
Brad: Yes. And then again, I think it’s … we’re working on, there’s a fertilizer called Early Bird fertilizer-
Margaret: Yes. Yes. And I’ll give the links about that, yes. [Update 12/19: The fertilizer is no longer available, the manufacturer says.]
Brad: Yes, doing some tests on to see what the dosage should be, and getting it out in the literature that it does work, and see if it works on cocoons as well. People are already going ahead and trying it, and I think are seeing some good results, but it can be kind of a messy process because the worms come to the surface and die.
Margaret: Oh, that stinks. [Laughter.]
Brad: And then you have worm guts laying around, which they don’t … the worm body doesn’t last forever so it’s kind of … if you can tolerate that.
Brad: So there’s some kind of chemical applications at a large scale. Frankly at a large-scale infestation we don’t have … that’s where our kind of gap is.
Margaret: In the natural habitats, yes.
Brad: In the natural habitat, right, right, right.
Margaret: Well Brad Herrick, I’m going to keep checking in with you. [Laughter.]
Brad: Great. Happy to talk.
Margaret: Because you’re my 411. Yes. But thank you so much and keep up the good work because I think this is one of the really, really important problems to solve and what you’re doing is shedding a light on some of the possibilities. So thank you.
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(Photos from UW-Madison Arboretum except as noted.)
MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 10th year in March 2019. In 2016, the show won three silver medals for excellence from the Garden Writers Association. 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 August 12, 2019 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).