By Tim Seastedt, For the Camera
November 4, 2005

Last spring, a 160-acre plot of Boulder County Open Space near Superior seemed doomed to become blanketed by diffuse knapweed — an invasive weed that by most accounts threatens native grasslands and greatly diminishes the worth of grazing lands.

As part of a study I've been conducting since 1997, my students and I estimated that the plot was home to 1.7 million knapweed plants. Each of those plants was theoretically capable of producing 150 flowers that could each produce 10 seeds. The math was scary: Those 1.7 million knapweed plants could produce about 2.5 billion knapweed seeds.

But this reproductive explosion was not to be. With a handful of lowly insects, we have been able to succeed in bringing knapweed to its knees where hundreds of thousands of dollars in herbicides failed.

The knapweed moved into the Front Range in the 1980s from sites west and north. The plant arrived in this country about a century ago in shipments of seeds or livestock forage and became established in grasslands of the Northwest.

The weed was not only largely inedible to native insects, wildlife and cattle, but studies indicated it even poisoned other plant species. The plant is susceptible to a number of herbicides, but these tend to be expensive and toxic to many native plant species and agronomic crops.

As the soil warmed in late May, an army of seed-head weevils called larinus minutus, imported from knapweed's homeland of Eurasia, emerged from the soil. About 200 of these insects had been obtained from the Colorado Department of Agriculture's Biocontrol Division in Palisade, and had been released on this site in 1997. They multiplied over eight growing seasons.

About the size of a large grain of rice, the insects climbed into growing knapweed plants and gnawed on the foliage.

Their numbers were startling. About 50 insects emerged per square yard of soil, yielding 20 per knapweed plant and a total of about 85 million weevils on the 160-acre parcel.

Meanwhile, a second species of weevil, Cyphocleonus achates, burrowed into the taproot of the plants. Fully one-third of the knapweed on our plot hosted these insects. About 600,000 of these insects would emerge as adults in July to continue feeding on the foliage and stems of the plant. This weevil was much larger than the seed-head weevil, and unlike most adult insects, this species lacks wings. It must walk to its next meal.

Over the summer, the knapweed did its best to grow and produce flowers. But when all was said and done, fewer than three seeds successfully matured per plant.

This autumn, there is less than one juvenile knapweed plant for every 60 square yards of grass, and the seeds produced by this year's crop are insufficient to enhance these now-low densities.

The seed-head weevils did their job. But now they have paid a price for their success. As many as 99 percent of the larvae starved this summer for lack of food. The root-feeding insects fared poorly as well: Most walked themselves to death in search of plants to house their eggs.

Juvenile knapweed plants found by the weevils will be fed upon this coming year by both species of insects. This suggests another bad year for the remaining knapweed.

Pockets of knapweed survive here and there, mostly in areas of human disturbance such as along roadsides. The weed will persist. But we will likely never again experience the densities of knapweed plants that occurred here in the earliest part of the 21st century.

The insects will see to that.

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

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