COLORADO

In the early 1990s, the U.S. Forest Service released approximately 500 flower-head weevils on the edge of Gunnison National Forest, near Almont, Colo., to control the invasive Canada thistle. Apparently, tastes change.

In the 12 years since a 1990 study suggested that the weevil preferred Canada thistle to native species, the insect has been widely released in the United States, particularly in Western national parks, forests and monuments.

But in 1999 and 2000, Charles O'Brien, an entomologist with Florida A&M University, and Svata Louda, of the University of Nebraska in Lincoln, collected plant samples from the Gunnison National Forest. They discovered that the weevil fed extensively on Tracy's thistle, a relatively uncommon species found only in western Colorado and eastern Utah. Worse, they discovered that the weevil had little effect on the non-native Canada thistle it was supposed to control.

"We need to test these methods more thoroughly," says O'Brien, who advocates biocontrol when it's monitored closely. "We shouldn't be compounding the problem."

Forest Service botanist Andrew Katz says scientists are much more careful about biocontrol these days, but he admits that once foreign insects get established, "it's bloody hard to call them back."

Copyright © 2002 HCN and Beth McElroy