NATURE NOTES:
In the Fight with Salt Cedar, Leaf Beetle Makes Headway

By Andrew Stuart

When it comes to the rivers of the Southwest, tamarisk, or salt cedar, has long been public enemy number one.

Introduced for erosion control, the non-native tree came to dominate riverside habitats where cottonwoods and willows once flourished. It’s been blamed for declining water quality and quantity on the Rio Grande and the Pecos River in West Texas.

Tamarisk was attacked with herbicides and bulldozers. More recently, scientists have imported one of the plant’s natural enemies – the tamarisk leaf beetle. Beetle larvae feed on tamarisk leaves.

A leaf beetle project launched in the Big Bend in 2006. And the results here have been promising.

An Old World native, tamarisk was introduced to the American West in the late 19th century. Where overgrazing had reduced native vegetation, tamarisk succeeded in holding soil in place.

But there were other effects. The plant spread rapidly, and, on riversides once lined with cottonwoods, monocultures of salt cedar took hold. The Big Bend stretch of the Rio Grande is a case in point. By the 1980s, from the Presidio Valley upstream through Hudspeth County, thickets of tamarisk lined much of the river.

Scientists described salt cedar as “gluttonous” – consuming far more water than native trees. Tamarisk leaves excrete salt, and the plant was blamed for increased salinity in soils and water.

Millions were spent to spray and mechanically remove tamarisk. But in the early 2000s, scientists turned to a new approach. They would introduce a predator, which, in its native lands, was known to defoliate vast stands of tamarisk. It was a beetle – of the genus Diorhabda.

Christopher Ritzi is an entomologist at Sul Ross State University in Alpine. He’s led a leaf beetle project in the Big Bend.

“These beetles are a fairly small size,” Ritzi said. “They’re a little bit less than a centimeter or so in size, kind of an olive-green color, typically with a couple strips on the back of their abdomen and a little bit of a yellowish tint associated with them as well.”

The beetles were introduced at four sites in the Presidio Valley in 2006. At first, the beetles were kept in mesh bags. In 2008 and 2009, scientists released the beetles into the environment.

As larvae, the beetles feed on the top layer of tamarisk leaves. They go through three or four generations each year – creating a sustained “lawn-mower effect” on the tamarisk, Ritzi said.

Tamarisk is a resilient plant. It can be defoliated multiple times without being killed. But scientists started to see progress in 2010 – particularly near Ruidosa and where Alamito Creek joins the Rio Grande.

“We have actually had success, where we’ve had dead stands of salt cedar being confirmed,” Ritzi said. “It took, in many cases, about four to five years of being defoliated up to four or five times per year, to be able to burn it down and get the beetles to do enough damage to take those plants out.”

Beetles have been similarly effective in Big Bend National Park and on the Pecos River.

By 2014 and 2015, beetle numbers in Ritzi’s research area had declined. Having knocked back the tamarisk, they were migrating upstream.

As the story of tamarisk makes clear, introducing non-natives is risky. As the beetles move upstream, they could defoliate critical nesting areas for the endangered Southwestern willow flycatcher. And while scientists don’t believe the beetles will extend their feeding beyond tamarisk, they’re keeping an eye on it.

Tamarisk is only one component in the transformation of the Rio Grande. The changes to the riverside habitat have as much to do with the damming of the river, and water use for agriculture, as they do with invasive species.

Some scientists now believe that claims of salt cedar’s “gluttony” were overstated, Ritzi said. And tamarisk control has not noticeably increased the water supply in the Big Bend stretch of the river.

But Ritzi said native plants are returning to areas where the leaf beetles have done their work.

“With more plant diversity, we should hopefully see more animal diversity,” he said, “and that should give us a much more interesting river, make more of the river wild and scenic and interesting for people to interact with.”

The eradication of tamarisk is unrealistic, Ritzi said. But the beetles are beginning to play an important role in controlling the plant – and creating more vibrant ecosystems along West Texas rivers.