Homeowner John Bourzac weaves in and out of the tall line of fragrant eucalyptus trees on his property in Clovis, California, and wonders what the fuzzy white balls on the leaves are. “I started to notice them about four or five years ago,” he says, worrying that while his trees seem to be tolerating them thus far, they can’t be good news.
Bourzac really likes his red gum (Eucalyptus camaldulensis) trees, which along with the blue gum (Eucalyptus globulus) are the two most common varieties of the over two-hundred species currently growing in the state. So ubiquitous and integrated into the landscape are they—planted as street trees in urban areas and groves in rural parts of the state—that many Californians assume they are native, though they were initially introduced as agricultural oddities, and later seen as a source of lumber. (Eucalyptus is in fact native only to Australia, extending into New Guinea, the Philippines and Indonesia.) Thirty years ago Bourzac planted red gums as a screen because they are quick growing and drought tolerant. Today they provide him with privacy from a busy road and give his property a secluded and tranquil feel.
But those white balls on the leaves indicate that Bourzac’s trees are under attack—and the prognosis isn’t good. The culprit is the redgum lerp psyllid (Glycaspis brimblecombei), a small green flying insect, also native to Australia, which was first discovered in 1998 in Los Angeles County before it spread through much of California. Peel away a fuzzy white ball and underneath lies a tiny yellow speck: the insect in its immature form, which is vulnerable without the protective coating it spins from sugar and wax. While the infestation is unlikely to kill Bourzac’s trees outright, it will suck sap from the leaves, causing some to drop off. This, in turn, stresses the tree, making it more vulnerable to attack by other insects. In the future, Bourzac can expect balding trees and sticky secretions underfoot, not to mention the sight and sound of cars roaring past.
But the redgum lerp psyllid isn’t the only pest that has been found attacking California’s eucalyptus trees in recent years. In the past three decades, sixteen new insect pests—all native to Australia—have become established. (Prior to 1983 there had been two since the introduction of blue gum to the state in the 1850s.) The arrivals have prompted a painstaking effort to find other Australian insects which are natural enemies of the pests, ones which can be brought in to save the trees. But of the sixteen, biological controls have been successfully introduced for just four.
The accumulation of so many pests in such a short time has led to a sneaking suspicion. For while California’s eucalyptus trees have passionate defenders, there are also those equally passionate about their eradication. While eucalyptus may be well adapted to an environment that contains few big shady trees, they are not well behaved everywhere. In some wetter coastal areas in northern California the blue gum has become an invasive, difficult-to-eradicate weed, crowding out California natives. Both the blue gum and the red gum have been designated “invasive plants” by the California Invasive Plant Council (Cal-IPC)—a non-profit whose “mission is to protect California’s lands and waters from ecologically-damaging invasive plants through science, education and policy”—with the blue gum’s ecological impact rated as “moderate” and red gum’s “limited.”
Contributing to the controversy is the threat of urban wildfire. Not only are eucalyptus leaves highly flammable, many species shed large amounts of bark and leaf litter, which may serve as a fuel source for fires. It begs the question: Could the pests have been deliberately introduced in an attempt to kill the trees? A case of biological sabotage by those minded to rid the state of an invader?
The man with the suspicion is Timothy Paine, an entomologist at the University of California (Riverside), who, for more than twenty-five years, has led the combat mission against the pests attacking California’s eucalyptus. Recently, after much hand wringing, he committed to the scientific record the idea that California’s eucalyptus may have been biologically sabotaged, publishing a journal article raising the possibility of bioterrorism. If he is right, it would be the first documented case of the intentional introduction of a plant pest in the U.S.
“We took all of the available information we had on the introduction of eucalyptus pests into California and the conclusion we drew is that there is a very high probability that someone was intentionally introducing them,” he explains, referring to himself and his co-authors. “There is likely intentional movement of insect pests of eucalyptus into the state. The patterns suggest that.”
I meet Paine at a café near the university before going to view his experimental eucalyptus groves and rearing laboratory, where he produces large quantities of biological controls for release. A tall and thoughtful man, he carries a magnifying glass on his key chain to better observe bugs up close. With a caution typical of a scientist, he chooses his words with extreme care, sometimes taking long pauses before his sentences. Occasionally he cups his head in his hands. As he tells his tale, it’s as if a weight is being lifted from his shoulders.
The story begins in 1985 when Paine’s department began receiving calls from county officials relaying concerns that large numbers of eucalyptus trees were dying and costing upwards of ten-thousand-dollars each to remove. First the calls came from residents in southern California but they soon began coming from the San Francisco Bay Area too. “At its peak, the University of California (San Diego) were saying they could spend their entire landscape budget for the whole campus just taking out eucalyptus trees,” he recalls. “It was a serious economic problem.”
Paine’s team identified the killer as the eucalyptus longhorned borer (Phoracantha semipunctata), an Australian wood boring beetle that resembles a cockroach but with extra-long antennae. Its first appearance—based on the earliest complaints—was traced to an old eucalyptus plantation in Lake Forest, California, one located near a marine air station. Paine assumed the borer entered via a packing create that happened to be made of eucalyptus wood. The borer is as Australian as eucalyptus but blue gum is grown commercially around the world for paper production and the pest has been found in other countries. “It could have come from anywhere,” he says.
Cobbling together funding from a variety of sources (including counties and cities, parks departments, universities, homeowners associations and the nursery industry), the UC-Riverside team began to study how their new insect foe might be managed, and soon began searching for an ally in the form of a natural enemy. “It was my first adventure in biological control,” recalls Paine. It seemed a logical strategy given that the trees were diffuse rather than in one spot. And in contrast to insecticides, the method has the advantage of being a permanent solution.
After a painstaking process testing the fitness and safety of a number of insects Paine’s team struck gold: a control that was so specific it would attack only the borer. Following the necessary federal and state government permits, in 1993 another Australian native, a tiny parasitic wasp (Avetianella longoi) the size of a grain of pepper, was let loose to combat the borer. It was a spectacular success. Laying its eggs inside those of the borer—so instead of a borer grub emerging a wasp does—the million tiny wasps Paine estimates his lab produced for release throughout California cut borer populations dramatically, which had by this time spread across the state. “Populations of that borer are so low it is really hard to find now,” he advises.
Paine was about to check off the problem and publish the success story when one of his undergraduate students working in the rearing laboratory noticed something strange. Some borers collected in southern California to boost the laboratory colony (they are needed to rear the wasps for release) had slightly different markings. “We realized we had a second borer species,” he laments.
The second borer, also from Australia and traced to southern California, wasn’t the only new kid on the block. Other pests specific to eucalyptus (including various species of leaf beetle and psyllid), all native to Australia, also showed up. “Every time we turned around there would be a new one,” recalls Paine. “We would solve one problem and we would have another.”
Ultimately successful biological controls were implemented for the first borer, as well as the eucalyptus snout beetle (Gonipterus scutellatus). Similarly, controls for the blue gum lerp psyllid and red gum lerp psyllid were developed by Donald Dahlsten, an insect biologist at the University of California (Berkeley), who, after his death in 2003, left Paine the only individual pursing the problem in earnest. (The blue gum lerp psyllid is now under excellent biological control while the control for the red gum lerp psyllid has only been moderately successful.)
Meanwhile, an attempt by Paine to eradicate the leaf-eating eucalyptus tortoise beetle (Trachymela sloanei) failed: his team released thousands of controls but were unable to get them established. Controls for two more psyllids were due to be released when they arrived of their own accord. “We are doing what we can,” says Paine, noting that pests have been prioritized based on the amount of damage they do.
Paine’s quest to find a control on the second eucalyptus longhorned borer (Phoracantha recurva) began around 1995. The expectation was that the parasitic wasp used for the first borer would work, but to his team’s surprise the second borer actually killed it. The answer has been a slightly different parasitic wasp (indeed all the controls introduced have been wasps), which they have been breeding in the rearing laboratory and releasing for the past three years. Paine isn’t yet sure how successful the control has been. While numbers of the second borer have dropped, his team hasn’t been able to find the wasps in the environment, which is the proof necessary to know the biological control is working. “It is frustrating,” he says. “We think we have a very good parasite but we can’t prove it because we can’t recollect it.”
The damage the borers can do is breathtaking. Tucked in the back of the university’s Agricultural Experiment Station behind the citrus experiments, Paine takes me to what he calls his “natural laboratory”—an abandoned eighty-year-old eucalyptus grove of various species of tree, which is used for observing and as a release site.
We head for the dead trees in the center—some killed by borers, others by psyllids—and examine a fallen one. Masses of squiggly trails are deeply engraved into the trunk, like the work of a crazed carpenter. The wood is permeated by holes. It is the tell-tale sign of borers. They lay eggs underneath the bark and when the grubs hatch, they begin audibly devouring the conductive tissue of the tree that brings the sugars down from the foliage to the roots. The tracks are left by the grubs which, at the end of the feeding period, sign their handiwork by tunnelling into the wood, making the holes from which they emerge as flying adults. “Basically they are just girdling the tree,” says Paine. “I have seen trees die in thirty days.”
Matt Ritter is a botanist at California Polytechnic State University (San Luis Obispo) who studies eucalyptus. The pests have really become apparent in the last ten or fifteen years, he says. “I don’t think you can look at eucalyptus in California anymore and not notice. You can actually identify the red gum by the fact that it has lerp psyllid on it and the blue gum very rarely has a leaf that is not partially eaten.” The longhorned borers, he confirms, have the potential to kill “a lot of trees,” which in turn adds to the fire risk.
Yet many Californians seem oblivious that their eucalyptus trees are being preyed upon. That is unsurprising, says Paine. There aren’t continuous swathes of eucalyptus forest in California that would make the insects’ damage stand out, and the only commercial production is on a very small scale—for foliage and the landscape trade. (The notion that it would be a timber tree was abandoned long ago after the young wood was found to twist and crack too easily.) A relatively small number of sick or dead ornamental trees are hardly eye-catching.
The Evidence for Bioterrorism
It was how the first longhorned borer spread when it arrived in California that first opened Paine’s eyes to the possibility of ecological sabotage. “It jumped,” he recalls. “It was infesting southern California and all of a sudden it started infesting the San Francisco Bay Area. It wasn’t a progression. We got two separate infestations and it filled in the rest of the state.” Rumors circulated, he recalls, which even state officials would repeat: an individual had been heard bragging about having moved the borer. But it was also entirely possible that that was a coincidence, a separate introduction from outside, so he put the rumors out of his mind. Yet with more pests arriving, Paine’s group began doing some detective work to trace the timing and location of the arrivals and started to wonder about the patterns they were seeing.
After a long period of sitting on the findings, Paine finally published the paper, Accumulation of Pest Insects on Eucalyptus in California: Random Process or Smoking Gun, in the Journal of Economic Entomology. The paper advanced the idea that it could be a case of biological sabotage—someone deliberately introducing the insects, motivated by the desire to get rid of the trees. The case is “a cautionary example of what could happen if a major food or fiber crop were intentionally targeted,” it notes.
Publishing was a difficult decision, recalls Paine. First, there is no direct evidence: “We don’t have somebody who has said ‘I did it.’ All we have is a series of coincident patterns that becomes suspicious,” he says. Second, he didn’t want to give anyone ideas about the potential for insects to become weapons, against eucalyptus or anything else. Bioterrorism is a concern, especially since 9/11, says Paine. “We didn’t want to plant any seeds.” But Paine—and co-authors Jocelyn Millar and Kent Daane—finally decided to go ahead and publish to raise the visibility of a potential problem. They also figured that if someone was sabotaging the trees, they might be deterred if they knew people were on to them.
Paine outlines the patterns. First, the introductions, which his team traced to between 1983 and 2008, were all—bar one—first detected in southern California, either in Los Angeles County or Orange County. If it was accidental—through the movement of goods or people—we would have expected more in the northern part of the state, says Paine. The Port of Oakland in the San Francisco Bay Area is one of the busiest container shipping ports in the country, rivalling the Port of Long Beach near Los Angeles. And while San Francisco’s international airport is smaller than Los Angeles International, it is also a large entry point for passengers and air cargo.
Second, the introductions have occurred in bunches: year-long periods of time when up to four insects would appear followed by lulls of several years before another wave. It’s a pattern that would be consistent with a villain making periodic trips to Australia to gather pests. Paine identifies four distinct periods of multiple species arriving: 1983-84 saw three; 1990-91 saw two; another four arrived in 1994-95; followed by three in 1998-99. The clusters then peter out with a single species in 2000, one in 2003, and two more in 2008.
Grouping the pests by their native range in Australia shows those in each cluster could all have come from the same east coast state, conceivably facilitating easier collection. Pests in the first cluster all occur together in Queensland, pests in the second in New South Wales and South Australia, pests in the third across all three of those states, while pests in the fourth bunch occur together only in New South Wales.
There are other patterns too. The blue gum and the red gum, the two eucalyptus species regarded as invasive, are particularly susceptible to the pests. Indicating the introductions are from Australia, most of the pests also seem to appear in California first before spreading to eucalyptus in other parts of the world. Seven are found only in the two locations.
The most obvious suspect—the nursery trade—which could allow immature pests to hitch a ride on eucalyptus plants, Paine thinks unlikely. What eucalyptus California’s nurseries sell is home grown and no new varieties or cultivars have been introduced for many years. “As far as we can tell, the nurseries have stopped importing eucalyptus. If they are moving stuff in it is as seed,” he explains.
Furthermore, asks Paine, why, if the movement of the pests was accidental, has it so disproportionately brought in enemies of the eucalyptus? (Of seventeen Australian pests to have become established in California since 1980, fourteen are specific to eucalyptus.) And where are the natural predators of the pests that might be expected to arrive with them? (Of the sixteen eucalyptus pests only two of their predators have arrived on their own.) And why haven’t any eucalyptus pests arrived from elsewhere? (While no Californian insects attack eucalyptus, species native to South America and Asia have colonized the eucalyptus that grows there.) The introductions “don’t seem to fit a random pattern,” says Paine.
He isn’t sure who might have done it or how but Paine thinks it would have taken somebody who knows their stuff. “I don’t know if they would need to be an entomologist but they would need to be skilful,” he says. Asked why he has never gone to the police, Paine sighs. There are no leads on who might be the culprit, and in any case, the evidence is circumstantial. “I can’t prove anything,” stresses Paine.
Other scientists are sceptical. “I know there are people out there that really dislike eucalyptus but just think what it would take to do it on purpose,” says Ritter. “You would have to go to Australia, find the pests, bring them here, and then release them in a way that they have the potential to continue on.” It is also a “dumb way” to control the tree, he adds. Other than the borer, a killer, most of the pests merely make the trees sickly. “I just don’t see compelling evidence,” he concludes.
Ted Center, an entomologist at the U.S. Department of Agriculture’s Invasive Plant Research Laboratory in Fort Lauderdale, Florida agrees. Globalization has ratcheted up the chances of importing pests and diseases from everywhere. Furthermore, he says, there are now more direct flights between Los Angeles and Australia than ever before, and pests entering the cargo holds of passenger planes need only survive fourteen or fifteen hours in order to reach California. Other destinations where eucalyptus occurs receive fewer flights or are less directly accessible, requiring connections. “In my opinion, the [Journal of Economic Entomology] paper is far too speculative,” he says.
Jacqueline Fletcher, an expert in crop biosecurity and director of the National Institute for Microbial Forensics & Food and Agricultural Biosecurity (Oklahoma State University), has recently been working on how to help farmers distinguish between intentional and accidental introduction of plant diseases. She isn’t necessarily convinced that there was intentional introduction, but she praises Paine for at least being prepared to raise the possibility. She identifies a tendency she refers to as “suspicion inertia.” “We tend to think of everything as just being natural—even an unfamiliar set of symptoms—and it is rare for someone to take the perspective that it might not be.”
Paine understands the scepticism. “We can’t demonstrate it conclusively from the data so the response from most people in the academic world is we need more,” he says. But, he notes, the conclusions haven’t been hastily or carelessly drawn. “We started wondering about it after the fourth or fifth introduction. It finally took sixteen that we could get enough of a pattern,” he says.
A genetic analysis matching the pests in California to specific populations in Australia could make the pattern stronger, but it still wouldn’t be concrete evidence, says Paine. “Without someone standing up [and claiming responsibility], we can never know for sure.”
Should the Trees Be Saved?
The case also raises a wider question: whether unintentional or sabotage, should eucalyptus be saved from the pests at all, given how damaging the trees themselves can be? What if the pests are actually the good guys?
Florida may be on the other side of the country, but it too has a problem with an Australian invasive weed. Melaleuca or paperbark (Melaleuca quinquenervia) causes extensive environmental damage in the Everglades by clogging waterways. Yet in stark contrast to California’s efforts to save its own problematic Australian, Florida’s approach is to use biological control to manage it. Thus far three insect species have been introduced and are successfully targeting the melaleuca.
The irony of California’s biological control program to save an invasive weed isn’t lost on Doug Johnson, executive director of the Cal-IPC. “It does seem a little surreal that you have some people working to get rid of eucalyptus and others working to save eucalyptus,” he says.
It’s an issue that has been raised with Paine. Some weed biological control experts have suggested bringing in pests might actually be a good idea, he says. The question comes up at public meetings, he adds. Many people want to know where they can get the controls to save their trees. But others want to know why the university is putting resources towards protecting the trees—or even how to rear and redistribute the pests.
The reasons California is saving an invasive weed aren’t straightforward. There are invasive plants in California where it is hard to find anything positive about them, says Johnson, but “in any given situation there may be more benefit to keeping eucalyptus than removing it.” Many people like big trees of any type and have grown up seeing eucalyptus as part of the landscape. It also has ecological uses; for example, monarch butterflies roost in it. The approach has been to manage it locally on a case-by-case basis—often to great controversy—rather than to use biocontrol.
But there has never been a proper debate to weigh the advantages and disadvantages of Paine’s work or whether it is the best way forward for the state as a whole. An ornamental tree that neither falls under the remit of forestry nor agricultural agencies, it’s an orphan problem with an interesting research angle that Paine set about trying to solve mostly on his own, without oversight. It is in the university’s remit to try and help solve such problems. “It is part of our responsibility,” explains Paine.
That riles people like Jake Sigg, the conservation chair of the San Francisco-based Yerba Buena chapter of the California Native Plant Society, a non-profit dedicated to protecting California’s native plants. “I think the university ought not to be going ahead with this research without considering all of the ramifications and hearing from all parties,” he says. “This is a serious public policy question. Someone ought to be considering it rather than just having the researchers importing the bugs and going ahead.”
It would be a “useful discussion” to have, agrees Paine, but he also points out that—as far as he is aware—there is no forum for doing that. “We have to go through a series of permitting processes to introduce something but nobody says, ‘No, you can’t bring this in because you are protecting a potentially invasive plant’,” he explains. And he notes values do change: what at one point might be considered a rational policy decision made by an informed public, a decade later may seem to have been short sighted.
But ultimately, admits Paine, his work is driven by his own belief that the trees are worth saving. In hindsight there are doubtless places where blue gums should never have been planted, he says, but you can’t go back in time. In his pragmatic view, the focus must be on the problem at hand: “You protect the resource that you have.” And the trees, which dominate the urban landscape and provide amenity, need help.
“Is it worth allowing millions of trees to die because there is the potential that some of them can be a pest?,” he asks. That’s exactly the value judgement both the researchers—and perhaps bioterrorists—are making.
Zoë Corbyn is a freelance science journalist based in San Francisco. Her work appears regularly in Nature, the Times Higher Education and The Guardian.
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