According to a recent study, in which two UCSB scientists were collaborating authors, the troublesome invasion of the tamarisk — a non-native tree which has overrun rivers in the western U.S. — can be controlled through defoliation via beetle.The hero of the study, the tamarisk leaf beetle, was released into tamarisk populations along the Virgin River in Nevada after years of quarantined testing. The result was impressive river water conservation due to massive defoliation of tamarisks — the beetles’ sole food source.
The project is part of a larger trend toward the use of biocontrol — the use of organisms for population management — to deal with pests such as the tamarisk. Beetles used as agents of biocontrol provide a solution to the tamarisk problem that is not as laborious or ecologically damaging than other options, such as uprooting the offending plants.
According to first author Robert Pattison, although the study’s approach to tamarisk control is simple, its results are nonetheless impressive.
“Trees use water by their leaves. That is how plants soak up water. Without leaves, they cannot soak up water. In that way, the study really is not that interesting,” Pattison said. “What is surprising is how quickly the beetles spread and how effective they were.”
The beetles exceeded researchers’ expectations by executing the task of defoliation with impressive speed and effectiveness. After two years, the beetles had defoliated all the tamarisks within 25 miles of their release site, according to Pattison.
Efforts to quell tamarisk invasion are related to water conservation, as the tamarisk populations pull water from the ground and deprive the local area of much-needed water.
“We are really interested in the biodiversity issues; but it is really the water resources issues that are driving the biocontrol of tamarisk,” Tom Dudley, an associate research biologist at UCSB who lived in Nevada with his wife and co-author Carla d’Antonio while conducting the study, said. “Those agencies that are trying to conserve water give support to these programs.”
Although the main issue driving the UCSB study was the tamarisks’ excessive thirst, tamarisk dominance also leads to issues such as the displacement of native plants, less habitable conditions for wildlife and increased fire danger. The areas in Nevada where most of the research was completed are not only low on water, but also on funding, according to Pattison.
The researchers found the beetles were not alone in their dependence on tamarisks. The southwestern willow flycatcher, an endangered bird which nests in the tamarisk trees, added a new, more complicated aspect to the project.
“That ended up creating a huge bureaucratic nightmare. The Virgin River is the first place where the beetle and bird are coexisting together,” Dudley said. “As a way to get out of that issue, there is a foundation doing restoration of native vegetation so that we can provide an alternate habitat for the willow flycatchers to nest in.”
Once native plant growth returns, not only will the birds no longer be in danger of losing their protected nesting sites, but new food resources will become available to them, according to Dudley. The pairing of beetle biocontrol and native plant restoration programs is expected to result in a win-win situation.
It is incredibly unlikely the beetles’ diet will adapt to include native plants, according to Pattison. The beetle communities survive only as long as tamarisk populations can support them.
“Fortunately, tamarisk is really unique in the western regions of the United States. Not much else is close to it taxonomically. [The] beetles die when they are fed other things,” Pattison said.
According to Dudley, however, there is still more to be done to protect these river ecosystems.
“It is not a perfect solution but it is part of a solution,” Dudley said, “We do have to do more to manage our rivers better in order for them to be more habitable for native species.”