What happens when an exotic bug is brought in to fight an exotic weed?
When Dan Bean and Tom Dudley were high school classmates in Southern California, their biology teacher offered extra credit for eating oddities of scientific interest. The duo dove into the challenge - Bean, a quiet man with a slow grin, allows that sea-urchin gonads were much tastier than he expected - and their combined score shamed all competitors. Which may partly explain why, on a clear fall day more than three decades later, Bean and Dudley are driving a rocky red dirt road on the Utah-Colorado border, on their way to watch a beetle kill some weeds.
"What about here, Doctor Bean?" teases Dudley, pointing out the truck window at a tangle of riverside trees and shrubs. Dudley, now a trim, sharp-featured ecologist from the University of California at Santa Barbara, is visiting his friend and colleague Bean, an entomologist at the Colorado Department of Agriculture insectary in nearby Palisade. Bean parks his white pickup and the pair clambers out, grabbing long, cone-shaped cloth nets as they head toward the trees.
After a few moments of net-swinging, Dudley shouts from the thicket. "Here's one ... one, two, three, four."
Bean and Dudley are searching for Diorhabda elongata, familiarly known as the tamarisk leaf beetle, newly arrived here from Kazakhstan by way of Utah, and, as it happens, stenciled in larger-than-life size on Dudley's T-shirt. First released in the United States in 2001, the beetles were set free in Utah, about 20 miles from this spot, in 2005, and recently began to march along the muddy waters of the Dolores River. Their target, as their name indicates, is the riverside shrub known as tamarisk or saltcedar, one of the most infamous invasive weeds in the West.
Six weeks ago, Bean says, there were no beetles in this particularly thick stand of tamarisk, cottonwood, and willow, but now, the tamarisk - which towers, treelike, over the scientists' heads - displays light brown tips, one of the first signs of beetle attack.
"The natives are already here," Dudley observes, looking at the native cottonwoods, some tall enough to form a canopy over the weeds. "They just need to be freed up." "And the beetles are going to free them," says Bean.
But their cheer is momentary. Though Diorhabda is a powerful weapon against tamarisk, it's a very complicated one. "Whenever anyone thinks they have a silver bullet," says Dudley, again diving into the brush, "it usually turns out to be a sort of off-gray color."
Tamarisk is one of the lessons in American West 101: About the time newcomers learn that the striking black-and-white bird on their porch is a common magpie, they find out that the feathery green riverside shrub with the pretty pink sprays of flowers is actually a nasty weed.
When and exactly how it arrived here is a matter of some debate, but the usual story is that tamarisk, a Eurasian species, was first planted in the West in the mid-1800s, both as an ornament and for erosion control on railroad beds and elsewhere. By the late 1800s, it had naturalized, and by the 1960s, it dominated vegetation along the Colorado, Rio Grande and Pecos rivers. Today, its Western range stretches from northern Mexico to Montana, and from Kansas to California, and covers over 1 million acres.
The conventional wisdom is that tamarisk, with its deep, tenacious root system, sucks up much more water than its native neighbors. Reality, as usual, is more complex: Recent analyses by researchers at the University of California at Santa Cruz show that in the drier areas atop riverbanks, tamarisk does use far more water than native plants. In wetter areas near streams, stands of cottonwoods and willows often use nearly as much water as their exotic competitor.
Yet there's no shortage of reasons to dislike tamarisk. Even though it offers some substitute food and shelter to native wildlife - a point we'll tackle later - research shows that in general, animals living in tamarisk are less diverse, fewer in number, and less healthy than their counterparts in native vegetation. The plant's trademark dense growth is thought to increase fire risk along riverbanks, and it resprouts quickly after burns. And as any boater can attest, the weed narrows streams and rivers, and chokes out campsites.
Since the 1960s, Westerners have worked to rid the region's rivers of tamarisk, hoping to salvage scarce water, protect wildlife, or fend off wildfire. Millions of dollars, and countless backbreaking hours, are spent each year on efforts to hack down and poison the plants. Perhaps the most ambitious project is headed by The Nature Conservancy, which has spent $600,000 removing tamarisk from the San Miguel River in southwestern Colorado. Over the past seven years, volunteers and contractors have used chainsaws, clippers, heavy equipment and herbicides to rid the watershed of the weed; they've covered 80 miles of the river and its tributaries so far, and organizers are happy to report a resurgence in native plants. They plan to complete the remaining 10 river miles in just over a year and use periodic, much less intensive retreatments to keep the watershed nearly tamarisk-free.
Along the Rio Grande and its tributaries in New Mexico, state-funded aerial herbicide spraying has killed tamarisk on thousands of acres, and on the Middle Rio Grande, heavy machinery has plucked the plant from the Bosque del Apache National Wildlife Refuge. On a long-neglected stretch of the lower Colorado River near the Mexican border, the city of Yuma, Ariz., and the neighboring Quechan Tribe have collaborated on a campaign to clear tamarisk and other invasive plants from the banks.
Near Grand Junction, Colo., a nonprofit group called the Tamarisk Coalition recruits volunteers to cut the plants out of a wilderness area near Colorado National Monument. The wilderness designation limits workers to handsaws, "and sometimes you feel like you're out there with a toothpick," says coalition staffer Clark Tate. But there are advantages to the group outings, she adds: "We have matched up some very happy couples."
Despite their successes, these intensive efforts can be reversed with just a few years of subsequent neglect, and are difficult - if not impossible - to execute on a regional scale. "There's just not enough money to get rid of it all with chemicals and bulldozers," says Dudley. Broader, lasting success, say many, requires reinforcements, preferably numbering in the millions, preferably willing to work for free. That's where biological control, in the form of Diorhabda elongata, comes in.
"Biological control is very risky," acknowledges Ruth Hufbauer, an invasive-species researcher at Colorado State University. "Yet when it's done carefully, done well, it's an incredibly powerful tool. It shouldn't be dismissed out of hand."
For a decade, Jack DeLoach searched for a worthy tamarisk adversary. DeLoach, a U.S. Department of Agriculture entomologist, has traveled to Israel, southern France, China, Kazakhstan and Turkmenistan, doing what is somewhat romantically known in the biocontrol business as "foreign exploration."
In his quest for a natural enemy for tamarisk, he teamed up with local entomologists to collect beetles from the weed's native range, sifting through the several hundred species known to attack the plant. He and other researchers tested about 20 species overseas, and brought about half back to his lab in Temple, Texas, for further research, finally whittling the possibilities down to one: Diorhabda.
Most important to biocontrol researchers - and their regulators - is that the insects they release have very particular tastes, proven to attack nothing but the weed in question (God forbid they eat food crops). Biocontrol researchers say that, in this respect, their profession has a clean record, and some dramatic successes to its credit. In the 1940s, when the introduced Klamath weed beetle decimated a rangeland weed in Northern California and southern Oregon, grateful ranchers and farmers in Humboldt County, Calif., even honored the insect with a commemorative plaque.
The most commonly cited biocontrol "mistake" in the United States, the 1960s introduction of a weevil that eats native as well as introduced species of thistles, was simply a product of its times: Researchers say that scientists and regulators knew the weevil's tastes, but thought native thistles weren't worth protecting. "Nobody would approve a release like that today," says Hufbauer.
The U.S. Department of Agriculture, advised by representatives of other federal agencies, now authorizes biocontrol insects for both agricultural weeds and ecological plagues. The numerous targets include leafy spurge in the Northern Plains and elsewhere, the invasive evergreen melaleuca tree in Florida, and several species of knapweed.
Much remains unknown, of course - over generations, it's still possible that the introduced insects could develop a taste for native species, or set off unforeseen, and effectively irreversible, cascades of consequences. Post-release studies of biocontrol insects are often spotty and inconsistent, adding to the uncertainty. But so far, these modern biocontrols have shown few, if any, side effects. In many efforts, the problem is not over-enthusiasm but underperformance: The insects simply die out, leaving the job undone.
Biocontrol, however, has an older, uglier ancestry. In the late 1800s and early 1900s, humans transplanted cane toads, mongoose and other species to new countries, naively expecting the newcomers to control domestic pests. People - and native wildlife - are still dealing with the consequences, and the most spectacular screwups have become cautionary tales.
"When people were doing things like introducing cane toads, people were making idiotic introductions all over the world," Hufbauer points out. "It wasn't just in the name of biological control - people were saying 'Oh, a boar in California! Wouldn't that be great?' People were doing things that we now think are insane. So we have to hope that today, we have a pretty good understanding of what's going on, and that we're not making mistakes that 50 years from now, we'll look back on and regret."
DeLoach and his team assured themselves that tamarisk, the sole member of its botanical family in North America, was the only target for incoming Diorhabda. What they didn't expect was that the weed itself might have some mourners.
Just as DeLoach was making final preparations to release Diorhabda in the mid-1990s, the southwestern willow flycatcher, a small songbird native to river corridors in Arizona and New Mexico, was added to the endangered species list. Biologists found that for some of the few hundred remaining pairs of nesting flycatchers, tamarisk could serve as a passable substitute for native willows. If the beetle worked as advertised, they worried, some flycatchers might be left with little to hold up their nests.
After all, no one was sure how quickly the beetle would chew through its targets, or exactly how far it would spread. And with the Colorado and many other Southwestern rivers long deprived of the natural floods that support native vegetation, the resurgence of cottonwoods and willows wasn't guaranteed. (The federal recovery plan for the willow flycatcher recommends that exotic species be removed in stages, without the help of biocontrol, and only in places otherwise hospitable to native plants.)
So followed several years of study and teeth-gnashing on all sides. Tom Dudley, then a researcher at the University of California at Berkeley, was a stream ecologist "sick of watching tamarisk take over rivers," he says, and he began collaborating with federal researchers on the ecological impacts of the beetle. When they realized that some varieties of Diorhabda only did well at certain latitudes, Dudley called his old school classmate, Dan Bean, who - after his adolescent culinary experiments in California - had become an expert in how beetles respond to different lengths of day and night.
Bean, working with other researchers, showed that beetles collected from northwestern China could only reproduce well north of the 38th parallel, which runs through southern Colorado and Utah, skirting willow flycatcher habitat. These beetles were eventually approved for release, and in the spring of 2001, with permits firmly in hand, researchers released members of this northern population of Diorhabda at several sites, including one in the Great Basin near Lovelock, Nev.
For more than a year, not much happened, and "biocontrollers were pretty well bummed out," remembers Bean. Then, in the summer of 2002, researchers in Nevada noticed small spots of browning tamarisk. By the following summer, about 400 acres of tamarisk along the Humboldt River had turned brown, and by the summer of 2004, Diorhabda had made its first pass through tens of thousands of acres of tamarisk-infested riverbank.
Jack DeLoach - now in his 70s - repeatedly postponed retirement in order to see Diorhabda on the march. "It's wonderful news," he says of the beetle's progress. "I never lost hope, but it was ... difficult."
Despite their impressive arrival in North America, the beetles work slowly. Tamarisk and Diorhabda, after all, are longtime adversaries in their native lands, and tamarisk plants bounce back from beetle attacks again ... and again ... and again. Diorhabda larvae, which do most of the tamarisk chewing, can defoliate a 15-foot-high tamarisk in four days, but it takes five or six attacks over multiple years to kill a plant.
And the beetles don't always win. "Not every release has worked," says Bean. "People who have been around successful releases can't believe the beetles aren't taking over the world, but a lot of them just flat-out fail."
On the Bighorn River in Wyoming, where a university researcher released beetles in 2001, large swaths of brown tamarisk weren't seen until 2006. Beetles at several sites on the West Coast were obliterated by ants. "Those poor beetles just got chopped up and handed out until there was nothing left of them," remembers Dudley.
No biocontrol is a complete cure. No matter how successful its campaign, the beetle will never rid the West of tamarisk, for that would mean destroying its own food supply. In optimistic moments, researchers envision that Diorhabda will knock tamarisk populations back by 75 or 80 percent, then settle into an equilibrium with the plant, helping create what one researcher calls "ecological fair play" by giving native plants more breathing room.
But the acres of brown tamarisk in Nevada were more than enough to encourage new releases. Beetles were released in western Utah, southern Colorado, and Dinosaur National Monument. In 2004, a county weed supervisor in southeastern Utah released some Diorhabda from Kazakhstan - neighbors of the Chinese beetles - along the Colorado River near Moab. They swarmed through the river corridor, colliding with tourists' T-shirts and windshields, eventually covering some 100 miles of the Colorado. By the end of 2006, they had reached the mouth of the Dolores, a tributary of the Colorado, and were poised to head toward the Colorado state line. This past July, says Bean, "they started flying upstream like crazy," defoliating about 25 miles of tamarisk on the Dolores, and by August, they had crossed the border into Colorado. A nervous editorial in the Moab Times-Independent reflected local ambivalence: "We're excited about the apparent success the beetle is having," the editors wrote. "But we're concerned about the unknowns ahead ... we hope Mother Nature won't punish us for our actions."
When Dudley and Bean make their fall foray down the Dolores, most of the beetles, taking their cue from the shorter days, are already snuggled between the leaf litter and the soil, stilled for the winter. But just weeks ago, many of the tamarisk along the river were draped with beetles, and the results are striking. On the state line, marked here by a small yellow sign and a barbed-wire fence, the thick tamarisk stands are almost completely brown. They're not dead yet, but the siege is under way: When the plants green up again next spring, Diorhabda will be waiting.
In a sunshine-filled laboratory at the Palisade Insectary, Jiana ten Brinke, a recent college graduate from California, is loading clear plastic containers of Diorhabda onto a rolling cart. Dan Bean, at a nearby lab bench, is inspecting a carton of weevils.
"Those aren't quite as cute as these," she tells him.
"Oh, come on," Bean says mildly. "They've got these little snouts ..."
"These are cuter," ten Brinke says, opening one of her containers, showing a crowd of sunflower-seed-sized beetles clambering over a tiny bouquet of tamarisk. "They're round and stripey, like little watermelons."
The beetles - which are, to an outside observer, not cute, but not as ugly as you might expect - are part of a veritable United Nations of beetles at the insectary. Populations from Crete, Kazakhstan, mainland Greece, Uzbekistan, and several areas of China are kept supplied with green tamarisk, all too easy to find here in the summer. In the winter, insectary workers turn to a few dozen scraggly-looking, nutrient-starved plants in their greenhouse. ("As soon as you try to grow a weed, you have all kinds of problems," says Bean.) When this emergency supply runs out, collaborators send buckets of green tamarisk from warmer climes and other greenhouses.
For more than a half-century, the insectary has supplied "beneficial" insects to local orchardists, and now, in cooperation with the U.S. Department of Agriculture, it's added the tamarisk beetle to its list of charges. Insectary workers have raised up and shipped out some 400,000 beetles from northwestern China to nine states, including various sites within Colorado. "We have a really big FedEx bill here," says Bean.
Bean and his colleagues are also studying how the beetles respond to new conditions. Diorhabda, as Bean observed when he first joined the project, is extremely sensitive to day length, and even small changes can cripple a population. But over time, the beetles do appear to adapt to different light regimes, and generation by generation, the captive beetles here at the lab are adjusting to the artificial light schedules they're placed in. That suggests that at some point - likely within a decade, Bean says - the beetles from China will be able to thrive farther south, beyond the limits of the 38th parallel. Regulatory quirks have also allowed researchers in Texas and New Mexico to release beetles from Crete, a population already well equipped for life in the Southwest. One way or another, the southwestern willow flycatcher will eventually contend with Diorhabda.
"People don't appreciate that biological systems aren't static," says Bean. "Anyone who thinks that the beetles are going to stop at the 38th parallel, and sit there for the rest of biological history, is going to be surprised."
So far, Diorhabda is behaving very well. The beetles are eating plenty of what they're supposed to, and, as watchful researchers report, nothing that's forbidden. They've survived and spread beyond expectations. But not even the most fervent biocontrol advocate calls their introduction a success - not yet. For no one is sure what will follow in their path.
"Killing tamarisk is a wonderful activity, but that's not the goal," says Tim Carlson, the executive director of the nonprofit Tamarisk Coalition, a group that works to connect tamarisk-control projects throughout the West. "We want to get rivers back to a healthy state, and we don't have a plan for doing that."
On the Dolores, and in other places where cottonwoods and willows still stand shoulder-to-shoulder with tamarisk, the beetle's work may be enough to encourage a boom in native species. But in other places, where natives are sparse or nonexistent, Diorhabda could easily hand the advantage not to natives but to other exotics, such as Russian knapweed and the fast-colonizing perennial whitetop.
The recipe for a healthy river is an elusive one, and differs for each site. But tamarisk adversaries agree that in most cases, the beetle is only the beginning, and that rivers will need more meddling, not less. In some places, rivers may need to be seeded or planted with native species. Beetle-killed dead tamarisk may need to be taken out with chainsaws or prescribed fire.
On dammed rivers, where flood timing can favor tamarisk and other exotic species, tamarisk may be only a symptom of a deeper malaise. Lasting success may mean changing water management - never an easy task in the West. "There's a belief that all you have to do is get rid of tamarisk, and the native vegetation will come back," says U.S. Fish and Wildlife Service biologist Greg Beatty. "But in many instances, our management choices are the reason why tamarisk is flourishing."
Like the long-running efforts to defeat tamarisk with hacksaws, bulldozers and poison, river restoration projects following tamarisk removal are small, underfunded and diffuse. There's no one agency in charge of river health, and most rivers flow through a patchwork of landowners and regulations. The Tamarisk Coalition is organizing local partnerships and encouraging state agencies to provide matching funds to groups and communities interested in river restoration. They're also negotiating with oil and gas companies to fund weed eradication and restoration projects, and pressing Congress to fund a 2006 law supporting tamarisk control and research. But for now, there's much more talk than action surrounding post-beetle restoration. "It's a giant experiment," Carlson acknowledges.
The experiment will almost certainly involve the southwestern willow flycatcher. Since the species was listed as endangered in the mid-1990s, flycatcher numbers have risen significantly, likely due to a combination of genuine habitat expansion and more intense study. But the promise of a Diorhabda debut in the Southwest within the next decade still worries wildlife researchers. "I doubt that saltcedar control would drive the flycatcher to extinction," says U.S. Geological Survey biologist Mark Sogge. "But without proactive restoration, recovery might be a lot harder."
On the Utah-Colorado border, in the river canyons of Dinosaur National Monument, botanist Tamara Naumann is pioneering one strategy for the future. Years before Diorhabda received its federal approval, Naumann focused her volunteer tamarisk-removal campaign on campsites and other highly visible areas - places where beetle-killed tamarisk, she knew, would be unpopular and dangerous. When she began releasing the beetles in the park in 2006, she chose remote places rarely reached by volunteers or visitors. "I consider them my newest volunteer corps," she says of the beetles. "Their job is to eat and have lots of sex."
Naumann, who has recruited well over 5,000 human volunteers over the 11 years of her tamarisk control program, fully expects her crews to keep boating the Yampa and Green rivers each summer. Their job may shift from tamarisk extraction to debris removal and native-plant restoration, or even to control of other weed species, but it won't end. "The beetle is never going to replace those volunteers," she says, looking horrified at the prospect. "I'd be an idiot to get rid of them."
On another fall day, in another redrock canyon near the Utah-Colorado border, Dan Bean extracts a small Coleman cooler from his truck. Inside are cardboard pint containers filled with thousands of Kazakh Diorhabda, recently captured in Utah and bustling toward freedom.
Bean and his two colleagues march into a crescent-shaped tamarisk thicket, a place as green and humid and buggy as any Eastern forest. They select a tree near the center of the stand, take a GPS reading, and, as if salting a steak, start emptying the containers onto the tamarisk's limbs.
If all goes as planned, the entomologists will see larvae here next spring, and the tamarisk will start to brown. Within a few years, these beetles will likely cross paths with those making their way up the Dolores, some 40 miles away. Bean surveys the tamarisk, some of which reach 25 feet overhead. "I think these will provide some pretty good food," he says. But here, there are few cottonwoods and willows: The pink flowers of Russian knapweed carpet the ground, interrupted only occasionally by native species like the skunkbush sumac, a shrub with leaves pungent as a beauty salon.
In the wake of Diorhabda, these species will begin a struggle for new territory, but for now at least, the tamarisk towers solidly over them all. The beetles drop from their containers, light as air, and vanish into the greenery, intent on surviving the winter.
Michelle Nijhuis is a High Country News contributing editor.