MOAB — If you walked into a conversation about tamarisk trees too late, you might think you were hearing the description of the alien antagonists from the latest sci-fi movie.
If you burn them, they grow back stronger. If you cut them down, they grow back thicker. You can't drown them. Poison has no effect on them. But scientists, after decades of fighting against them, may have at last found their silver bullet: a small, black and brown leaf beetle, a natural enemy of the tamarisk tree.
The tamarisk tree (tamarix chinensis) is native to Central Asia and the Mediterranean. It was introduced to the Western United States in the early twentieth century for use as an ornamental, as a windbreak and for erosion control.
It is well adapted for the hot, arid climates with alkaline soil of its native habitat and found these same condition in the riparian areas — the bank of a river or lake — of the Western United States, and as a result they have grown wildly out of control.
It's now common to see river banks along the Colorado Plateau densely populated with them. According to the U.S. Department of Agriculture (USDA), tamarisk trees may cover as a high as 2 million acres in the Western United States.
According to the U.S. Geological Survey (USGS), tamarisks can be found in all Western states and they're especially prevalent in southern Utah, northern Arizona, western Colorado and western New Mexico. Native plants find it hard to compete with tamarisk trees.
Tamarisk populations typically grow in dense thickets, with as many as 3,000 plants per acre. Tamarisk trees, a type of salt cedar, draw more salt from the groundwater than native vegetation, and their leaves deposit this salt back into the ground when the leaves fall. This often results in a substantial rise in soil salinity, which then prevents the germination of many native plants.
Dan Bean, director of biological pest control for the USDA, says the tamarisks push out native plants that other species of birds, mammals and insects in the ecosystem utilize for survival. "They're only bad because they tend to reach really dense population levels," Bean said. "In numbers that large, they change the ecosystem."
Tamarisk trees reproduce prolifically. They can spread both vegetatively by sending out roots, which then grow into new trees, and also sexually by spreading seeds. In addition, they are very difficult to kill. A mature tree can resprout after fire, flood, cutting and poisoning with herbicide.
A main factor aiding the tamarisk's success in the West has been a lack of natural predators. Since no other eradication method proved successful, scientists became interested in identifying natural enemies from the tamarisk's homeland that could aid in the tamarisk battle.
In the 1980s, the USDA looked for insects that are natural enemies of tamarisk trees. They looked in China, Tunisia, Greece, Kazakhstan and Uzbekistan, eventually choosing a tamarisk-eating specialist called the tamarisk leaf beetle.
Before releasing the beetle into populations of tamarisks in the Western United States, the USDA first tested it in a quarantined environment. Testing began in 1992, with wider field testing happening in 1999.
According to USDA reports, the beetles were subjected to an extensive range of host plants, and tests showed that the beetles favored tamarisk trees to native plants to the point that they would rather starve to death before attacking a native plant. Happy with these results, the USDA released the beetles into the wild on a very limited, closely monitored basis in 2001, and on a much wider scale in 2004, releasing beetles into tamarisk groves outside of Moab and Grand Junction, Colo.
Since then, beetles have been released in more than 100 different areas throughout the West, Bean said.
And now the beetles are spreading of their own accord and they're in the process of stressing millions of acres of tamarisks on public and private lands, Bean said. The beetles have spread far enough on their own that there's now a moratorium on new beetle releases.
The tamarisk leaf beetles, feeding in large numbers, have an amazing ability to defoliate an entire stand of tamarisk trees in a matter of days, Bean said. Once the plant has been defoliated, it can no longer photosynthesize and store food in its roots.
Once a tree has been defoliated, the beetles then move on in search of green trees, leaving behind them a path of brown tamarisk trees. Even though the trees look dead, they're not. The following spring, the tamarisk will send out new leaves from energy stored in its roots. But so far, the beetles have returned each year to feed on the trees again, each time defoliating the trees. Eventually this repeated defoliation exhausts a tree's ability to revive and the tree dies. According to early results, it takes four to five years to fully kill a tree.
Before the arrival and success of the tamarisk leaf beetle, resource managers, tasked with the job of removing the trees from their respective resource areas, found it an almost impossible task. Methods common for killing a normal tree didn't work with tamarisk trees.
When the resource managers cut down a tamarisk tree, it just sent up several sprouts from its newly cut trunk. After it matured, it was actually denser than it had been before.
The beetles have been so effective, decimating tamarisk stands so quickly and efficiently, that many people now worry that the USDA's new-found silver bullet, once they've eliminated their primary objective, may go in search of new targets.
The Tamarisk Coalition's Season Martin is confident that the beetles won't spread to native plants. "In all the studies (the USDA) did, they didn't spread to any native plants. There's no risk of this happening," he said.
The spread and effectiveness of the beetles is being monitored by the Tamarisk Coalition, a nonprofit agency that partners with land managers from any area that is affected by the beetle. The land managers report to the Tamarisk Coalition the effect that the beetles are having on their lands and any new changes that may have occurred, Martin said.
In addition, Martin said that the Tamarisk Coalition visits and monitors different affected areas once a year, between late May and the end of September, when the beetles are most active.
After the tamarisks have been killed, they'll still be left standing in large, dead masses. In some cases, the dead trees will be removed by whatever agency oversees the land, Bean said. And in some cases, after removal of the dead trees, native plants will be replanted, most likely willows, cottonwoods and other native species that grow in riparian areas. But reaching the affected areas can be time- and cost-prohibitive.
Mark Miller, chief of resource stewardship and science for Canyonlands and Arches national parks — areas heavily affected by dead tamarisks — says they simply don't have the funding or staff to remove all the dead trees. "We have funding for one six-month employee for four Park Service units in all of southwest Utah," Miller said.