Subscribe for 17¢ / day

KALISPELL - An army of European insects is on the march, invading Montana with a mission to get at the root of the state's spotted knapweed problem.

Science calls them Cyphocleonus achates. Most folks know them as root weevils. Knapweed calls them trouble.

On Wednesday, Montana Department of Transportation crews will cut loose a colony of root-boring weevils in Lincoln County, the first of several releases expected in the ongoing war on weeds. The new tools of the trade in that war are biological weapons such as C. achates, bugs with a taste for invasive weeds.

The weevils that favor the flavor of knapweed were first released in the United States back in 1987, part of a Montana program to knock back both spotted and diffuse knapweed.

The knapweeds took hold across the northern United States in the early part of the last century, traveling from Eastern Europe and Asia to put down roots in disturbed soils and overgrazed pasture. In some places, knapweed has choked out 100 percent of cattle range, leaving the land awash in a sea of purple-flowered plants the cows won't eat.

But root weevils will.

The bugs are big, most of an inch long, speckled in a brownish-gray that blends both with Montana dirt and the seedhead on a knapweed plant. They don't fly, but can run fast, and have been known to play dead when poked or prodded.

Ronald Lang, at the Forestry Sciences Lab on the Bozeman campus of Montana State University, says the weevils eat knapweed exclusively from cradle to grave. The borers spend the winter in a larval stage, snug as a bug in the taproot of a knapweed plant.

They feed on the roots until round about August, when the adult weevils emerge to graze tender knapweed leaves. Through late summer and early fall, female weevils eat and mate, eat and mate, laying upward of 100 eggs on the plant's roots, just below the soil's surface.

Within a week or two, the youngsters hatch and begin the cycle again, chewing away at the roots, weakening the plants.

Bacteria and fungus then invade the weevil holes, and by the next summer season the knapweed is dead.

The weevils are, in fact, a big reason knapweed isn't an invasive problem in its native Europe. Indigenous to Austria, Greece, Hungary and Romania, the weevils have followed the knapweed to colonize much of Eastern Europe and Asia.

But with the plant now well established in North America - where it has no natural controls - enemies must be imported.

Of course, it's not ideal to have two foreign armies battling it out on Montana soil - especially when one is a proven colonizer and the other seems to have no known pesticide susceptibility.

But the weevils already have been unleashed in Arizona and California, Minnesota and Idaho, Nebraska, Oregon, South Dakota, Utah, Washington. Perennial populations are firmly established in Colorado, Wyoming and Montana.

Weed czars say the bugs have been remarkably effective, especially in places where herbicides are impractical or just generally undesirable. But the weevils won't work just anywhere.

They prefer toasty warm soils, and can't stand a short growing season. The eggs and larvae don't do well underwater, and a spring flood can wipe out the weevils, if not the knapweed.

Fortunately, the hot Montana roadsides so swamped in summer knapweed are often just the spot for C. achates, and highway officials hope the insects can help slow the spread of weeds into eastern Montana.

While they admit root-boring weevils cannot eradicate knapweed patches, state officials hope the bio-agents can be an important part of the arsenal now being deployed in their war with the weeds.

Reporter Michael Jamison can be reached at 1-800-366-7186 or at

You must be logged in to react.
Click any reaction to login.