Wayne Slaght is a rancher. He manages the 10,000-acre Two Creek Ranch in Powell County, Montana, the ranch he grew up on and that his father managed before him. It's in bear country, and he knows every grizzly that passes through this rolling patch of Big Sky country about 50 miles east of Missoula. It's elk country too, and one bitter November day he was pounding postholes in the frozen ground, erecting an eight-foot-high elk fence around a larder of hay that will feed his 800 cattle during the long winter that lies ahead. But the biggest threat to his livelihood doesn't even have teeth. "It's knapweed," he says without hesitation. Uncontrolled, "it could put me out of business in ten years as easily as a grizzly could take down one of my cows."

Spotted knapweed, as the plant is more formally known, is a national menace, a weed of mass destruction. In Montana alone, it covers some 4.5 million acres and costs ranchers more than $40 million annually in herbicide and lost productivity. Native from central Europe to Siberia, spotted knapweed reached North America in the late 19th century. It invades pastureland and renders huge tracts commercially useless, because cattle, horses and most other animals turn up their noses at it. The purple-flowered pest, which some nonranchers regard as beautiful, has become so rampant that elk have changed their migration routes to avoid it.

New research points to an unusual reason for the plant's success. Ragan Callaway of the University of Montana in Missoula, who studies how plants interact with one another, and Jorge Vivanco of Colorado State University have found that spotted knapweed conducts chemical warfare on its neighbors—the first comprehensive evidence of an invasive plant using an offensive chemical weapon.

Several thousand foreign plant and animal species, including zebra mussels from Eurasia and Asian long-horned beetles, have colonized the United States. About one in ten turns invasive, or spreads to such a degree that it disrupts existing ecosystems. All told, invasive species cost the nation upwards of $140 billion a year. But few rival spotted knapweed for sheer nastiness. It's "one of the worst invasive plant species in the United States, one that we really don't have much of a handle on yet," says Eric Lane, Colorado's weed coordinator.

Spotted knapweed arrived in Victoria, British Columbia, in 1883 either as a contaminant in imported alfalfa or in soil used as ship ballast. Since then, the weed has spread throughout Canada and into almost every state in the United States—and every county in Montana. In late summer and autumn, its one-inch, thistle-like flowers cast a pink-and-lavender hue across dry meadows, pastures, stony hills, roadsides and flood plains. "There are very few species of plant, even invasive ones, that so completely take over a landscape the way that spotted knapweed does," says Callaway. Worse yet, it's tough for native plants to grow back when spotted knapweed has been eliminated. "Many herbicides can kill it easily," Vivanco says. "The real problem," he speculates, "stems from the fact that spotted knapweed releases a chemical that leaves the soil toxic to the native plants it displaces."

The idea that plants might engage in chemical warfare against each other is even older than spotted knapweed's residence in North America. In 1832, Swiss botanist Alphos-Louis-Pierre-Pyramus DeCandolle speculated that noxious weeds may exude chemicals from their roots that inhibit the growth of other plants. Allelopathy, as the theory is known, has had its ups and downs among scientists; lately, it has been a hypothesis non grata. Still, Callaway and graduate student Wendy Ridenour decided to see if allelopathy could explain spotted knapweed's success in out-competing native plants.

Ridenour grew spotted knapweed together with Idaho fescue, a native bunch grass, in a clear pot filled with sand, enabling her to measure the growth of the plants' roots. Four days after the plants had germinated, fescue roots sharing a pot with knapweed were half the size of those grown alone or with other, native species. Then, when Ridenour spiked the pots with activated charcoal, which absorbs organic chemicals and would neutralize any poison that the weed released, the fescue roots grew at a nearly normal rate despite sharing quarters with spotted knapweed.

Vivanco took on the search for spotted knapweed's chemical arsenal. He and a researcher in his lab, Harsh Bais, found that within two to three weeks of germinating, spotted knapweed seedlings were already churning out a substance that killed any other plant that they exposed to it. The toxin, they found, is a chemical called (-)-catechin (pronounced minus-CAT-e-kin). Most chemicals occur in one of two forms that mirror each other, like left and right hands. Green tea and cotton plant roots produce the "plus" version of catechin, using it as a potent antibiotic to keep bacteria from nibbling on them. But spotted knapweed produces the "minus" form, which almost immediately destroys the roots of other plants. Botanists have long known that plants use chemicals to defend themselves against insects and other predators, says Alastair Fitter, a biologist from the University of York in England. But in this case, the toxin appears to be arming a plant for invasion.

Vivanco and Callaway went on to show that spotted knapweed-infected North American soils had two to three times more (-)-catechin than soils from the plant's native habitat in Europe, where spotted knapweed grows in harmony with thousands of grasses and other perennials. This suggested to the researchers that either European soil microbes had evolved a way of eliminating (-)-catechin or that American spotted knapweed populations produce more of the toxin. The plant's European neighbors are also less sensitive to the chemical; in contrast to their North American counterparts, the native grasses thrived in (-)-catechin soil.

Recognizing that European plants had perhaps evolved a means of coping with the chemical led the scientists to search for North American plants with the same ability. So far, Callaway and Vivanco's groups have found about ten different species that show resistance. Ridenour is now attempting to breed bluebunch wheatgrass, with the goal of producing a native species for replanting efforts.

Another approach is to control the knapweed with natural predators. Jim Story, an entomologist at Montana State University, has gone to Central and Eastern Europe in search of insects that attack spotted knapweed. Through his efforts, some 13 species of beetles, moths and other bugs have been released on spotted knapweed infestations in North America. His current favorite is a Romanian root-boring weevil known as Cyphocleonus achates, a half-inch-long insect that feeds on spotted knapweed leaves and whose larvae feed on the plant's taproot. Story says extensive testing has been done to try to ensure that none of the spotted knapweed predators has a taste for North American plants.

Yet there's reason to believe that the predator strategy will backfire. Callaway, Vivanco and several colleagues have shown in an unpublished greenhouse study that spotted knapweed responds to some insect attacks by increasing its production of (-)-catechin.

A study of one "natural" insect method of controlling knapweed suggests that the cure may be nearly as bad as the disease. Since the early 1970s, European seed head flies have been released to control spotted knapweed throughout the nation. U.S. Forest Service ecologist Dean Pearson has found that deer mice were gorging themselves on fly larvae inside spotted knapweed flowers. The mice were each eating up to 1,200 larvae per night during the hard Montana winter, getting as much as 85 percent of their diet from them. Deer mouse populations doubled and even tripled in spotted knapweed-infested areas.

The problem is that deer mice are carriers of hantavirus. Since 1993, this highly lethal germ has killed more than 100 people, mostly in the Western United States, with a few cases as far away as Maine and Florida. In the spotted knapweed-infested hills surrounding Missoula, where the European seed head fly is ubiquitous, the number of hantavirus-infected deer mice has skyrocketed. And given that the European seed head fly is now being released in the Eastern United States to control spotted knapweed, it raises the possibility that hantavirus could escalate there as well, says Pearson.

For now, Missoula is attempting to control spotted knapweed on its 4,000 acres of parkland and open space with a 300-strong flock of sheep. "Unlike other grazers, sheep love knapweed, and we're finding they do a pretty good job of controlling the infestation," says Marilyn Marler, Missoula's open space weed coordinator.

Would Wayne Slaght ever think about grazing sheep on the Two Creek Ranch? There's a historic animosity between sheep and cattle ranchers, stemming from range wars in the late 1800s over grazing rights. Perhaps, then, it should come as no surprise that the question triggered a raised eyebrow, a thoughtful look and a quiet shake of the head: "I think I'll wait for something better to come along."