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Chewed out: an invasive plant called tamarisk has been killing off cottonwoods and destroying river habitat for decades. Now park rangers are hoping a beetle can turn the tide.

When John Wesley Powell made his renowned 1869 trip down the Colorado River, he described the river banks as "set with willows, box-elders, and cottonwood groves." In fact, his exploration party often camped among willows. But 150 years later, the Colorado River corridor looks different--its banks are now dominated by an aggressive, invasive plant that is choking out the native vegetation.

Tamarisk was introduced to the United States from Eurasia in the early 1800s as an ornamental plant. A water lover, the tree spreads along waterways up to 12 miles per year, and it has displaced native vegetation on an estimated 1.6 million acres of land in the western United States. Because it burns so intensely, tamarisk accelerates wildfires. The shrubby trees also block river views and boater access to shorelines. "If you're hiking on trails along the river, you can't even see the water," says Jeff Troutman, chief resource manager for Canyonlands National Park.

The Colorado River winds through Canyonlands for about 30 miles upstream of its confluence with the Green River, then the two rivers join for another 14 miles through the park. Tamarisk stands along the route are among the largest and densest in the West. Park staff and volunteers have used a variety of methods to extinguish the plants, including removing them by hand, using chainsaws, and spraying herbicides. But few roads offer access to the Canyonlands river corridor, and trees typically grow back if not rigorously monitored. In 1984, insect ecologist Tim Graham worked on a tamarisk control crew in Horseshoe Canyon, an isolated unit of Canyonlands, where they completely removed the species. But 25 years later, he says, rangers are still pulling up seedlings. "You have to continually watch for it--if you miss it when it's a centimeter tall, the next year it will be a meter high," he says.

Five years ago, the park's eradication efforts got a boost--in the form of a tiny beetle. Tamarisk leaf beetles, which the U.S. Department of Agriculture (USDA) identified and collected in China in the late 1980s, feed on tamarisk until the plant is defoliated. After repeated feeding attacks, the root system shrinks, and the trees typically die within three to four years of beetle infestation.

Although the beetles legally can't be released on federal land, they are being introduced to areas outside several Southwest parks, including the Grand Canyon and Canyonlands. In 2004, with approval from the USDA, Grand County, Utah (which includes part of Canyonlands), released leaf beetles at several sites along the Colorado River, upstream of the park boundary. "Within two years, we saw the beetle moving down the river and into the park, and within four years it had pretty much exploded through the entire river corridor and was starting to move up side canyons," says Troutman. "We knew the beetles were going to move, but the surprise was how quickly they spread and had a significant impact on the trees," says Graham. Additional beetle releases followed in 2005-06.

Prior to the releases, the USDA did extensive testing, which showed that the beetles didn't harm native vegetation and would, in fact, starve in search of tamarisk. "The chance that these beetles will make a host shift to something native and run rampant through the riparian zone denuding everything is very slim," says Graham. And successful precedents exist: the Chrysolin beetle was used to control St. Johnswort in California and Oregon, and the Aphthona beetle helped reduce leafy spurge in several western states.

Although the beetles have munched away miles of tamarisk, it's impossible to predict where the bugs will go, says Stacy Kolegas, executive director of the Tamarisk Coalition, a nonprofit dedicated to restoring riparian zones. "In some places you'll see lots of defoliated tamarisk, and then right across the river the trees look healthy."

And the USDA's studies couldn't test for things like how the beetles' arrival might change the local food web. "One of our rangers noted that spiders are more abundant around the beetles, because it's a food source they haven't been able to exploit before," says Troutman. "These kinds of impacts are much harder to predict." To better understand the impacts, field crews monitor beetle activity, as well as bird populations and vegetation changes in the release areas. At the University of Utah, a research team is using satellite data to map areas of defoliated tamarisk, which can help determine where and how fast the beetles are moving.

Although the insect's rapid impact on the tamarisk is just what park managers hoped for, some park visitors are troubled by the sight of trees dying along the riverbanks. "Brown trees aren't very pretty, and the shade people used to count on in the campgrounds is gone," says Troutman. "So there are some changes we're going to have to tolerate until we get the native plants re-established."

Graham says that tamarisk is an ideal candidate for biological control because it's so different from any North American flora, which minimizes the chance that the beetles will find something native to chew on once the tamarisk is gone. And, aside from monitoring, it's a relatively maintenance-free removal method. "You don't have to do anything besides release the beetle," he says. "It didn't cost the parks a dime."

Laurie J. Schmidt is a freelance writer based in Fort Collins, Colorado, who specializes in earth and space sciences.
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Author:Schmidt, Laurie J.
Publication:National Parks
Geographic Code:1USA
Date:Jan 1, 2010
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