[Un]Natural Selection Ep. 1: A Necessary Weevil?
Ecologists were hopeful. They’d found an exotic weevil to beat back invasive thistles that were invading farm pastures and prairies. The problem was, the little beetle didn’t exactly do its job. The weevils were jeopardizing the pitcher’s thistle, a federally threatened species. In the Great Lakes dunes, more species of pollinators rely on this rare native thistle than any other plant. Now, scientists are working to right their wrongs.
Credits for this episode:
Hosts: Dan Wanschura and Morgan Springer
Producer: Patrick Shea
Editor: Morgan Springer
Consulting Editor: Peter Payette
Music: Podington Bear, Max Dragoo, Marlin Ledin
Logo: Erin O'Malley
DAN WANSCHURA, CO-HOST: From Interlochen Public Radio, this is [Un] Natural Selection. A new series about the benefits and pitfalls of tinkering with the natural world. Are we solving problems, or just making more?
I’m Dan Wanschura.
MORGAN SPRINGER, CO-HOST: And I’m Morgan Springer. We’ll be exploring that question for the next seven episodes. Today, Episode One: A Necessary Weevil.
PATRICK SHEA, BYLINE: With a question mark.
SPRINGER: That’s reporter Patrick Shea. Hi Patrick.
WANSCHURA: Where are you going to take us today? Where do we start?
SHEA: 200 million years ago. That’s when the supercontinent Pangea started to split apart. And biologists say, that started separate evolutionary timelines.
Those pieces of Pangea would become the seven continents we know today. And as they floated out to sea, unique webs of life were woven in isolation.
But long after the continents drifted apart, humans started drifting together.
As people crossed the oceans, they brought seeds with them– both by mistake, and by design. And sometimes, a run-of-the-mill plant on one continent becomes an aggressive weed on another.
Some of those plants were thistles. Today’s story starts with those pesky plants.
ERIC EGELER: If you’re walking through our pasture and you hit one, you’ll know.
ALLISON EGELER: Oh, you’ll know. It’s nasty stuff. It is nasty stuff.
SHEA: That’s Eric and Allison Egeler. They raise grass fed cattle outside of Ludington, Michigan. And on their farm, a species known as Canada thistle has been–thorn in their side.
ERIC EGELER: It’s kind of stringy, spiny– the cows will definitely not eat into them. They’ll eat their way around them more.
SHEA: And as cows avoid thistles, the plants keep seeding and spreading, taking up more and more grazing real estate.
ALLISON EGELER: You know it’s almost to the point where just to prevent that spread you might just have to use chemicals. Which is not our first option at all.
The Egelers would prefer not to spray herbicide because it can kill other plants in the pasture. Plants they want. It’s also not particularly healthy for the person mixing and applying the chemicals. Then on top of that, there’s the financial cost.
ERIC EGELER: I can remember a few years back, we had a cornfield. We actually had to spray the thistle, it was so bad in that cornfield. I think it cost us close to three grand just in chemical. And it stunted it, didn’t kill it.
SHEA: The Egelers are not alone in their frustration with Canada Thistle.
Despite its name, this plant is from Europe. But it's been a nuisance to North American farmers for hundreds of years. It's so despised that many states have laws requiring landowners to control it and keep it from spreading. Vermont passed a law like this more than 200 years ago. And now, noxious weed laws in states like Colorado, Iowa and Oklahoma list a few species of European thistle as public enemy number one.
TOMMY PUFFINBARGER: Musk thistle in the state of Oklahoma has been listed on the noxious weed list, so it is the landowner’s responsibility to control that.
SHEA: That’s Tommy Puffinbarger, director of Oklahoma State University’s Alfalfa County extension. In his area, musk thistle is a big problem. In a video from OSU, he explains one way to keep the thistles in check: Biological control, which means you try to control a pest by introducing a natural predator.
PUFFINBARGER: So then we rely on our biological control, which is the musk thistle head weevil.
SHEA: A weevil is a type of tiny little beetle that happens to eat thistle seeds.
Back in Europe, they’re natural predators of these thistles. So, starting in the 1960s, state and federal agencies began releasing these European weevils across the country, hoping they’d beat back invasive thistles.
The weevil attack goes like this: they lay their eggs in or around a thistle’s flower head. And from those eggs hatch larvae.
PUFFINBARGER: And then the larvae will feed on the flower for 25 to 30 days.
SHEA: During that time, the larvae eat all the seeds inside. And every seed eaten is one less thistle down the road.
These Weevils might seem like the perfect, chemical-free solution to a nationwide weed problem. But that’s not the end of the story.
These little bugs are causing new problems in the sand dunes of the Upper Great Lakes.
That’s because the weevils aren't the picky eaters scientists hoped they’d be. They don’t just feed on the thistles they were supposed to. The weevils are eating a rare native plant and stopping it from reproducing– a plant called pitcher’s thistle.
SPRINGER: So now we’re getting to the heart of it. That part in the story where we see the problem with our own environmental management. The unintended consequences of our actions …
PATRICK: Yeah, and even the intended consequence– biological control–didn’t really work out.
Some thistles send out what’s called runners - tiny horizontal roots underground.
Then new thistles can shoot up from those runners, grow, flower, do their thing … So even if a weevil eats the seeds from the original plant, new plants can grow.
Pitcher’s thistle, on the other hand, it doesn't do that. No runners underground. Seeds are all its got.
DAN: So the weevil wasn’t super successful at killing the invasive thistles. But what about the native pitcher’s thistle. What were the consequences for it? Does it matter that the weevils are hurting this plant?
SHEA: I’ll let an expert explain that. Claudia Jolls is a conservation biologist who’s been studying… Pitcher’s Thistle for the better part of 30 years.
CLAUDIA JOLLS: “Well, as a Michigoose – I’m a Michigan native – we certainly knew the dune systems. And I knew there were distinctive plants, but it wasn't until my research program at the University of Michigan’s field station that I came to see the pitcher's thistle’s role. I came to understand that rare doesn’t mean unimportant.”
SHEA: And the more researchers look into pitcher’s thistle, the more important this rare plant turns out to be. In one of her studies, Jolls looks at how the thistle fits into the “network topography” of the sand dunes ecosystem.
JOLLS: Well certainly, it’s scientific jargon but it’s something that anyone can appreciate if you go out to these spectacular freshwater dunes. Just on a sunny afternoon, sit down at a pitcher’s thistle plant and watch what happens.
You’ll see anywhere from 30 to 50 different species of insects come to visit those flowers for resources like pollen and nectar.
Pitcher's thistle, simply put, is visited by more species of insect than anything else in the dune landscape. To lose it will impact the entire community as well.
SHEA: Keep picturing that sand dune. It’s pretty bare. There aren’t really a whole lot of flowers that can live in this environment. It can get really windy, there are crashing waves and there aren’t a lot of nutrients available in the sand. It’s a harsh place, and it takes a special kind of plant to make it here.
And this special plant can flower all summer long: from June to September. So there are times when pitcher’s thistle is just about the only thing on the menu for pollinating insects.
But there’s a problem. This critical plant is in big trouble and not just because of the weevil.
JOLLS: I think the really sad part about my career is that I’ve been studying this plant long enough to see its threats increase.
SHEA: The pitcher's thistle has been listed as a federally threatened species for more than 30 years. It’s decline has been largely due to habitat loss as people build homes and communities near the lakeshore, and it’s only gotten worse.
JOLLS: So, 25 years ago, we didn’t have to worry about an invasive biological control agent.
SHEA: She’s talking about those weevils. Remember, they were spread on purpose, as a biological control agent. But in this case, the agent went rogue.
SPRINGER: So what does this mean?
SHEA: These weevils are really doing a lot of damage. They’re preventing reproduction of the pitcher’s thistle, and they seem to be spreading. The first signs of this problem were noticed in southeast Wisconsin, in the early 2000s. But now the weevils are targeting pitcher’s thistle all over the Great Lakes. Scientists think that only the northernmost dune systems are still weevil-free.
So the outlook is pretty grim. One of the reasons this problem is so devastating has to do with the plant’s life cycle. Pitcher’s thistle only flowers once in its lifetime.
It spends most of its life as a small circle of leaves on the ground. Soaking up the sun, building up energy, and growing its taproot down into the sand. Then, after four to eight years, it finally shoots up a stock, forms seedheads and flowers. And then those insects looking for food have a meal.
And that one season is its only chance to reproduce. It gets one shot, then it dies.
SPRINGER: So when weevils eat all those seeds they lose that one shot completely.
SHEA: Right. It’d be like if you were working on some big seven-part podcast series, and you had hours and hours of tape, and it all got deleted, just like that.
SPRINGER: Don’t even go there!
WANSCHURA: What a nightmare! Is there any hope for this plant? Can anything be done?
SHEA: I guess I’d say there are people doing what they can. First off, some botanists are putting pitcher’s thistle seeds into seed banks just in case the species does go extinct. And unfortunately, it looks like that’s where this is headed.
But that doesn’t mean that scientists are just throwing in the towel already.
I’m at Indiana Dunes National Park, at the southern tip of Lake Michigan. And I’m here with Noel Pavlovic, a research ecologist with the U.S. Geological Survey. Like Claudia Jolls, he’s been studying pitcher’s thistle for decades, and he’s also noticed a recent decline.
NOEL PAVLOVIC: When I first got here, there was some pitcher’s thistle over there, and over there– and now it’s gone.
Noel is walking along the beach with his coworker, Johanna Nifosi.
They’re worried that if the weevils keep feasting, there won’t be enough seeds for pitcher's thistle to make it here.
That’s why they’re on this walk– to plant new seeds and look for damaged seedheads.
Johanna finds a pitcher’s thistle and calls Noel over.
JOHANNA NIFOSI: There’s an adult right there. You can see seedheads there.
Noel peels back the outer layer of a seed head to look for signs of weevil larvae.
PAVLOVIC: This one looks normal. Yeah, this one looks normal. Well, the bottom is usually gray but this looks black to me. So it looks like frass.
SHEA: Bug poop?
PAVLOVIC: Bug poop.
SHEA: Weevil poop, to be exact.
Noel first started noticing weevil-damaged seed heads about ten years ago. Since then, he’s been trying to learn everything he can about the insects. In their greenhouse, he shows me some samples he’s collected.
PAVLOVIC: Ok this is a great specimen actually. So this is the larinus thistle head weevil. See, this guys got this long snout. See that? So that’s larinus.
SHEA: Was this one of the species that’s been released as a biocontrol agent?
PAVLOVIC: Yes, for Canada thistle.
SHEA: Was it successful in controlling Canada thistle?
PAVLOVIC: Not that I’m aware of.
SHEA: That’s just one of three weevil species Noel and Johanna have found here in the park. So they’re hurrying to get more seeds into the sand, and winter is the time to plant.
These two won’t be stopped by snow, sleet or even Congress.
PAVLOVIC: A few years ago, when the government shutdown,I was at home, not allowed to work. I called up Johanna and said “holy crow, we gotta plant those seeds!” And we went and planted seeds. It was cold, wasn’t it?
NIFOSI: I love it.
SPRINGER: So they’re basically racing the weevil.
WANSCHURA: They have to plant more pitcher's thistle than the weevil can get too
SHEA: Yeah, they hope their planting efforts will buy some time, and help pitcher’s thistle survive while scientists search for a way to control the weevils.
One method being studied is pheromone traps. Those would mimic the specific scent of a female weevil, to catch and kill males in search of a mate. But that research is still in the early stages.
And if land managers have learned anything from all this, they won’t be hasty when it comes to that control method. Because that’s how we got here in the first place.
PAVLOVIC: We as humans want to have quick answers to things.
NIFOSI: And our lifetimes are very short. So it’s very important that you continue research for many many years. These biological processes are not necessarily attached to our human, short lives.
SHEA: In the meantime, Noel and Johanna will keep planting seeds, and crossing their fingers.
PAVLOVIC: Don’t want to lose any of these seeds.There we go – grow babies grow.
NIFOSI: Yes, that’s the magical chant. They don’t grow if Noel doesn’t say that.
SPRINGER: Is that all for this week, Patrick?
I want to wrap things up with Claudia Jolls again, the researcher we heard from early on. She agrees that saving this plant won’t happen quickly. And these weevils show how quick fixes can backfire. But I really like her takeaways.
JOLLS: We can maybe take a cue from our friends in medicine, in that our guiding principle should be “first, do no harm.” And if we find that we have done harm, to try and rectify it.
So yes, ethically, morally we are obliged to be good stewards of the earth, because the earth has been a good steward to us.