Anyone who has taken a trip down the Colorado River has likely noticed banks covered with tamarisk trees, one of the region’s most invasive species. At the regular meeting of the Grand County Council on July 3, Wright Robinson, tamarisk bio-control research technician, and Tim Higgs, weed supervisor for Grand County, presented 11 years’ worth of data on tamarisk control to the council. Their study sought to better understand why and how beetles kill certain trees and not others.
Tamarisk, also called salt cedar, was originally imported from its native dry areas in Eurasia and Africa to stabilize stream banks. It accomplished the job, but without any natural predators it spread throughout the American West and choked out many native plant species. Since its unrestrained distribution, tamarisk has proven highly difficult to exterminate. Introducing a predator was the most natural solution.
Unless the root crown is destroyed, which can often be many feet underground, tamarisk will grow back. Within weeks of being cut or burned, sprouts will reappear. Therefore, burning and cutting is largely ineffective against them. “When you cut, when you burn, they love it. It clears the competition and they’ll send up new shoots,” said Robinson. Herbicides can work, but spraying the whole county with chemicals is not an appealing option. A more creative solution was found in the tamarisk beetle, which can control tamarisk naturally.
Beginning in 2004, tamarisk beetles began being released in Grand County. Releases continued in 2005 and 2006. The success of the beetles was not immediate, but over time the results have become clear. In 2005, total browning of tamarisk in Grand County was less than two hectares. By 2008 the beetles had spread throughout the county and in 2013 the browning figure had risen to 650,000 hectares. “Wherever there was tamarisk, there was beetles,” said Robinson.
Browning does not necessarily mean the plant is dead. The beetles open the leaves and drink sap from them, which desiccates the leaves. The result is the plant cannot carry out photosynthesis and must rely on nutrients stored in the roots. Robinson described browned tamarisk as “on a perpetual diet… eventually it dies.” Beetle larvae reach a critical mass and cause the tree to crash. However, some tamarisk do recover. Notably, in terms of fire danger, tamarisk present a high risk whether they are green, brown, or completely dead. The point of the study presented to the council was to figure out which factors determine tamarisk survival.
Researchers conducted mortality studies at over 1,000 locations around the county. They hypothesized that tamarisk defoliation would follow a downstream pattern, would be greater where beetles had fed longer and would relate to plant stress variables such as soil texture and salinity and surface water availability. After years of gathering data, none of those factors were proven to directly influence tamarisk survival rates. “We’re still trying to understand why and how beetles kill some tamarisk but not others,” Robinson said.
Though the study didn’t result in definitive answers, it did produce “the most extensive, most continuous landscape-level data sets for beetle movement, tamarisk response and tamarisk mortality in the country,” claimed Robinson. A graduate student from the University of Denver will be publishing an article on the study in the peer-reviewed scientific journal Biological Invasions. After 11 years of constant monitoring, “the big question is: will our field mortality studies be able to continue in 2018?” said Robinson.
While we may not understand exactly how they’ve been effective, the results are clear – tamarisk beetles are making a difference. Success of the tamarisk beetles has gone in cycles though. Beetle populations will crash when they over-consume their food source, which allows the tamarisk to come back. Luckily, once the tamarisk recover, so do the beetles. The beetles have undoubtedly been a boon for native species. Willows have reclaimed much of their old habitat on the riverbanks. According to council member Rory Paxman, the increase in willow coverage has led to a drastic rise in the prevalence of beavers.