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Tamarisk-munching beetles travel the Rio Grande

Head north of Albuquerque and look over toward the Rio Grande and its forest, or bosque. Within that green ribbon of trees, you’ll also spot leaves that are reddish brown. Even from the Interstate, the dying trees are obvious.

Those leaves belong to tamarisk, or salt cedar. More than a century ago, the trees were introduced to control erosion and act as windbreaks. But they have overtaken riverbanks across the southwestern United States, sucking up water and choking out native species like cottonwoods and willows.

Across the region, people try and fight the invasive trees with bulldozers, chainsaws, and poison. 

But that’s not what’s killing these trees along the middle Rio Grande.

“I’m sure some people would be like, ‘Eww! Bugs!’” says Ondrea Hummell, an ecologist with the U.S. Army Corps of Engineers.

This summer, Hummell was checking the riverbank in Albuquerque for diorhabda, a beetle that eats tamarisk leaves.

More than a decade ago, the beetles were released inside cages on test sites in seven states including Colorado and Utah. They were brought from China and Kazakhstan—where they don’t live south of the 37th latitude. Scientists assumed that would be true in North America, too.

But that hasn’t been the case, and the beetles are heading south.

“What’s happened is they’ve gotten out of the Four Corners area where they were originally released, and they’ve come down to the Rio Grande,” she says.  

The beetles Hummell is looking for here in Albuquerque were released in Utah six years ago. They travel fast, flying from tree to tree. “We’re told they can move 50 miles at a stretch,” says Hummell. They don't kill the trees right away. Rather they keep defoliating them again and again until the tree eventually dies.

With a big white net, Hummell heads toward a tamarisk tree behind Albuquerque’s Bio-Park. It’s the end of July—hot, with cicadas screaming from the trees. Their noise competes with traffic from the Central Bridge, where it crosses the Rio Grande.

“See how it looks kind of brownish and dry? That could just be the drought,” she notes. “But usually that’s a sign.” She whacks the tree a few times, then points into the net: “All right, we’ve got some late larvae.”

So great, right? Tamarisk trees are dying in New Mexico. That’s a good thing.

Well, it depends who you ask.

When the U.S. Department of Agriculture first wanted to release the beetles, the U.S. Fish and Wildlife Service gave the go-ahead—as long as the beetles weren’t released along the Rio Grande in New Mexico or Texas. That’s because the southwestern willow flycatcher, a rare bird, builds its nests in tamarisk when it can’t find native willow trees.

The Fish and Wildlife Service said the beetles could be released, just not within 200 miles of any nesting flycatchers.

“People stayed out of that range. But the bugs didn’t,” says Gina Dello Russo, an ecologist with the Fish and Wildlife Service. She works at Bosque del Apache National Wildlife Refuge, which has about 20 pairs of the flycatchers during nesting season. She’s also the New Mexico Invasive Species Strike Team Coordinator.

With fewer willows along the Rio Grande than in the past, the birds sometimes nest in tamarisk trees. Dello Russo explains: “If you can picture very dense vegetation with a structure that has a lot of crossing branches when you get up to six, nine, ten feet high…and they put their cup nests in those crossing branches.”

Dello Russo actually doesn’t sound all that worried about the flycatcher. Unlike southern Arizona, where tamarisk has completely overtaken streams, here, she says, the flycatchers still have some willows. And places like the refuge are doing restoration projects—removing tamarisk and then planting willows and other native trees.

She is concerned that people may be putting too much faith in the diorhabda to take care of the tamarisk problem. “I’ve been asked already, ‘Then why are we putting money into tamarisk control if we’ve got this leaf beetle that’s going to take care of it for us?’” she says.

But she believes there is still work to be done, even after beetles devour the tree’s leaves. The dead trees need to be pulled out by the roots and removed—and native trees need to be planted in their place.

“I don’t think we should change the way we do things or [change] our efforts that are underway. Yet,” she says. “Until we see what part the bug actually plays in our management of tamarisk and our management of southwestern willow flycatcher habitat.”  

Meanwhile, Robin Silver isn’t surprised that the beetles have already made it halfway through New Mexico. The environmental group he co-founded, the Center for Biological Diversity, opposed the beetle releases.

“There was a long and very ugly battle to stop the release,” he says.

The group worried about the flycatchers, which Silver says return each year to the same nesting spots. Without leaves to hide their nests, the birds are vulnerable to predators.

The beetle hasn’t yet made it to where flycatchers nest in Arizona, so there is still time, Silver says.  “Your window in New Mexico is slamming shut,” he says. “But in Arizona, we have a short window where we might be able to get some restoration activities going and then seeing if we can get some alternative nesting habitat for the flycatchers, before there is nothing.”

In fact, the two species—the beetles and the birds—are about to converge in central New Mexico.

Back in July, when Hummell was whacking tamarisk trees, she learned the beetles had made it to Albuquerque’s South Valley. Now, two months later, the beetles have traveled even further south along the Rio Grande—to the Pueblo of Isleta.

And that’s exactly where two pairs of flycatchers nested this season. From there, the bird’s habitat stretches to the south, to the Bosque del Apache National Wildlife Refuge and on along the lower Rio Grande.

Right now, no one knows right now how the standoff will end. And the beetles are still on the move.