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Target meal. An introduced weevil was supposed to dine selectively on Canada thistle (above), but it's developed a taste for a native thistle as well.

Weed-eating Insects Munch Wrong Plants

HILO, HAWAII--An nonnative insect released in Colorado in 1992 to control the weed Canada thistle has started munching on native thistles instead, researchers reported here 31 July at the Society of Conservation Biology meeting. The finding strengthens some ecologists' claims that the current screening process for insect releases doesn't protect native species.

Nonnative organisms have been introduced to combat another alien species in the United States for almost a century. Although the releases were not well-regulated in the early part of the 20th century, in recent years, such plans have been subject to more and more testing before insects are set free. Advocates of biological control have claimed that recent reports of released insects harming native species are the result of releases made 20 or more years ago.

However, in 1999, ecologist Svata Louda of the University of Nebraska, Lincoln, found the biocontrol weevil Larinus planus feeding on a native thistle in the Gunnison National Forest of Colorado. "First I was flabbergasted--and then I was upset," she says. By collecting and dissecting seed heads of native Tracy's thistle and nearby Canada thistles, she and her collaborators found the weevil on native plants more often than on the target. In fact, biocontrol agents are to blame for more than 50% of the insect damage to seeds of Tracy's thistle.

The weevil was evaluated for release in 1990 using contemporary protocols intended to prevent harm to native species. Finding the insect chowing down on native plants demonstrates that the current criteria are not stringent enough, says ecologist Daniel Simberloff of the University of Tennessee, Knoxville, who has been critical of biological control efforts. He says of Louda's discovery, "I'm not surprised."

Larinus planus page
Canada thistle control by insects
Canada thistle fact sheet