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How did Montana become infested with knapweed?

Melissa Maggio, coordinator of the Montana Biocontrol Coordination Project, stands among dried knapweed plants looking for weevils in the flower heads.
Kathleen Shannon
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Melissa Maggio, coordinator of the Montana Biocontrol Coordination Project, looks for weevils in dried knapweed flower heads.

Freddy Monares: Today, reporter Kathleen Shannon takes us into the Bitterroot to talk about a rather infamous plant: Spotted knapweed.

Kathleen Shannon: This question came from Liz Nielson. She's been living in the Bitterroot for 30 years, at times raising cattle, listening to Montana Public Radio, and now she has mules. I called her up to ask where she's running into spotted knapweed.

Liz Nielson: Everywhere. We have about 13 acres here and it is a never ending battle. And, you know, you don't have to live here very long to hear about spotted knapweed and how horrible it is. And it is, it is evil incarnate of the plant world.

Freddy Monares: Those are some strong feelings to have about a plant.

Kathleen Shannon: Yup, and Liz is definitely not alone. We'll get to why that is. But first, she posed this question for us.

Liz Nielson: I've heard so many different stories about how it got here and who's responsible, and I've never really heard the legitimate reason, and I thought that's a good question for you guys.

Kathleen Shannon: So how did spotted knapweed get here and why is there so much of it? To answer those questions, let's make sure we're all caught up on what spotted knapweed is. Freddy, are you familiar with it?

Freddy Monares: No, I'm not.

Spotted knapweed
Chris Evans, University of Illinois, Bugwood.org
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Spotted knapweed

Kathleen Shannon: They have purple flowers that can bloom from June through October. One plant stands about 2-4 feet tall with long, wiry, hairy stems. They like dry, disturbed soils in open areas, places like pastures and along trails. They've got a hearty taproot, so big plants can be hard to pull up. And some people think the flowers are pretty!

Freddy Monares: So they're pretty, but people still don't like them?

Kathleen Shannon: Right. It’s not native here and in the 1960s, the state Legislature classified it as a noxious weed because it can harm agriculture and natural resources. And it wasn’t the only newcomer. In fact, in 1969, the state Legislature required all counties to develop a weed management district. Enter Jerry Marks.

Jerry Marks: Hi, I'm Jerry Marks. I'm department head for the Missoula Extension Office and Weed District.

Kathleen Shannon: He was around for the rise of spotted knapweed in Montana.

Jerry Marks: So I've been here quite a few years.

Freddy Monares: So, where did it come from?

Kathleen Shannon: I thought this question would be a big mystery. But it turns out people have been tracking how non-native species got here for a long time. The U.S. Forest Service pins introduction in North America to 1883. It first hit Victoria, British Columbia, which is surrounded by water, and then Washington's coast line. It came from somewhere in knapweed's native habitat, which is central Europe. And, again, it's the late 19th century, so a lot of boats are landing on our western coast.

Freddy Monares: Ah, so it traveled by sea?

Kathleen Shannon: Right. It was likely mixed in with a shipment of either alfalfa or clover seed. Or, it could have been in the ship ballast, which is just piles of rocks or dirt that got shoveled into the bottom of ships to keep them level while offloading cargo.

Freddy Monares: Sneaky little things!

Kathleen Shannon: Ha, exactly. So knapweed keeps spreading. Fast forward a few decades, and boom, it lands in Montana.The first plant found in the state was ID'd in Darby in 1916. It took another decade to show up in Bozeman, soon followed by Missoula County and more. I asked Marks what the scene in western Montana was like when spotted knapweed took root.

Jerry Marks: Well, there was certainly more agriculture in those days. Missoula and the Bitterroot was a hub for actually growing fruits and vegetables that was sold to the mining communities.

Kathleen Shannon: And that agricultural market was likely how knapweed started to move across the state. Then a shift in the 1960s really accelerated its spread.

Freddy Monares: How so?

Kathleen Shannon: That's when Interstate 90 was in the works. And all that construction means disturbed soils. You know who loves disturbed soils?

Freddy Monares: Spotted knapweed?

Kathleen Shannon: Yeah, you got it. The highway helped push knapweed from northwest Montana east through the whole state. And it really took off. At its peak population around 2003, there were roughly five million acres of spotted knapweed in Montana. That's a chunk of land just about the size of Massachusetts. But back as it’s spreading in the middle of the 20th century there's important global context happening, and that's World War II. Chemical weapons developed then led to chemical warfare at home in the form of herbicides.

Jerry Marks: That become kind of the tool at the time. But no matter what the county did and what the landowners did, it continued to spread.

Freddy Monares: Why didn't the herbicide nip it in the bud, so to speak?

Kathleen Shannon: So this is where biology steps in. Each plant can produce as many as 10 thousand seeds! And those seeds are sticky little suckers. So they can spread on our clothes, animals can ingest them, even trucks driven through a knapweed field can take them to new places. Plus it produces a toxin that slows the growth of neighboring plants, paving the way for knapweed to take over entire fields. And this is the crux of why people like Nielson are so fed up with knapweed.

Liz Nielson: It's impossible to stop it. It's like a purple wave.

A knapweed infested field.
Caleb Slemmons, National Ecological Observatory Network, Bugwood.org
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A knapweed infested field.

Kathleen Shannon: And once that purple wave takes over, it’s not so great. It's not nutritious for cattle or elk, so they'll avoid it, and it drives out native species that they'd otherwise be eating.

Freddy Monares: Got it. So it sounds like we’ve answered Nielson’s question on how knapweed got to Montana. But that also leads me to wonder what we can do about it?

Kathleen Shannon: So there’s herbicide, like I mentioned before. Goats and sheep will graze on it. Mowing at the right time of its life cycle can knock it back, too. Tilling and just good ol' pulling them up by hand can help. But there's also what I think is a super cool way to reduce knapweed and I went down to Lolo to learn more.

Melissa Maggio: I would say this field that we're standing in, do you think it's like, five football fields?

Kathleen Shannon: Melissa Maggio runs the Montana Biocontrol Coordination Project.

Melissa Maggio: I'm guessing we're looking at like 60 acres. It's pretty contiguously infested with spotted knapweed.

Kathleen Shannon: We're standing right along Highway 93, south of Missoula, surrounded by nothing but knapweed. It’s mid-September, so the plants are mostly dead and the whole place has a real skeletal vibe to it. Maggio's job is to manage noxious weeds through biocontrols. So, back in Eurasia where knapweed comes from, it has some native predators, namely, several types of bugs that feed on parts of the plant and keep its population in check. We don't naturally have those same predators here. So we imported them.

Freddy Monares: Just brought them in?

Kathleen Shannon: Well, it took a while. Researchers and the federal government spent years making sure those bugs would be safe to introduce here, and by the 1990s they’d brought three critters to the scene.

Freddy Monares: Tell me about them.

Kathleen Shannon: So, we've got a seed head weevil, a gall fly, and a root boring weevil that are all working on these plants. Our extension agent friend Jerry Marks said it's like building a condo: you've gotta have one working on the flower upstairs, another on the main floor leaves and stems, and a third working on the roots in the basement.

Freddy Monares: Wow, that's a lot of action!

Kathleen Shannon: Yeah, these little guys are busy. And here in Lolo, all three of the insects munched on this field through the summer. Maggio wanted to show me evidence of the root weevil’s work.

 Melissa Maggio, coordinator of the Montana Biocontrol Coordination Project, stands among dried knapweed plants looking for weevils in the flower heads.
Kathleen Shannon
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Melissa Maggio, coordinator of the Montana Biocontrol Coordination Project, looks for weevils in dried knapweed flower heads.

Melissa Maggio: So sometimes you can pull these plants back and just, like, find them hanging out in the soil. And see how this plant just completely broke off?

Kathleen Shannon: Yeah.

Melissa Maggio: It's because there's so many of these root weevils attacking it. And you can see all the tunneling in this root.

Melissa Maggio: Oh, here's one.

Kathleen Shannon: Oh. Oh, hey little bud.

Melissa Maggio: Isn't he just the cutest thing ever? Some of my seasonal nicknamed them grass puppies because they're so cute. And I always tell people to pet them because they're like velvet.

Kathleen Shannon: Oh yeah.

Freddy Monares: You sound really excited.

Kathleen Shannon: I was! Holding that little weevil felt like meeting a friend in a field of foes!

Freddy Monares:  So do all these critters just live out their lives in this field?

Kathleen Shannon: Actually, part of Maggio’s job is collecting them and sending them off elsewhere, so the insects can go to work on other knapweed infestations. She’s essentially running a hands-off breeding program. This year, Maggio shipped root weevils to 46 counties in Montana and 12 other states.

Freddy Monares: So are these bugs actually affecting the knapweed?

Kathleen Shannon: I would say, slowly. Maggio says it could be five, eight, maybe 10 years before you start seeing impacts to the knapweed.

Freddy Monares: That sounds like a long time to wait.

Kathleen Shannon: It's the long game! But Maggio pointed out, it's called "biocontrol," not "bio-eradication." So the goal isn't to eliminate every last plant. It's more to achieve balance between an accidentally introduced plant and its intentionally-introduced plant-eating predators.

Adult lesser knapweed flower weevil.
L.L. Berry, Bugwood.org
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Adult lesser knapweed flower weevil.

Freddy Monares: So it sounds like knapweed is here to stay. Will Montana be battling it forever?

Kathleen Shannon: Probably, but these efforts to control spotted knapweed have made a big difference. The Montana Natural Heritage Program database recorded a nearly 70 percent decrease in knapweed observations in 2021, compared to seven years earlier. One cool thing experts told me as I researched this story is that landowners and managers are becoming more aware of knapweed and what they can do about it, which means they’re also learning more about the ecology of where they live. Like, if you want your knapweed gone, it helps to plant a native grass in its place. So spotted knapweed is here to stay, but we’ve got a foot on the brakes.

More info on knapweed and bio-control of weeds:

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Freddy Monares was a reporter and Morning Edition host at Montana Public Radio.