CHEYENNE, Wyo. - Concern about an endangered bird has caused the U.S. Department of Agriculture to declare a cease-fire in its biological war against saltcedar, an invasive tree that has taken over riparian areas across the West.

The USDA Animal and Plant Health Inspection Service last week formally ended its program of releasing saltcedar leaf beetles to eat saltcedar, also known as tamarisk, in 13 states: Colorado, Idaho, Iowa, Kansas, Missouri, Nebraska, Nevada, North Dakota, Oregon, South Dakota, Montana, Washington and Wyoming.

The reason for the program's demise is the southwestern willow flycatcher, an endangered species found in scattered pockets around the Southwest. The bird nests in saltcedar, as well as in native willows and cottonwoods.

Concern that beetles could destroy much of the bird's nesting habitat was why the USDA excluded New Mexico, Arizona and California from the beetle-release program, which began in 2005.

Now, scientists think the beetles are likely to spread from the states where they were introduced. They say it could be just a matter of time before the insects chew through saltcedar all the way down the Colorado River drainage in Arizona and eastern California.

"The beetles move around. They don't stay where you put them," Alan Dowdy, director of invertebrate and biological control programs for APHIS in Riverdale, Md., said Monday.

The USDA moved to end the beetle program last year, he said. A June 15 memo from Dowdy told APHIS state directors that APHIS no longer endorsed releasing saltcedar leaf beetles and stated that doing so could be prosecuted and punished by a fine up to $250,000 per violation.

The change has environmentalists who opposed the use of saltcedar leaf beetles from the beginning saying "I told you so." They also said it might be too little, too late to prevent one artificially introduced species from destroying another and wiping out an endangered native species in the process.

"It's very serious," said Robin Silver, with the Center for Biological Diversity.

The Tucson, Ariz.-based group and the Maricopa Audubon Society sued APHIS and the U.S. Fish and Wildlife Service last year over the release of saltcedar leaf beetles in southern Utah in 2006. The released beetles proliferated, the groups said, destroying several saltcedar trees containing southwestern willow flycatcher nests.

The release also opened a door for the beetles to spread southward, the groups said.

Saltcedar grows up to 30 feet tall. The tree was introduced to the West during the Dust Bowl of the 1930s and has since spread along streams throughout the region.

One of the problems with the tree is it concentrates salt in its leaves. When those leaves fall, salt can concentrate around the trees and prevent anything else from growing.

Saltcedar has been successful in part because of the dams built in the West during the 20th century, said Matthew Chew, an assistant research professor at Arizona State University.

The dams altered the natural flow of Western rivers like the Colorado, giving saltcedar trees an advantage over native willows and cottonwoods.

"They are adapted to this new regime - this new, artificially managed regime," Chew said. "We created a habitat. We created the perfect conditions."

The federal government's view that saltcedar leaf beetles could do no harm was an "illusion," said Jeff Ruch, executive director of the Washington, D.C.-based group Public Employees for Environmental Responsibility.

"Perhaps the best hope for the endangered southwestern willow flycatcher is for it to develop a taste for leaf beetles," Ruch said.