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Public Radio's Environmental News Magazine (follow us on Google News)

Garlic Mustard Raises a Stink

Air Date: Week of

It may look harmless, but garlic mustard is wreaking havoc on ecosystems across the country. (Photo: Glenn Miller, Oregon Department of Agriculture)

A gourmet introduced the garlic mustard plant to his Long Island garden in 1869. One hundred forty years later, the weed has spread across 34 states and into Canada with no signs of stopping. Ecologist Adam Davis of the Agricultural Research Service of the USDA tells host Steve Curwood how biological control offers a promising way of dealing with the garlic mustard problem.


CURWOOD: In the kitchen, garlic and mustard are favorite ingredients to flavor and spice our food. But the garlic mustard plant – well, that’s more like a stinky kettle of fish. This unwelcome import from Europe has taken over millions of acres of forest floors in America and is far too widespread to get rid of easily. So, scientists are now looking to biological control to address the garlic mustard problem. Joining me now is Adam Davis. He’s an ecologist with the Agricultural Research Service of the USDA, and teaches at the University of Illinois. Hello, Professor Davis!

DAVIS: Hello.

CURWOOD: Tell me about the garlic mustard plant.

DAVIS: Garlic mustard is a biennial plant, which means it takes two years to complete its life cycle. It’s home range is in Europe, extending into Western Asia. It’s a very hardy plant; it comes out pretty much first thing at the end of winter, beginning of spring, and dominates the forest floor.

CURWOOD: Just how, when, and where was it introduced to North America?

DAVIS: It was actually brought in intentionally – 1869 – it was brought in as a culinary herb in Long Island. But unfortunately it doesn’t taste very good, like a really sharp mustard green.

CURWOOD: Since it came, it’s what, in 34 states and 4 Canadian provinces? And it just keeps on going.

DAVIS: Ya. Well it does best within temperate forests in kind of the northern part of the U.S. But it’s getting more and more prevalent within each of the states where it’s found.

CURWOOD: This plant is considered a pest. Why is it so unstoppable?

DAVIS: It seems to have a few different mechanisms that make it particularly invasive. It’s cold-hardy and shade tolerant, so comes up early in the spring when most plants aren’t able to grow. It also appears to secrete what are called allelochemicals into the soil. Allelochemicals are chemical compounds that one plant introduces into the growing environment to suppress the growth of another plant. So it’s kind of chemical warfare against the native plants.

CURWOOD: Now typically with a week, a plant you don’t want, you just pull it up and you get rid of it. What about the garlic mustard?

On the frontlines of a garlic mustard invasion. (Photo: Glenn Miller, Oregon Department of Agriculture)

DAVIS: Within a given year, you can certainly kill the adult plants by pulling them up. But the problem is that the garlic mustard seeds are quite long-lived, and once the seeds enter the soil seed bank, they can remain there for over ten years. So if you want to get rid of a population by hand, it means that you’ve got to pull it every year for ten years.

CURWOOD: As I understand it, some researchers in Switzerland have discovered a predator that has a particular taste for garlic mustard.

DAVIS: Yeah, this predator is called Ceutorhynchus scrobicollis, and these weevils are very well adapted for dealing with garlic mustard. Garlic mustard has some potent antifeedant compounds that it produces, and in order to eat garlic mustard these weevils have to detoxify it.

CURWOOD: I gather there’s some pressure to bring this weevil in and sic it on plants, but of course when one does that, there could be unintended consequences.

DAVIS: Sure. In recent years biological control has received some negative press because of the unintended consequences. And as a new generation of biocontrol specialists, we’re trying to develop ways of minimizing risk to non-target species.

CURWOOD: I understand you use computer modeling to assess using this weevil to control the garlic mustard plant. How did that work out?

DAVIS: It worked out quite well. We simulated transitions between different garlic mustard life stages and across different parts of its range and tried to figure out how much damage and what types of damage would be necessary to control garlic mustard.

CURWOOD: And the computer told you what?

DAVIS: The computer told us this particular weevil is very very specific. It’s monophagous – it just eats garlic mustard.

CURWOOD: What about the ability of this weevil to adapt? If it figured out how to eat garlic mustard, maybe it could figure out how to eat chrysanthemums, something like that.

DAVIS: This weevil coevolved with garlic mustard over millenia, and so if it burns itself out in a local population, it’s just not going to have the time to adapt.

CURWOOD: What would be a successful outcome?

DAVIS: I think completely eradicating it is really unlikely. But the goal of classical biological control is to have the agent and the plant pest come to some new sort of equilibrium at a much lower population density.

CURWOOD: I imagine people listening to us are going to feel somewhat apprehensive despite your assurances it’s very low probability – and the problem is this – typically we don’t know what we don’t know, we just don’t know what the weevil might try to do to adapt in response.

DAVIS: One of the suggestions for further reducing risk associated with biological control is to do preliminary caged releases, where you could see if the agent starts behaving differently in the introduced range than in the home range. It’s not really about eliminating risk, it’s about managing risk and thinking more in a risk-benefit framework.

CURWOOD: Adam Davis is a plant ecologist with the agricultural research service of the USDA and teaches at the University of Illinois at Champaign-Urbana. Thank you so much.

DAVIS: Thanks for having me.



National Invasive Species Information Center


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