Weevils zap the ‘wicked weed of the West’

As startling claims about knapweed’s virulence are retracted, CU researchers show that weed-eating bugs can help control invasive species without herbicides

By Clint Talbott

It’s not often that plants, even foreign ones, are described as diabolical, but spotted knapweed has that rare distinction. A 2004 issue of Smithsonian magazine, for instance, dubbed it the “wicked weed of the West,” a “national menace,” a “weed of mass destruction.”

The same year, The Denver Post reported that Colorado forests were being “eaten alive” by invasive plants, which, the paper said, had “killed nearly 10 percent of the state’s native species.”

The reports were overstated and incorrect, but the press wasn’t making this stuff up. It was summarizing research results published in leading academic journals.

For instance, the journal BioScience stated in 1998 that the spread of alien species was second only to habitat loss as the “greatest threat to biodiversity.”  An influential paper in Science had reported that knapweed produced poisons that killed native species.

In the academic and agricultural world, the prevailing view was that invasive species like spotted knapweed were wiping out prairie and farmland, that they had to be eradicated or at least controlled, and that the best means of doing this was through the use of herbicides.

Tim Seastedt, a professor of ecology and evolutionary biology at the University of Colorado

Tim Seastedt, a professor of ecology and evolutionary biology at the University of Colorado, has spent about 14 years studying the spread and control of knapweeds. His research contradicts the view that knapweed is a floral WMD.

Further, his work indicates that weed-eating bugs such as certain flies and weevils—the knapweed’s natural predators—can keep and are keeping the weed under control.

As Seastedt and CU researcher David Knochel reported in their 2010 progress report on the Spotted Knapweed Eradication Project in Spruce Gulch near Boulder:

“Spotted knapweed in the presence of multiple biological control agents and in the presence of competing vegetation no longer functions as an aggressive, invasive species. The plant not only has limited potential to spread, but declines in abundance in existing infestations.”

The Spruce Gulch landowners asked Seastedt’s group for help because they didn’t want herbicides in their water supply. With only biological controls—meaning weed-eating insects—one area of Spruce Gulch has shown an 80-percent decline in plant densities since 2007.

In several recent studies funded by the U.S. Department of Agriculture, Seastedt and Knochel have replicated these results.

And yet, Seastedt said in a recent presentation, “The current popular, basic and applied-scientific literature on  …  the knapweeds in North America is dominated by a compilation of half-truths and distortions.”

Some of that misinformation stems from widely cited scientific papers that were later retracted or amended. The revelation that the sky was not falling—and that the news of leafy WMDs was greatly exaggerated—did not make headlines.

“Scary stuff sells,” Seastedt observes. And the scare, which was a long time coming, takes awhile to recede.

Eurasian invasian

Knapweed is native to Eurasia and was accidentally carried into the United States in the late 1800s, according to the USDA. It has spread over millions of acres of land in North America, out-competing native species.

For instance, knapweed can crowd out alfalfa and other plants on which cows graze. Cows don’t like knapweed, which means that feeding them can become more laborious and expensive when knapweed moves in.

The weeds’ spread seemed to accelerate in the 1990s.

Knapweed can be killed with herbicides, and that is the default response of many land managers. Boulder County began spraying diffuse knapweed, a close relative of spotted knapweed, from helicopters in 1996. In 1997, Seastedt testified before the county commissioners, telling them that spraying made little sense.

Spraying herbicides on 1,000-acre parcels in a sea of tens of thousands of acres of knapweed would not have any long-term benefit, he said. Besides being toxic and expensive, he said, herbicides must be re-applied regularly.

The county let Seastedt conduct experiments on public land, trying to control knapweed with some of the plant’s natural predators, which had not arrived with the weed itself.

“We started with about 50 insects against about a million plants in 1997,” Seastedt recalls. The bugs took awhile to make an impact. On one parcel of land, for instance, the number of knapweed stems per square meter more than doubled between 1997 and 2000.

By 2001, however, the knapweed density had dropped about 80 percent. It fell even more the following two years. Density rose slightly in 2004 and ’05, then fell again to near-zero levels.

Knapweed’s natural predators were part of the picture. Another was that as knapweed populations struggled, other species bounced back. “We demonstrated that plant competition mattered,” Seastedt notes. “The invaders didn’t really do well if competing plant species were present.”

Still, “The surprise was how well the biocontrols worked and how quickly they worked.” Those results have been replicated from Colorado to British Columbia, he adds.

‘Biochemical weapons’

A group of weevils feasting on knapweed.

Meanwhile, other scientific results were making even bigger headlines. In 2003 and 2004, articles in Science and in Frontiers in Ecology and the Environment reported that knapweed can produce “novel biochemical weapons” that poison native plants.

In a 2003 issue of Science, the reputed weapon was cited, rather clinically, as “allelopathy and exotic plant invasions.” In the 2004 Smithsonian, however, knapweed was “the wicked weed of the west” that was “costing ranchers millions.” In 2008, the Duluth News Tribune in Minnesota reported that knapweed was killing not only native plants, but also trees.

Other researchers tried to find these “novel biochemical weapons.” They did not succeed.

Seastedt surmises that scientists were scrambling to understand the spread of knapweed. By 2003, “people were looking for an explanation for why a plant that’s not dominant in its home country is taking over here,” Seastedt notes.

But poison was not the culprit.

Academic journals published several retractions and amendments to papers that reported evidence of green WMDs. In 2010, the Annual Review of Plant Biology published an “erratum” article noting that “there is now little evidence that allelopathy facilitates knapweed invasions.”

Similarly in 2010, Science published an erratum notice on its widely cited 2003 paper. “The authors have had difficulty replicating the high and consistent levels of (the poison) found in soils surrounding (knapweed) plants as originally reported.”

Knapweed’s proliferation had a more prosaic root, Seastedt says. “The answer is that the herbivores that exist in the home country didn’t exist here, but there’s this lag time” between the bugs’ introduction and plant population control.

He adds, “Biological systems are notoriously non-linear.”

Knapweed seems to have been controlled in native lands by its traditional predators, and it appears to be manageable here, too, Seastedt adds. “It still exists, but it’s not an ecological threat.”

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