In an effort to control spotted knapweed, the Missouri Departments of Agriculture and Conservation and UMC Extension are releasing weevils that eat the seeds and attack the roots of spotted knapweed.

Missourians, to arms! An invading army is sneaking across Missouri’s borders hidden in bales of hay, poisoning Show-Me natives and taking over thousands of acres of prime pasture and wildlife habitat. Now is the time to attack, while they are most exposed.

That sounds overblown, especially when you discover the “invaders” are wildflowers. However, according to agriculture and conservation experts, Missouri is, indeed, in the midst of a serious invasion, and summer is the best time for landowners to mount a counteroffensive.

The invader is spotted knapweed, Centaurea stoebe. The 2-foot-tall perennial, bedecked with attractive, fringy pink blossoms, is a member of the aster family. It probably arrived in the United States in the late 1800s in contaminated hay or seed from Eurasia. Since then it has spread over 45 states. It can survive in a wide range of conditions, and it thrives in heavily disturbed sites, such as roadsides and agricultural field margins.

Unlike aster species native to Missouri, spotted knapweed’s roots produce chemicals that are toxic to other plants. It produces up to 1,000 seeds per plant. Once established, seeds accumulate, often exceeding 5,000 per square foot of soil. The seeds remain viable for at least eight years.

With dual strategy, spotted knapweed’s conquest can be surprisingly rapid. Seeds strewed along roads by trucks transporting contaminated hay and by roadside mowing quickly sprout in roadside ditches. Two years later a phalanx of flowery invaders is ready to storm bordering pastures and woodlands.

“This is a very bad plant,” says Tim Banek, invasive species coordinator for the Missouri Department of Conservation. “It can go from small infestations to being out of control almost before you know it. Controlling an infestation when it first starts is much easier than tackling one that covers dozens of acres.”

Looking at a stand of spotted knapweed, with pink blossoms waving in the wind, you might wonder what all the fuss is about. Banek said the invader, which is on Missouri’s official list of noxious weeds, is a problem for several reasons.

“It is bad news for wildlife because it tends to root out native vegetation and form monocultures over huge areas. It is not a good wildlife food plant, and biological diversity takes a nosedive when spotted knapweed takes over.”

Spotted knapweed is bad for agriculture, too. Pastures infested with it become less palatable and nutritious for livestock. In Montana alone, it caused $42 million in agricultural losses in 1996.

Early discovery of spotted knapweed infestations requires a close look at the ground. The plant produces only a flat rosette of leaves during its first year, as it builds a deep taproot.

Banek said spotted knapweed is an excellent candidate for “IPM” – integrated pest management. This means attacking invaders with a combination of physical measures, such as pulling plants, applying herbicides and using biological controls.

“The infestation in southern Missouri has gotten too large to control with herbicides alone,” said Banek. “Biological controls don’t produce quick results, but they provide long-term control that complements other methods. At this point, it looks like an integrated approach is the only way to deal with spotted knapweed.”

The Missouri Department of Transportation is using herbicide to control infestations along state highways. The conservation department, the Missouri Department of Transportation and MU Extension have been releasing two species of weevils that eat spotted knapweed seed heads and bore into the plants’ roots. The conservation department is evaluating the insects’ effectiveness on Cover Conservation and Tingler Prairie Natural Area in Howell County and along Highway 142, which runs near the Arkansas border in south-central Missouri.

The U.S. Department of Agriculture has tested these insects on native plants and approved them for release. However, a permit is required before they can be released. The insects have been effective in other states. So far, they have not been found to affect any non-target plants.

The weevil trials are among the first in Missouri. Wider use of these and other biological controls could follow if the trials produce good results.

Photos of spotted knapweed in different growth phases and information about controlling infestations are available from MU Extension centers or at Missourians who would like to know more about spotted knapweed should contact Banek by calling 573-522-4115, ext. 3371, or by e-mail at

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