Wicked Weed of the West

Spotted knapweed is driving out native plants and destroying rangeland, costing ranchers millions. Can anybody stop this outlaw?

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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 Callaway, Vivanco and several colleagues have shown in an unpublished greenhouse study that spotted knapweed responds to some insect attacks by increasing its production of (-)-catechin.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."


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