Weevils Wage War on Invasive Weed

By Amanda Leigh Haag

Within a mile of the sprawling suburb of Superior, Colo., a war is being waged. In sight of shopping malls and town homes, an army of hungry insects is bearing down on its adversary: a noxious weed, known as ‘diffuse knapweed.’

University of Colorado biologist Tim Seastedt led a class of CU journalism students through the battlefield in September. Standing amidst native wild rose, prairie grass and skeletal remains of knapweed, he explained how he monitors the progress and counts the casualties. His insects, called ‘biocontrols,’ have reduced the invasive knapweed on the Superior study site from 30% cover in 1997 to perhaps a tenth of 1 percent cover today, according to Seastedt. Popping open the remains of a knapweed shoot, Seastedt revealed its inhabitant: a thriving weevil. “He was gonna sit inside that stem all winter. But now I’ve rained on his parade,” said Seastedt.

In 1997, Seastedt released a small platoon of plant-munching insects: a few hundred in number, they consisted of a species of stem-boring weevil, two species of gall flies, one species of root-feeding beetle and one species of root-boring weevil. Today, his troops number in the billions and have dispersed across roughly 10,000 acres of land in Boulder County. “Wherever there is knapweed in Boulder County, there now are bugs,” said Seastedt.

According to Seastedt, diffuse knapweed arrived on the scene “with a vengeance” in the 1980s. This member of the sunflower family was introduced from Europe and Asia and has made itself at home in Boulder County open spaces. In the absence of natural predators, the opportunistic weed has infested 83,000 acres along the Front Range, according to Boulder County figures. Seastedt estimates that knapweed can overtake up to 50 percent of grazing lands and prairies by displacing native plants. Cattle dislike knapweed, making it a grievance to ranchers. Disturbed soils from cattle ranges, development sites, and off-trail hiking are especially vulnerable to invasion.

When Seastedt got involved, Boulder County was footing a $50,000-$100,000 bill each year to control knapweed via broadcast spraying of herbicides. A nearby resident with multiple chemical sensitivities attempted to enlist Seastedt, exclaiming, “They’re gonna spray the prairie!” Seastedt declined to get involved until he learned that the county was not monitoring the effects of the herbicides.

Seastedt observed that while the chemicals would knock the knapweed back for a year, the rogue weeds would show right back up again within a couple of years. Seastedt noted, “The prairie is a kind of sink—the reason the weed is out there is because of the seed source.” Essentially tumbleweeds as adults, knapweed can “tumble and bounce a quarter mile across the landscape, most of the seeds hopping out along the way.”

Seastedt had a hunch that the biocontrols might help cut back knapweed. So he talked Boulder County into an experiment: to release his insect brigade on 157 acres of ‘prairie turned knapweed’ near Superior.

Six years later, Seastedt speaks fondly of his insects. On this experimental slice of restored prairie, knapweed has retreated, its seed survivorship reduced to a fraction of 1 percent. Referring to the seed casualties, Seastedt said, “My poor little weevils got up in the spring with nothing to eat. Where’s lunch?” Since knapweed reproduces purely by seed, seed casualties are fantastic news. According to Seastedt, the diminished seed survivorship has also been observed on the 10,000 acres of Boulder County that have been stormed by the insects.

That the insects singled out knapweed has been fortuitous for Seastedt and Boulder County. So far, the insects have only had eyes for knapweed and not natives such as prairie grass. This may hold enormous promise for Seastedt’s biocontrol brigade to treat knapweed across the state. Kathy Damas, Integrated Pest Management Coordinator for the City of Boulder, stated that she thought Seastedt’s experiment had been a great success and that the city had chosen not to broadcast-spray diffuse knapweed this year.

Amanda Haag is a CU master’s student studying environmental journalism and a participant in the university’s Carbon, Climate and Society Initiative.