ALBUQUERQUE, N.M.—Salt cedar trees have a reputation as water hogs, and they’re known for pushing out native vegetation along rivers and in wetlands across the West.
Land managers have tried everything from burning them and ripping them out by the roots to spraying herbicide on their pink plumage. For the last several years, they’ve experimented in some states with exotic leaf beetles—the only natural enemy of the nonnative, invasive trees.
While the beetles have successfully defoliated tens of thousands of acres of salt cedar in Colorado, Texas, Utah and Wyoming, researchers in New Mexico are stumped.
That’s because beetles released along the Pecos River in the southeastern part of the state remain limited in number and don’t seem to have much appetite for the variety of salt cedar found there.
“Maybe it’s something about the trees or just the beetles’ adaptation to the area,” said Kevin Gardner, a senior research specialist at New Mexico State University’s Department of Entomology, Plant Pathology and Weed Science.
Researchers first released leaf beetles in New Mexico in 2001, a year after similar releases in other Western states.
The U.S. Department of Agriculture had the foreign insects quarantined and studied for more than a decade to make sure they would not be a threat to anything other than salt cedar.
The beetles released in other states have proven to be healthy eaters, but researchers say those near Artesia, N.M., have yet to defoliate more than one tree and haven’t been prolific reproducers.
Gardner has some working theories.
One is that herbicide treatments along the Pecos in recent years have forced predators—like ants—into remaining stands of salt cedar. Another is that the Artesia area has been stricken by drought, leaving little moisture in the baked soil, and the air is less humid than in the bugs’ native Crete.
Gardner also is investigating the beetles’ palate and whether they favor certain varieties of salt cedar.
Salt cedar, also known as tamarisk, was brought to the United States more than a century ago as an ornamental plant and soil stabilizer. It wasn’t long before the trees took over, gulping up water, killing native plants and growing into impenetrable stands that became fire hazards.
Researchers transplanted a dozen varieties of salt cedar in cages at New Mexico State’s experiment station in Artesia. They put the beetles in the cages and spent the summer watching them.
The beetles appear to have taste buds for salt cedars in Nevada. Once the favored trees were stripped down, Gardner said the insects moved to New Mexico varieties found along the Pecos and the Rio Grande.
“The study definitely indicates that there are differences in preference,” Gardner said.
A survey last week near Artesia turned up a large pocket of beetles, giving Gardner some hope that the insects will outgrow their pickiness.
He’s also hopeful because beetles have been found for the first time in northwestern New Mexico. They already have defoliated several hundred acres of salt cedar along the San Juan River.
Those beetles are likely migrants from earlier releases of a Chinese variety in southeastern Utah and southwestern Colorado.
“These beetles have moved in very rapidly because of the large populations they’re moving from. We’re trying to get a grasp on exactly how far they can move with these surveys that we’re doing,” said Gardner, who spent part of the week looking for beetles between Shiprock and Farmington.
State and federal land managers are carefully watching the insects’ movement because of their potential impact on the habitat of the Southwestern willow flycatcher, an endangered bird typically found in riparian areas.
Gardner and others say protecting the bird has become a Catch-22 for land managers because some flycatchers have been spotted nesting in salt cedar.
“Until you do something to get rid of the salt cedar and get the native vegetation back to a healthy state, you’re not going to save that bird,” Gardner said.
Eddie Williams, a noxious weed coordinator for the Bureau of Land Management, said it takes years of regular treatment to knock back salt cedar to manageable levels. He said the agency spends about half of its annual $500,000 weed budget on getting rid of the trees in New Mexico.
The BLM has treated about 12,500 acres over the last five years, Williams said.
On the Delaware River in southern New Mexico, it has taken about seven years to see a significant reduction in salt cedar.
BLM range conservationist Ray Keller said there were about 3,000 plants per acre when the project was started in 2002. Now, that’s down to about a dozen plants per acre and native grasses and cottonwoods have come back.
