Diorhabda elongata loves invasive tamarisks as much as the Grand County Weed Department hates them.
Or rather, the tiny insect that’s also known as the tamarisk leaf beetle loves to feast on the trees’ leaves and drink tamarisk sap, eventually causing many — but not all — of them to shrivel up and die.
The question of why some tamarisks green up again — while others don’t — when the biological pressures are taken off them is the subject of an academic paper that was accepted this week for publication in the peer-reviewed scientific journal “Biological Invasions.”
“We’re not really sure what’s going on,” Grand County Weed Department tamarisk bio-control research technician Wright Robinson told the county council on Tuesday, July 3. “… We’re still trying to understand why and how beetles kill some tamarisk, but not others.”
To predict the impacts of tamarisk defoliation, Robinson said that University of Denver PhD student Annie Henry and others turned to local data gathered over 11 years of “systematic and constant” beetle monitoring throughout the county.
“Currently, Grand County has the most extensive, most continuous landscape-level data sets for beetle movement, tamarisk response and tamarisk mortality in the country,” he said. “We are the ones with the data.”
Grand County approved the use of the beetles as a biological control in 2004, and the insects were initially released over the next two years, to control the Eurasian invader. Tamarisk, which is also known as salt cedar, was first brought to North America to stabilize streambanks. Farmers also used the trees as windbreaks, and many colorful tamarisks wound up in yards as ornamental trees or shrubs.
“And then, everything got loose,” Robinson said. “… So it just grew, unabated.”
Throughout their native range in the “stans” of Central Asia, he said, tamarisks don’t grow out of control because natural predators like the beetles keep them in check. But their numbers are out of whack throughout much of the Southwest, where they’ve overtaken riparian areas across more than 1 million acres, and thrive even in alkaline and saline soils.
Like other noxious and invasive species, tamarisks have an advantage over many native plants, trees and shrubs: For instance, Robinson said that “millions” of tamarisk seeds can disperse when a car drives by the trees; the seeds also fall in waterways and travel downstream, where they quickly gain footholds in other riparian areas.
The ability to disperse quickly is one thing that tamarisk beetles have in common with their favorite snack.
“These beetles can fly and get blown in any direction, and they’re going to bump into a tamarisk tree,” Robinson said. “It’s like a smorgasbord out there for them.”
The release of the beetles in Utah and other Western states has been met with controversy elsewhere, particularly in Arizona, one of several key areas where the endangered southwestern willow flycatcher breeds in riparian areas that tamarisks have invaded. But in Grand County, weed department officials view them as a favorable alternative to mechanical cutting or burning, which come at a steep cost — and involve heavy labor — without necessarily achieving the desired results.
Robinson noted that the trees are deep-rooted, and if they aren’t cut below the root crown, they will keep sending up shoots.
“That’s how tamarisk survives,” he said. “So when you cut (and) when you burn, they love it.”
“You can cut tamarisk, but within two weeks, we’re back to this,” he said, gesturing toward images of new growth sprouting out of trunks. “… We can burn tamarisk; within a couple of weeks, we’re back to this.Within one year, they were 7 to 8 feet tall.”
Herbicides can be effective, Robinson said.
“But we don’t want to spray the whole county with (them),” he added.
A single female tamarisk beetle can lay 300 to 500 eggs that hatch within a week. From the eggs, the larvae will begin to feed voraciously on the trees, which begin to “brown” after the beetles open up a tamarisk’s leaves and drink the sap.
By exposing the leaves to desiccation, the beetles force much of the tree to recede. At that point, the tree can’t continue to photosynthesize, so it draws on the storage from its roots, trying in vain to get energy from sunlight.
“When it can’t do that, it’s on a perpetual diet, and eventually, it dies,” Robinson said.
One year after they were first released in the county, the beetles had browned a grand total of less than 2 hectares, or about 5 acres. By 2006, an estimated 400 hectares had been browned, and that percentage had increased tenfold, to an estimated 4,000 hectares, by the following year.
When 2008 arrived, Robinson said, “everything cuts loose,” and the county was inundated with the beetles.
Over the next five years, the weed department continued to monitor the insects, and observed their spread across two-thirds of the entire county, excluding the upper reaches of the Book Cliffs, the La Sal Mountains and other high elevations. (Tamarisks don’t grow above 6,500 feet in southeastern Utah.)
“(The beetles) took everything,” Robinson said. “… Wherever there were tamarisk, there were beetles.”
At the end of that five-year period, researchers launched their local tamarisk mortality study, examining 80 sites across the county.
About 60 percent of tamarisks were green that year, but that number dropped by almost 15 percent in 2014, and continued to plummet the following year. In 2016, though, the numbers of green tamarisks were back up — to 55 percent.
“The beetles ate so much of their food, their population went down,” Robinson said. “(Then) we don’t have the beetles to control the tamarisk; the tamarisk came back up.”
This year, however, Robinson said that researchers are starting to see the beetles come back in larger numbers.
That isn’t necessarily the news that some environmental groups or federal and state wildlife officials in Arizona would like to hear, though.
In 2016, a U.S. Geological Survey (USGS) study found that the beetles pose a threat to southwestern willow flycatchers by defoliating tamarisk during the birds’ nesting season, exposing their young to higher temperatures and predators. The agency predicts that over the next decade, the beetles will destroy 36 percent of flycatcher habitat along the lower Colorado River and 55 percent along the upper Gila River.
But in Grand County, Robinson said that he and others have observed positive signs at beetle release sites, as native willows are returning to locations where tamarisks have keeled over and died.
“We started seeing this all over the county,” he said. “And we were also seeing this willow growing up through the tamarisk … light is able to get down into the ground, and the willow comes up. Now, real estate is shifting from tamarisk over to willow, and a lot of the green you see along the river today is willow, privet, other plants like that.”
Grand County Council member Rory Paxman said he routinely observes those positive signs firsthand during his excursions along the Colorado River with his family’s tour company, Canyonlands by Night and Day.
In the time since he first began to see tamarisks fade away and native sandbar willows come up in their place, Paxman said he’s noticed more beaver on the river than ever before.
“When I started riding boats, I’d probably see one or two (beaver) a day,” Paxman said. “And right now, I can go downriver and I can show you over 150 of them on every sunset trip I do, so I think it’s kind of a neat way that it’s kind of helped them a little bit, too.”
Grand County Council vice chair Curtis Wells credited the department and its employees for their work to keep tamarisk populations in check.
“Tamarisk seems to be a common enemy, so it’s good to see that you guys are on the tip of the spear getting it under control,” he said.
The question of additional funding this year for the department’s tamarisk research work remains is another matter, though.
Grand County Weed Department Supervisor Tim Higgs told the county council that many of its funding sources have dried up. But Higgs said his department has asked the Utah Division of Forestry, Fire and State Lands for $2,500 in funding to continue that work; he hopes to have an answer in the near future.
Invasive trees have overtaken riparian areas across more than 1 million acres in the Southwest
“These beetles can fly and get blown in any direction, and they’re going to bump into a tamarisk tree … It’s like a smorgasbord out there for them.”