Tamarisk beetle: A case study in the dangers of biological controls to eradicate non-native species

Our readers were introduced to Matt Chew in his guest post about the economic interests of ecological “restorations.”  Dr. Chew is a faculty member of Arizona State University’s Center for Biology and Society and an instructor in the ASU School of Life Sciences. 

The most recent newsletter (see page 8) of the California Invasive Plant Council (Cal-IPC) informed us that the beetle that was introduced in Arizona to eradicate tamarisk has spread to California, where it was not introduced.  When the beetle was originally introduced, its spread beyond where it was introduced was not predicted, based on climatic restrictions on its life cycle.  As usual, evolution overturns the best laid plans.  According to Cal-IPC, Rapid evolution in this developmental trait, however, allowed beetles to stay active later in the season and thus facilitated their expansion southward…”   

Tamarisk defoliated by tamarisk leaf beetle along Colorado River, near Needles, California

The rapid defoliation of tamarisk throughout the southwest, including California, is an immediate threat to the endangered Southwestern willow flycatcher, which long ago adapted to tamarisk in the absence of its native host, willow.  The native willow requires a great deal more water than tamarisk. Therefore, willow died off when water throughout the southwest was diverted out of riparian corridors for human consumption and agricultural production.

Dr. Chew is an expert on tamarisk and the role it plays in the ecosystems of the southwest and so we asked him to write another guest post for us on this topic.  He has generously obliged with this detailed history of biocontrols and their use to eradicate non-native species. 

Biocontrols are also topical because a new biocontrol was recently approved by the USDA to eradicate cape ivy.  This biocontrol was eagerly anticipated by native plant advocates and is likely to be widely used by land managers in California.  Therefore, this is a timely opportunity to learn about the pros and cons of biocontrols.  How long will it take the introduced insect to start feeding on the many other species of ivy that are not considered “invasive?”

Evolution and natural selection are wild cards in attempts to eradicate non-native plants and animals.  Although there are many dangerous consequences of using pesticides, the role that evolution plays in rendering pesticides useless is less understood and taken into consideration.  Much like the hungry beetle that now is running rampant in the southwest, the weeds that are continuously sprayed with herbicide are also adapting and evolving defenses against the chemicals being used to eradicate them.  There are now millions of acres of agricultural crop land infested by weeds that are immune to the pesticides that were sprayed on them for decades.  Our pesticides are now useless on these “superweeds.” Instead of getting off the pesticide treadmill, we are developing stronger—and therefore more toxic—herbicides.

There are many reasons why we object to the eradication of non-native plants and animals.  The tamarisk beetle is an example that illustrates a few of our objections:

  • Many of the plants being eradicated are providing food and habitat for animals. The animals that depend upon them are being harmed by their elimination.
  • The methods used to eradicate non-native species often have unintended, negative consequences, such as breeding “superweeds” that cannot be eradicated.
  • The puny tools of humans are often powerless against the much stronger forces of nature, such as natural selection and evolution. These forces of nature should be treated with greater respect, particularly by people who call themselves “scientists.”

Million Trees


Southwestern willow flycatcher

From California to Texas and occasionally beyond, tamarisks are among the most talked-about introduced plants in the US. Most of that discussion consists of familiar anti-alien dogma, augmented by the long-obsolete assertion that tamarisks are profligate water-guzzlers. Suffice for now to say that anti-tamarisk sentiment led to state and federal suppression policies beginning around 1940, and eventually to legislation at both levels. Little more than accumulated bad reputation of tamarisk and its presence in the region of interest led the US Fish and Wildlife Service to include tamarisks among the supposed threats to the persistence of the Southwestern Willow Flycatcher (Empidonax traillii extimus) when that subspecies was formally listed “endangered” in 1993. All of that meant both political will and appropriations were applied to the US Department of Agriculture’s search for biological control agents to deploy as “counter-pests” against tamarisks. By 1998 they had their critter, an Asian leaf-eating beetle that putatively specialized on tamarisks and would rather die than eat anything else.

By that time, though, circumspection had set in, because especially in southern Arizona the endangered birds had taken to nesting in tamarisk stands. USDA promised USFWS that their armored foreign legion would not jeopardize flycatcher populations. USDA argued that the beetles they were about to propagate and release by the multi-millions were genetically incapable of surviving below 38° North latitude. In addition to famously dividing North from South Korea, that frontier runs from near the tip of Point Reyes through Stockton and Mono Lake; just south of Tonopah, Nevada; south of Canyonlands Nation Park; through Moffat and Swink, Colorado; on through the Garden City Kansas and increasingly irrelevant points east. Southern Arizona would surely never see a tamarisk leaf beetle. “Because SCIENCE!” Hold that thought.

In 1952 the otherwise obscure and perhaps pseudonymous writer Rose Bonne copyrighted a succinct cautionary account of biological pest control. Perhaps it was read or sung or shown to you as a child: I know an Old Lady [who swallowed a fly].  Ms. Bonne denied knowing how or why the old lady swallowed the fly, but considered it portentous: “Perhaps she’ll die!” Subsequent actions had definite (if sometimes puzzling) rationales. The next four animals consumed represented a hopeful trophic cascade: the Old Lady swallowed a spider to catch the fly, then a bird to catch the spider, a cat to catch the bird and a dog to catch the cat. At that point, distended and incoherent, she panicked, swallowing a goat to catch the dog, a cow to catch the goat, then finally, fatally, a horse. (Revisionists inserted a pig between the goat and the cow. If you doubt me, Google it.)

The history of biocontrols

We can barely pause to consider the long and checkered history of biological control. Its inception required a few conditions, which may have arisen in different orders in different places.  A sense of ownership, territorial claims or resource collection rights seems necessary, as does dissatisfaction with the dictates of fate. Why attempt to affect an outcome without expecting to benefit from the effort? A bit of empirical, practical natural history knowledge is also indispensable. Together they add up to the possibility of acting on the basis that “the enemy of my enemy is my friend,” to garner a greater share of whatever natural product seems desirable. Dogs to guard flocks and cats to discourage rodents are biological controls. The more organized and concentrated agriculture became, the greater the need for knowledge of “natural enemies” to enlist as economic allies. Even after revolutions in industrial chemistry offered alternatives, better living was still sometimes available through biology.

With private property rights come boundary disputes, complaints about trespass and spillover effects of management decisions. Public property, especially where subject to intensive multiple use mandates, adds complexity and diversity (if not novelty) to the mix. Rights collide with powers and authorities. Politically compromised jurisdictions—like U.S. state authority over wildlife except where superseded by federal laws and treaties or licensed to private parties—are endless fodder for litigation and finger pointing. All the while, science reconstructs what is known or considered knowable, changing expectations, affecting policies and destabilizing political balances.

Modern civilizations depend upon the plants they have introduced

Modern agricultural, horticultural and forestry practices are all legacies of the Renaissance, Reformation and Enlightenment motivations underpinning European colonialism. Empires were assembled and contested primarily for their economic advantages. During the past half-millennium they generated new wealth and new social classes that developed new governments. Among the array of actions those governments continue to undertake is facilitating the redistribution of valuable plants and animals. A visit to any retail food market reveals our near-total embrace of that redistribution. Almost every staple ingredient in every foodstuff is raised or grown far from its “wild” point of origin. Even insistent locavores prefer locally raised food, not locally evolved food. A negligible fraction of us recognize never-transported, never-domesticated edible organisms. Fewer still could survive on them as hunter-gatherers. Such are among the generally intended, hoped-for, positive outcomes of imperial colonialism. Famine is unnecessary, though it is a political tool, deployable as a weapon.

Fish, meat and leather, plant and animal fibers, timber, pulp and derived products can still be wild harvested, but are mostly and increasingly farmed. Anything worth gathering is worth cultivating, from redwood trees to bison to sugarcane to minks to soybeans to insects, yeasts, and bacteria. Even aspirational exceptions like native plant gardening are actually impossible to accomplish: seed intentionally transported from one location to another has been biogeographically rerouted; plants sold by native plant nurseries are raised in multi-source, formulated soils in plastic pots. Even simply deciding to leave a plant where it was found can render it an artifact, and there may no longer be any wilderness so remote that the configuration of its biota remains uninfluenced by human agency.

Benefits of introduced species often outweigh harm

We are told that some of the consequences of all this redistributed and reconfigured biota are marginally negligible. Others are cutting into the profits. Some organisms are moved around unintentionally and unknowingly (zebra mussels, various “blight” fungi) often because unaware transportation technology designers and operators never prevented their distribution. Many intentionally abducted and marooned populations are behaving in unexpected ways, thriving without always accomplishing their intended purposes (alligator apples and cane toads in Australia; house sparrows and wild carrots in North America) or even significantly over-achieving (“Asian” carps and kudzu in North America; rhododendrons and grey squirrels in Britain). Even where post-colonial inclinations to recover and reinstate pre-colonial values are tolerated, they hardly withstand translation into economic choices.  We are adeptly, fundamentally invested in moving things around. We are likewise invested in competition, and building coalitions and alliances to help us win competitions. Especially competitions we thoughtlessly or accidentally set in motion.

Tamarisk on the Colorado River

The Old Lady who swallowed the fly would probably have been fine had she not overthought the problem. The fly was doubtless well on its way to being digested by the time she found a spider, which was likewise moribund before a bird came to hand. Maybe should could have swallowed a willow flycatcher (already protected by the Migratory Bird Treaty Act) and skipped the spider? Had the US Army Corps of Engineers, the USDA and others not overthought the problem in the late 19th and early 20th centuries, they might have come up with suitable alternatives to planting tamarisks to stabilize Texas barrier islands, deepening Four Corners arroyos and fly-away Dust Bowl topsoils. Yes, tamarisks, too, were brought to us to biologically control problems of our own making and conception. Then we needed a beetle…

As things turned out, USDA scientists were either mistaken or disingenuous regarding the latitudinal limits of their tamarisk leaf beetles. Likewise, even about the identity of the beetles, which is why I haven’t inflicted their Latin epithets on you yet. By 2010, sniping between USFWS and USDA, abetted by various conflicting conservation NGOs, led to a new “Biological Assessment” for the federally imposed tamarisk leaf beetle invasion. (I usually avoid using “invasion” in such circumstances, because invading exceeds many capacities of so-called “invasive species.” This was a real invasion, though, planned and carried out by people, not beetles. Beetles merely bred and spread.) One species of beetle became five, four which had been introduced: Diorhabda carinulata; D. elongata; D. sublineata; and D. carinata. Some were quite well-adapted to life in southern Arizona (31-32° N) and beyond. Furthermore, the endangered birds were also nesting in tamarisks in southern Utah, c. 37° N. USDA washed its hands of the federal program and revoked federal permits to release beetles; but that had no effect on the State of Colorado, which was heavily invested in producing them and continues to do so.

Distribution of tamarisk leaf beetle. Tamarisk Coalition
Tamarisk leaf beetle

Fast-forward to 2017. Tamarisk leaf beetles have been spreading along Arizona waterways at rates up to ten times faster than their most ardent cheerleaders imagined they could, and from multiple directions. They will arrive in almost every known Southwestern Willow Flycatcher nesting area sometime this year. By next spring those riparian thickets will be defoliated just at the point when the nestlings most require thermal cover (i.e., shade). Thanks to Reclamation-Era water diversion projects, attempts to re-vegetate those areas with willows will require constant gardening. Reclamation replaced willow habitat with tamarisk habitat. Nevertheless, the birds persisted. Beetle releases suppressed the tamarisks, but will almost certainly fail to eliminate them entirely. Beetles are just another evolutionary pressure on a tamarisk population that is already unlike any other in the world due to unforeseen hybridizing among several species. New tamarisks and new beetles are evolving. Maybe the beetles will try a bite of something else. They’re in California now; could they find something there? Maybe the birds will evolve to eat the beetles, although that hasn’t happened yet. Perhaps the day will come when the Southwestern Willow Flycatcher gives way to the Tamarisk Beetlebird. It might not even take very long. But don’t bet on it. And don’t bet on biologists, bureaucrats or any other ambitious adults to re-learn the lesson of unintended consequences they laughed at as children, then (like so many other lessons) forgot.

Matt Chew

Invasion Biology: Confusion about cause and effect

We have said before on Million Trees that eradicating non-native plants will not result in the return of native plants because the underlying conditions that supported those native plants have changed and they are no longer competitive within their historic ranges.  In those earlier posts we have focused on higher levels of CO₂ and the resulting climate change as the environmental variables to which non-natives are better adapted.  Changes in water quality and flows have also resulted in changes in animal and plant populations and we will provide a few specific examples in this post. 

Water levels in the Sacramento River delta have been hotly debated for decades and that debate has recently heated up as a commission gets close to making recommendations that will be legally binding.(1)  On one side of the debate, the cities of Southern California and agriculture throughout the state want more water from the delta.  They have been getting a lot of it for decades, but they want much more of it.  On the other side of the debate, environmentalists object to exporting “our” water because they believe that the decline in the populations of native fish such as smelt and salmon is a direct result of the reduction in water flow from the delta to the ocean via the San Francisco bay.   They object to further diversion of delta water and have legally challenged historic levels of water diversion using the Endangered Species Act. 

USFWS Recovery Plan for Native Fish in the Sacramento River Delta

The non-native bass in the delta are the proverbial red herring in this debate.  Those who want yet more water diverted to agriculture claim that the bass are to blame for the declining salmon population.  They demand that the bass be eradicated and they predict that the salmon population will recover once their non-native competitor is removed.(2)  

The diversion of fresh water flow from the delta reduces the speed of the flow of the water, making it turbid and brackish as the ocean water overwhelms the fresh water from the Sacramento River.  The warmer temperature of the water also promotes the growth of water weeds and algae.  The bass benefit from these conditions, but the salmon do not.  Eradicating the bass will not change these underlying conditions.  Salmon populations are unlikely to rebound unless these underlying conditions are changed.

This is not an isolated example of the fallacy of invasion biology.   There are as many examples of similar arguments as there are non-native animal and plant species now occupying spaces previously occupied by natives.  Native plant advocates and their allies want non-native turtles eradicated because they believe they are responsible for declining populations of native turtles.  They want to eradicate non-native bull frogs which they believe would benefit the native red-legged frogs.  Etc., etc., etc., ad nauseum.

And there are as many examples of how such eradication strategies may not benefit natives as there are demands for eradication efforts.  Here are just a couple. 

The Tamarisk or saltcedar tree is one of hundreds of non-native trees that are considered invasive by native plant advocates.  Here’s a description of an expedition on the Colorado River to eradicate Tamarisk that was published by the Sierra Club magazine.(3)

“’Kamikase!’  The most enthusiastic team members start to yell…and fall upon the larger plants with samarai fervor…’Kill tammys!’  someone yells.  ‘Boy, that was satisfying.’ says a fellow tammy warrior…”  And these are their tools of the trade:  “…a veritable armory of tamarisk-killing tools, 32 gallons of herbicide, more than 40 cases of beer…and a Virgin Mary votive candle that…the camp cook has christened with a label reading, ‘Our Lady of Biodiversity.’”

Herbicide is being used in one of the country’s most important watersheds, yet there is no evidence that the Tamarisk is harming the environment:

  • One study found the “mean values for 22 of 30 soil, geomorphology, and vegetation structure traits did not differ significantly between saltcedar and Fremont cottonwood stands.”(4)
  • The same study found that saltcedar increased floristic biodiversity.
  • Another study stated, “As for the claim that salt cedar has little or no value to insects, birds, and mammals, that has been obliterated by available data.”(5)
Tamarisk in natural habitat in Isreal, taken by Michael Baranovsky, Wikimedia Commons

But more importantly, eradicating the saltcedar is not likely to result in the return of the native cottonwoods because the natural flood cycle upon which the cottonwood depends has been altered by man.  The saltcedar thrives in the reduced water flow.  Unless the water flow is restored, the native trees will not return no matter how many saltcedar are destroyed.  Not only are we wasting our time and effort trying to eradicate saltcedar, we are also poisoning our water in the process.

In our final example, cause and effect were not confused, and a restoration was successful.   The Yuba Pass area in California is one of the most important migratory bird routes in the state.  The breeding population of Willow flycatchers disappeared from one of the wet meadows east of the pass.  The native willows upon which the flycatcher is dependent were disappearing from the meadow because channels caused by man along the edge of the meadow diverted water out of the meadow and dried it up.  Ponderosa pines and sage, which prefer the drier conditions, were taking over the meadow.  If native plant advocates had been in charge of remediating this situation their reaction may have been to eradicate the “invading” pines and sage.  That would have been fruitless effort; conditions in the meadow were suitable for pines and sage, not for willows.   But in this case biologists provided a more sophisticated solution.  They eradicated no plants.  They redirected the water from the channel back into its original slow flow through the meadow.  The meadow is again wet, the willows are now thriving, and the Willow flycatcher has returned.

Willow flycatcher, USFWS

“Invasion biology” is an ideology, not a science.  It frequently confuses cause with effect.  A proper diagnosis of what may superficially appear to be an “invasion” requires an understanding of the complexity of nature.  Most often the underlying reasons for an “invasion” are man-made conditions such as pollution and competition for scarce resources that are extremely difficult to fix.  It may be convenient to scapegoat a plant or animal for what man has caused, but it is unlikely to reverse the conditions that create an opportunity for a non-native plant or animal that is better adapted to those new conditions.


(1) “Delta plan may do more harm than good,” Oakland Tribune, 11/5/10

“Effort Falters on San Francisco Bay Delta,” NY Times, 12/15/10

(2) http://www.sfgate.com/cgi-bin/article/article?f=/c/a/2010/12/11/EDG21GN1MJ.DTL

(3) http://www.sierraclub.org/sierra/200407/grand_canyon.asp

(4) Stromberg, JC 1998, “Dynamics of Fremont cottonwood and saltcedar populations along the San Pedro River,” Journal of Arid Environments, 40:133-155

(5) Anderson, BW 1998, “The Case for Salt cedar,” Restoration and Management Notes, 16: -130-134, 138