ALGODONES – The rust-red leaves in the tamarisk trees on the banks of the Rio Grande north of Albuquerque are the unmistakable sign that the trees’ nemesis, a leaf-eating beetle known as Diorhabda elongata, is here to stay.
“The law of unintended consequences,” mused Adrian Oglesby.
Introduced in the 19th century to protect railroad bridge abutments, praised for its ability to protect riverbanks from erosion, vilified for alleged water-sucking ways while simultaneously defended as wildlife habitat, the story of the Eurasian tamarisk – also known as salt cedar – is a textbook example of unintended consequences.
The beetle, introduced in small populations in an attempt to control the tamarisk, is the latest example. Brought from Europe to Utah and Colorado a decade ago, along with small populations in Texas, the beetle has run amok, spreading far beyond the narrow range biologists predicted.
After initial beetle arrival in 2012, the beetle rapidly spread uninvited up and down New Mexico’s rivers.
“Last year was really the year of the beetle,” said Oglesby, an attorney at a University of New Mexico water policy think tank and board member of the Tamarisk Coalition, a nonprofit tracking the beetle’s spread. “It came charging down the Jemez. It came charging down the Rio Grande, and now it’s charging up the Pecos as well.”
The beetles lay their eggs on tamarisks, with their larval offspring eating the leaves, quickly turning green patches of trees brown. Depending on local conditions, they often do not kill the tree outright, leaving it bristling with dead growth that nevertheless can sprout new leaves the following year.
Getting rid of tamarisk always has been an article of faith along Western rivers, but the dying trees along rivers’ edges in New Mexico and around the West are raising new questions – about fire risk and lost habitat for birds and other creatures that have made their homes in the artificial forests.
Oglesby saw the entire panoply of the tree’s history on display as he and a group of colleagues kayaked down the Rio Grande north of Albuquerque one recent fall afternoon.
Tamarisk swarmed over the river’s banks, crowding out native vegetation. In some areas, humans had intervened at great expense to clear them, creating an open bosque cottonwood forest.
But everywhere the scrubby tamarisk remained, there were signs of beetles chomping their way through them.
Formed to pursue habitat restoration along Western rivers, the Colorado-based Tamarisk Coalition now has become the de facto chronicler of the beetle’s spread. The group’s 2014 monitoring efforts are not yet complete, said Ben Bloodworth, who is overseeing the effort for the group.
But preliminary reports suggest that the beetle has become firmly established in Bernalillo and Valencia counties, and that a second population of beetles introduced in Texas has made its way through Las Cruces and is moving north up the Rio Grande.
The first beetle introductions, in Colorado and Utah, were approved by the U.S. Department of Agriculture, with the belief that the beetle’s impacts would be local.
Once it became clear the beetle was spreading much farther than expected, the agency stopped the program, but the beetle has continued to spread, undeterred.
The goal was water conservation.
“Originally, it was thought that tamarisk consumes a lot more water than natives,” Bloodworth explained.
The scientific research on that question, though, is not at all clear cut – first because native plants like cottonwoods and willows that come back in the tamarisk’s place also consume a lot of water, and second because a complete lack of vegetation in response can lead to increased evaporation.
A hope of water savings continues to drive tamarisk removal efforts, including calls for expanded beetle introduction, but the mixed evidence suggests that “such actions offer no panacea,” a team of researchers led by the University of Arizona’s Juliet Stromberg wrote in a recent report summing up the evidence.
That was the case in New Mexico, where the state’s water managers hung their Pecos River water supply hopes on tamarisk removal.
“It didn’t work,” Oglesby said.
The effort failed to generate the promised water, Texas sued because New Mexico failed to deliver its legally required share down the Pecos at the New Mexico-Texas border, and New Mexico ended up spending $100 million buying up and retiring agricultural water rights instead.
Here in the Albuquerque reach of the Rio Grande, the beetles arrived in 2012 and appear much more firmly entrenched in the region’s trees this year, said Rowan Converse, a biologist with the Bosque Environmental Monitoring Program, based at Bosque School.
Converse said she and her colleagues are seeing the classic beetle pattern – dead vegetation as beetles sweep through a stand of trees, followed by tiny shoots of green at the base of the tree late in the year as it struggles back to life.
It is unclear what land managers will do in response to the dying trees along the state’s riverbanks.
Research suggests the fire risk from beetle-hit trees is not a major problem, according to Bloodworth. Healthy tamarisk is already a major fire risk, and that risk does go up slightly because of the dead foliage when the beetles first hit a tree. But in subsequent years it drops substantially, below even the danger of its beetle-free kin.
Some worry about increased erosion as a result of the loss of trees currently anchoring the banks of the region’s arroyos and rivers, Bloodworth said. In areas where the beetle is more firmly established, land managers have had to deal with other weedy plants moving in to take advantage of the niche left by dying trees.
For now, action in the middle Rio Grande is limited to monitoring the beetle’s progress. “People are just waiting and watching,” Oglesby said.