In 1997, Boulder County officials challenged Tim Seastedt to find a way to beat back one of the most noxious and pervasive weeds in the West.
Eight years later, the University of Colorado ecologist has declared victory against diffuse knapweed on his 160-acre test plot near Superior—and he believes his method can bring the weed under control throughout the region. His secret weapon? A couple of little weevils.
"We threw out a few hundred bugs against a few million knapweed plants," said Seastedt, a professor in the Department of Ecology and Evolutionary Biology at CU, and research fellow at the Institute of Arctic and Alpine Research. "The odds were not really fair."
But it turns out that the odds were really against the knapweed.
Seastedt said since he introduced his "gorgeous little insects"—the seedhead weevil and the knapweed root weevil – in 1997, they have multiplied from hundreds to billions. At that density, he said, no knapweed could survive their onslaught.
Diffuse knapweed is the most aggressive of the four knapweed species found in Colorado. Not only does it destroy native grassland, it is unpalatable to most grazing animals.
Introduced with alfalfa crops from Eurasia in the early 1900s, this invasive weed found few natural enemies in North America. It now thrives in semi-arid grasslands of the West, where it has colonized more than 3 million acres, including 145,000 acres across Colorado's Front Range.
Generating tens of thousands of seeds per plant, diffuse knapweed rapidly builds its population in a new area. The weed also spreads its seeds great distances as a tumbleweed.
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| Diffuse knapweed "tumbleweeds" along a Front Range fence (Photo/Tim Seastedt) |
Herbicide spraying has been ineffective because knapweed's plentiful reserve of seeds in the soil allows more plants to grow in the place of those that were treated, said Seastedt.
Previous biological controls have also been unsuccessful. By 1996, 10 insect species had been released in Colorado, to little effect.
But Seastedt said his addition of the two new weevil species made the crucial difference. Seedhead weevil larvae attack knapweed's reproductive output by consuming its seeds, while the adults weaken it by devouring the foliage. "The root weevil," said Seastedt, "is the icing on the cake to the bio-control story." Root weevils inhabit the roots, preventing knapweed from flowering until the insects are mature. This delay in the plant's life cycle allows seedhead weevil numbers to increase to a level where they consume vast amounts of seeds.
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| A Cyphocleonus (lower left) and Larinus weevils on a knapweed plant(Photo/Mark Giebel) |
"Once you shut down its seed production to a low level, it does not have the ability to persist in density," said Seastedt.
From 1997 to 2000, Seastedt found that the number of seeds per square meter on his test plot fell from 4500 to fewer than 10. The population of young knapweed plants also decreased from 50 per square meter to less than one. Consistent low numbers since 2000 and a lack of knapweed on his test plot this year makes Seastedt confident that the reserve of seeds has been exhausted and diffuse knapweed has been brought under control.
Another part of the success story is the weevils' ability to migrate throughout Boulder County, negating the need for a widespread controlled release. The weevils have been so successful that Seastedt said they have almost eaten themselves out of house and home. Seastedt said he is currently working with a city council member from Lyons, however, to coordinate the release of the insects there.
Other areas in North America have reported similar results. Scientists in Oregon, Montana and British Columbia have also had success at controlling diffuse knapweed with these little weevils. As for the pesky plant, "We've moved it from the noxious to the obnoxious list," Seastedt said.
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| Professor Tim Seastedt ruminating in the field (Photo/Tim's Web site) |
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