Introduced Beetle Bids to Curb Water-Sucking Tamarisk

The tamarisk plant has been stealing water from other and increasing fire risk, so its Asian predator has been introduced to control it in Southwest
tamarisk, drought, leaf beatle, Diorhabda carinulata

Wikimedia Commons/David Hawgood

Along the rivers and streams of the Southeast, an invasion of tamarisk plants has been spreading, largely unchecked, for the last 200 years. Native to Kazakhstan and adjoining regions of northwestern China, the plant has in many places outcompeted other species like willow and cottonwood, sucking up water in a region where it is in short supply and constant demand.

To curb that invasion, scientists have engineered another. A decade ago, they began introducing populations of the tamarisk's natural predator, the leaf beetle Diorhabda carinulata, to prey on the plants and bring their numbers into check.

And as a recent study by researchers at the University of California, Santa Barbara, suggests, the beetles are adapting to their new role as biocontrol agents even more readily than anticipated.

When scientists first introduced the tamarisk leaf beetle to the Southeast in 2001, the insect was disoriented by the shorter daytime cycle resulting from the region's lower latitude, explained Tom Dudley, lead investigator at UC Santa Barbara's Riparian Invasive Research Laboratory. With daylight shortening to 14.5 hours in late summer, the beetle would begin hibernating when it should have been feeding and reproducing, he said.

As a result, the insects would burn through their carbohydrate stores before the arrival of spring, causing mass die-offs.

But after only a few years in the Southwest, they appear to have adapted to the change in geography. Fieldwork by Dudley and co-author Dan Bean suggests that the leaf beetle has adjusted its hibernation cycle to accommodate its new conditions, possibly by reverting to a temperature cue rather than a daylight cue for hibernation.

Learning from mass die-offs
"This is one of the clearest cases of rapid evolution," said Dudley. At the same time, it is not wholly unexpected, he added, since mass die-off events have trimmed the population down to only those that can cope in their new environment.

As a result of their successful adaptation, the beetles have spread quickly through much of the area affected by tamarisk, their reach expanding from Utah down through Arizona and to the banks of Lake Mead. Carried by the wind, they can travel up to 50 kilometers from their place of birth, Dudley said.

Though their passing is often marked by yellow riverbanks thick with defoliated tamarisk plants, there are always some plants that remain, Dudley said.

"It's never the aim of biocontrol to completely wipe out a species," he said. The goal is to suppress it to a controllable density so that native plants like willows and cottonwood trees can again compete, he added.

After nearly two centuries in the American West, the plant has to some extent integrated with the surrounding ecosystem. Coupled with other local species, it can provide refuge for the willow flycatcher, an endangered bird native to the Southwest. And it still provides some of the functions for which it was originally introduced to the region, forming windbreaks and preventing soil erosion.

One tough competitor meets another
But its unchecked spread has also generated serious concerns. The tamarisk takes in water voraciously, and is better adapted to withstand prolonged drought than many of its other riparian neighbors.

Climate projections of increased aridity could further play into the tamarisk's hand, helping it choke off competition from the other plant species that vie for the region's scant water supplies.

Even when green, the plant burns readily, Dudley said, raising concerns that riverbanks where it dominates could be transformed from fire barriers to fire corridors.

These concerns all helped to win approval for the biocontrol project, though the technique has met with stiff resistance and deep skepticism from many parties, including prominent environmental groups.

Though the beetles themselves are a relatively new addition to the ecosystem, they, too, have found a niche, and appear to have become a food source for other local birds, Dudley said.


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