Satellite Views of Beetles Show Salt Cedar Thirst Overstated

"By 1938, infestations were found from Florida to California and as far north as Idaho. Saltcedar continues to spread rapidly and currently infests water drainages and areas throughout the United States," says the U.S. Animal and Plant Health Inspection Service.

Tamarisk has a bad reputation as a water-sucking plant that dries springs, lowers water tables and reduces stream flows. According to some estimates, the West is losing up to 4.5 million acre-feet of water per year because of tamarisk - enough to supply more than 20 million people with water for one year or to irrigate over 1,000,000 acres of land.

But new research using satellites to monitor beetles is showing that tamarisk is less of a water hog than previously thought.

To combat the plant invaders, the salt cedar leaf beetle, Diorhabda elongata, was brought to the United States from Kazakhstan. After an environmental assessment, the federal government approved the beetles for tamarisk control.

A saltcedar leaf beetle on a tamarisk plant (Photo by Bob Richard courtesy APHIS)

Thousands of the beetles were released in Utah during summer 2004, then again in summer 2005 and 2006 at locations along the Colorado River near Moab. Widespread defoliation of tamarisk in the area was observed during summer 2007.

Now, University of Utah scientists say their new study shows it is feasible to use satellites to monitor the extent of the beetle's attack on tamarisk, and whether use of the beetles may backfire with unintended environmental consequences.

"We don't have any idea of the long-term impacts of using the beetles; their release may have unexpected repercussions," says Philip Dennison, an assistant professor of geography and first author of the study scheduled for online publication later this month in the journal "Remote Sensing of Environment."

"The impact of this defoliation is largely unknown," says study co-author Kevin Hultine, a research assistant professor of biology at the University of Utah. "The net impact of controlling tamarisk could be positive or negative."

Dennison and Hultine say recent research indicates tamarisk's thirst is overstated.

"Some of the earliest research on tamarisk water use suggested tamarisk uses dramatically more water than other tree species," Hultine says. "So a lot of estimates on water loss over entire river reaches are based on information that now has been discredited in the scientific literature."

Hultine believes that unless aggressive programs to restore defoliated areas are implemented, tamarisk will be replaced by other invaders, such as Russian knapweed, Russian olive and pepperweed, that may use more water than tamarisk.

Eradicating tamarisk with beetles also may reduce bird habitat, he warns.

While some tamarisk has died in Nevada where the beetles first were established, "we don't understand whether repeated defoliation eventually will kill most of the trees, or will they reach some point where they'll just have less leaf area over the entire year," Hultine says.

The researchers also used the satellite to estimate the evaporation of water from soil and the transpiration or use of water by plants to learn more about how defoliation of tamarisk affects water use. For comparison, Hultine measured sap flow through trees, which reflects how much water is used by the trees.

Satellite estimates of tamarisk water use declined modestly as the plants were defoliated, Dennison says. The findings also were consistent with earlier research indicating tamarisk uses less water than scientists had thought.

Dennison says he and his colleagues did the study to test the feasibility of using satellites to monitor tamarisk defoliation on an ongoing basis. That, he says, could be done by federal agencies such as the Bureau of Land Management, Bureau of Reclamation and U.S. Geological Survey.

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