In arid West, a foreign legion of beetles takes on a thirsty invader

Scientists say the beetles released on Southwest riverbanks could tame the water-sucking tamarisk trees.

By , Staff writer of The Christian Science Monitor

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    Defoliated tamarisk bushes line the Virgin River in the northwestern corner of Arizona. Just three weeks earlier, beetles known as Diorhabda elongata passed through the area, feeding on the green leaves of the tamarisks and leaving behind these browning bushes. The result pleased scientists who helped introduce the foreign beetles to parts of the American West as tamarisk-control measure.
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Armies of foreign beetles are on the march along the river systems of the desert Southwest, and ecologist Tom Dudley greets them as little green liberators.

He is among a group of scientists and land managers who are bringing the beetles to America to wage war on tamarisk, an invasive plant that now dominates the Southwest riverbanks. The program holds out the promise – and potential peril – of shifting the environment in the west for trees, birds, and humans alike.

The tamarisk has become a pariah in these parts for crowding out native willow and cottonwood trees, and worsening wildfires. But it’s the plant’s thirst that earns it the most ire. By some estimates, the slender-branched shrub uses up more of the Colorado River than the residents of Las Vegas and southern Nevada.

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“We wanted to improve the habitat for native vegetation,” which has the added benefit of using much less water, “maybe half as much,” says Mr. Dudley, a professor at the University of California, Santa Barbara.

That puts ecologists like Dudley, who are interested in repairing stressed habitats, in league with government agencies that want steady and sustainable water sources. Lake Powell and Lake Mead, the water tanks of the Southwest, are below 50 percent capacity after years of drought.

“We’re not talking about getting extra water that everyone divvies up, we are talking about getting those [lake levels] up” to avoid rationing, says J.C. Davis, spokesperson for the Southern Nevada Water Authority. “To the extent you keep the water in the reservoir rather than in the roots of the tamarisk, then everyone on the river benefits – including us.”

When picky eating is a good thing

In a gorge carved by the Virgin River, a tributary of the Colorado, Dudley swings a net over some tamarisk and peers into his catch: dozens of green, tic-tac-sized beetles. The beetle, Diorhabda elongata, eats one thing only – the tiny, scaly green leaves of the tamarisk.

It’s a point researchers spent years confirming before anyone was allowed to release the bugs, brought over from places like Kazakhstan and Crete into the US. It’s not unusual for insects to be such picky eaters – that’s what makes them valuable for so-called biocontrol efforts such as this, says Dudley.

The first permitted releases of beetles began on Western rivers in 2001, and these early sites are just beginning to yield information about the long-term impacts of the more than 1 billion beetles now in the US.

Their burgeoning numbers speak to some concerns about the idea. “Biocontrol is not a very fine-edged tool,” notes Mark Sogge of the US Geological Survey. “It’s something that once you release it, you don’t have control over where it goes, how fast it acts....”

The beetle populations only recently exploded on the Virgin River, following their release two years ago in St. George, Utah, some 15 miles upstream from the gorge. Now they are munching en masse downriver, across the northwestern corner of Arizona toward Nevada, as if feasting along one enormous Vegas buffet line.

In their wake lies a trail of browned-out, skeletal tamarisk. The bushes are by no means dead – it will take typically four years of defoliation to finally kill them off. Surviving plants aren’t frowned on: the goal is to tame, not destroy, the tamarisk.

What exactly will happen next isn’t fully known. And that’s when the controversy begins. “The very simplistic story is that if we remove the tamarisk, native plants will come back. If that happens, great. But there will be, I believe, a lot of places where that won’t happen,” says Mr. Sogge,

Parts of the Southwest, he says, have such poor hydrology that native plants will not return. Instead, other invaders like Russian thistle could move in, or maybe nothing at all – leaving a dead thicket of tamarisk and a denuded riparian zone.

Those scenarios trouble Sogge, an avian expert whose research has found some 50 species of birds using the tamarisk, which is also known as the salt cedar. Among the birds that nest in these trees is the southwestern willow flycatcher, a federally endangered species.

“We like to have absolute good guys and absolute bad guys. The salt cedar doesn’t fall into that dichotomy,” says Sogge.

Many of these birds used to nest in native willows and cottonwoods and would prefer to nest in them again if those natives revived. Sogge worries, however, that the birds will soon not have the choice of either.

Dudley counters that a lot of these systems are in “sufficiently decent condition that natural germination is likely to occur.”

Early signs from a beetle release on the Colorado River around Moab, Utah, show that tamarisk defoliation is giving native plants a sudden second life. Now, Dudley and his colleagues are scrambling to set up test sites farther downstream on the Virgin to experiment with revegetation efforts once the beetles pass through. The tests could show whether native plants will need human help such as prescribed fire, controlled flooding, or reseeding.

Little push for revegetation
Unfortunately for Dudley, the beetles are marching faster than the bureaucracy surrounding his grants.

Rather than miss his window of opportunity, he has plunked down $20,000 of his own money to keep the study moving, in the hopes he will be reimbursed once government agencies get on the same page.

Now that the beetles have been released, the federal government’s lack of follow-through on revegetation bothers many people involved in the project. Sogge says that once the permit to release beetles was given, the commitment to monitoring work went down.

Dan Bean, insectary manager for the Colorado Department of Agriculture, puts it this way: “What happens with biocontrol is the release and initial defoliation is a big deal. Senators and congressmen VIPs come out and get their pictures taken, and then – it really falls off the radar.”

“There is no big pool of money for revegetation that I know of,” adds Mr. Bean. Instead nongovernmental organizations like The Nature Conservancy are putting up some funds.
Aside from the high costs and low flash of revegetation work, some might prefer that nothing replace the tamarisk, as that would free up the most water, notes Dudley.

So far, areas with defoliated tamarisks are showing some modest gains for water.

“You won’t see the Colorado River go to twice its flow. It’ll be subtle. You could be talking about hundreds of thousands of acre feet,” says Bean. “It’ll have an impact, but I don’t think it will radically change the way we have to worry about water in the Southwest.”

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