But caution needs to be used in deploying those non-native organisms, University of Nebraska-Lincoln botanist Svata Louda said after making an unexpected discovery in Nebraska's Sandhills.
Since 1984, Louda has studied the Platte thistle, whose range is restricted to the Sandhills, in the Nature Conservancy's Niobrara Valley Preserve in north central Nebraska. In 1993, she and her team of researchers came across an insect species they had not previously seen in the region - the flowerhead weevil (Rhiocyllus conicus), a Eurasian species that had been introduced in eastern Nebraska in 1972 to control musk thistle, another Eurasian species that was introduced by accident in the 19th century.
The invasion of the weevil has been devastating for the Platte thistle. The insect, Louda said, found the Platte thistle to be a more-than-adequate substitute for the musk thistle (which Louda has not seen in her study area).
"They've reduced seed production to the extent that it's only 14 percent of what it was before," she said. "It's already limiting plant numbers."
The possible extinction of the Platte thistle involves more than the loss of a native species, Louda said. It could have economic consequences, too.
"It's not a weed, it's not a bad plant," she said. "Ranchers laugh when I ask if it is. It's butterfly food and bees depend on the nectar - some of the native bees that have become really important. If something goes wrong with honeybees, and they've had problems with mites, native pollinators become even more important."
Louda said her findings should send up a yellow flag - not a red one - to advocates of biological controls.
"People had hoped that biological control would be an environmentally sound alternative to chemicals in the control of weeds, especially in our region where we have large rangeland areas where it's often too expensive to try to control weeds chemically," she said. "When you have a bad weed, the idea of biological control is a really good one. It's based on fundamental ecological principles, but we haven't been as careful about the selection of exotic insects as I think we should have.
"It's almost like the advocates hoped they had found a silver bullet. 'This is the answer, that we're going to be able to solve all the exotic weed problems by moving in all these exotic insects.'"
Louda said there are two essential questions biologists and land managers must answer before they can use biological controls effectively. If they can't answer them in the affirmative, chemicals or simple hand weeding are better solutions.
The first question is the "diet breadth" of the species to be introduced. "Will it eat a native species? If it won't, you have a much better case for biological control," she said.
The other question, she said, is "How bad is the problem? Is it really affecting native vegetation and native lands in such a way that it's worth releasing something that can't be stopped once it gets going? You can always stop using chemicals, like we did with DDT, but you can't stop an exotic species once it has a foothold."
Better yet, the best solution for problems like musk thistle might be to get rid of them the old-fashioned way.
"My husband is a farmer and he controls these exotic thistles by hand," Louda said. He goes out and cuts them and after he's done that for a few years he doesn't have problems. You spend an afternoon or two in the growing season controlling them. And if you don't overgraze, you don't have large problems."
Louda and her team's findings were published in an article,
"Ecological Effects of an Insect Introduced for the Biological
Control of Weeds," that appears in this week's issue of Science,
the magazine of the American Association for the Advancement of
Science. Her co-authors are Deborah Kendall of Fort Lewis College
in Durango, Colo.; Jeff Connor, resource manager for Rocky
Mountain National Park in Colorado; and Dan Simberloff of the
University of Tennessee, Knoxville.
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