New West Feature

How a Bug’s Dinner Could Help the West Win Its Battle With Knapweed

By Kylee Perez, 4-21-11

	A weevil feeding on knapweed, one of the few biological controls to the invasive plant.

Knapweed, the much-maligned plant that has dominated whole swaths of the Western landscape, may have less of a hold thanks to some tiny insects.

Research at the University of Colorado at Boulder suggests knapweed may not be as uncontrollable as once thought. Tim Seastedt, a professor of ecology and evolutionary biology, has been using the knapweed weevil and the seedhead weevil, both native to southern Europe and eastern Russia, to control the weed.

“The plant was spun as the wicked weed of the West and it’s clearly been oversold,” Seastedt said.

Spotted knapweed and diffuse knapweed are nonnative, invasive species that have the ability to take over entire ecosystems and crowd out native plants, said Steve Sauer, Boulder County weed supervisor for the Colorado Department of Agriculture.

Both the diffuse knapweed and the spotted knapweed have become widespread throughout the country since their introduction in the 1880s, but are particularly predominate in the West.

TIM SEASTEDT, professor of ecology and evolutionary biology at the University of Colorado at Boulder.

When ground is disturbed, usually by cattle grazing or by humans bulldozing areas, the knapweed seeds germinate faster, allowing it to gain a foothold over other native species in the area. Once the knapweed is established, it’s difficult for other species to grow in that area because the knapweed is using all of the resources. In the past, herbicides have been the main weapon used to fight the vigorous plant, something that has become a multimillion-dollar battle, Seastedt said.

The weevil larvae eat the knapweed’s seeds and roots and the adult weevils eat the leaves. This causes damage that weakens the plant and makes it difficult for it to recover, especially during drought years when it’s already stressed, Seastedt said.

Once the knapweed is weakened, native plants get a chance to reclaim an area. When the knapweed seeds later competes against other plant species that have grown robust, they don’t do as well, a key element of the biological control program, Seastedt said

“If we just had the insects alone without plant competition, I don’t think we could knock out the knapweed,” Seastedt said

The weevils are slow workers though. Once cattle were banned from Seastedt’s research area in Lefthand Canyon near Boulder, it still took the insects four years to produce an 80-percent decline in knapweed.

“The insects are playing catch-up with the plant,” Seastedt said. “Once they get going it takes a long time.”

But one researcher in Montana doubts the weevils are having a significant effect on knapweed populations.

Dean Pearson, a research ecologist at the U.S. Forest Service’s Rocky Mountain Research Station, has been studying the weevils’ effect on knapweed for the past four years.

Pearson’s research at a site at the University of Montana in Missoula shows that even though the weevils might be able to kill individual plants, another knapweed variety tends to emerge.

“At the population level, they compensate,” Pearson said. “So for every plant that dies, another one takes its place.”

Pearson has found one natural control that may be effective in controlling knapweed: drought. During dry conditions in 2000 and 2001, knapweed populations declined in the natural world while other native plants did well, Pearson said.

Seastedt sees benefits with drought, as well. He points to drought as an event that can make the weevils even more effective. The weevils feed on the knapweeds’ root system, making it harder for the plant to get water from the soil. With less water in the soil during drought years, that makes the insect-ravaged weed’s ability to thrive that much tougher.

Finding a biological control that works can mean a lot less money spent on chemicals in the West.

“If we’re correct about this biological control, it means the cost associated with the plant should go away,” Seastedt said. “Those resources can now be used to deal with other spices that don’t have the controls.”

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