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GUY RAZ, HOST:

And if you're just tuning in, this is WEEKENDS on ALL THINGS CONSIDERED from NPR News. I'm Guy Raz. Long before all the post-911 security precautions in the U.S., a person or a group of people carried out a series of bioterror attacks in California. And the target? The menthol-scented eucalyptus trees. Someone may be trying to kill them.

Now, before you wonder why you hadn't heard of this, well, it's because the story may not be true. It's a hypothesis and a theory promoted by a famed California entomologist and eucalyptus expert named Timothy Paine.

(SOUNDBITE OF LEAVES CRUNCHING)

TIMOTHY PAINE: Well, where we are right now is an area of eucalyptus that have been planted, and they probably are 100 years old, some of the trees in here.

RAZ: Paine walks through a grove of eucalyptus trees on the campus of UC Riverside where he teaches. This is his outdoor laboratory, a test site. And on one of trees, balls of white fuzz cover the leaves. Timothy Paine walks over and snaps off a leaf to reveal the culprit. Under that fuzz, an insect called a lerp psyllid.

PAINE: They're feeding on the upper surface of the leaf. So what I can do is just put my fingernail right on the edge of the lerp and give it a little bit of a tug.

RAZ: And there it is, a flat black and yellow bug that literally sucks the life out of the tree. The lerp psyllid is just one pest attacking the eucalyptus, but it's not the only one. All over this grove, insects are killing these trees.

PAINE: In 1985, '86 in California, we started seeing eucalyptus trees dying.

RAZ: It began with a longhorned borer. That's a beetle that looks like a cockroach. As soon as Timothy Paine found a natural enemy to fight off the beetle, he got word of another pest.

PAINE: We were seeing insect after insect coming in. We started working on the longhorned borer, and then another would come in, and another one would come in.

RAZ: All together, over a period of 25 years, 16 non-native insects arrived in California, all of them with one main target, the eucalyptus. Paine got suspicious. He thought it was just too coincidental. So he started working on models, and what he discovered astounded him. Every four years or so, a cluster of insects from a particular place in Australia would show up in Southern California.

PAINE: They will be from Queensland, or they would be from New South Wales, or they'd be from south Australia.

RAZ: And for insects - like the lerp psyllid - it's not so easy for those fuzzy white balls to hop onto a flight to Los Angeles. And that's when Paine began to suspect that these pests might be getting a helping hand.

PAINE: If you see something one time, you accept it. You see another pattern, you wonder if it's a coincidence. You start seeing five or six different patterns that all point in the same direction, you start to raise questions.

RAZ: Over the years, Paine has become more and more convinced that someone or some group has brought these killers from Australia to California. And the purpose? To get rid of California's eucalyptus trees. You're saying, though, that this was malicious.

PAINE: We can't imagine that it was done for any other reason than that, yes.

RAZ: In other words, he believes it's a case of biological terrorism. But if you're wondering why anyone would do such a thing, well, it turns out that the eucalyptus is actually quite controversial. Before 1850, not a single one existed in California. That year, some seeds from Australia were planted in the state. And today, it might as well be California's state tree. The state is covered with eucalyptus groves. But some environmental groups consider the trees invasive.

Zoe Corbyn is a science journalist who recently wrote about Paine's theory of biological terrorism for Failure magazine.

ZOE CORBYN: California's got fantastic and lovely native plants. And the native plant advocates argue that the trees sort of prohibit those from flourishing.

RAZ: Corbyn also says that native plant advocates point to another problem with the eucalyptus, their dry, brittle leaves are a huge fire hazard responsible for a particularly devastating fire in Berkeley in the early 1990s. Corbyn says that if Paine's theory is correct, this could be the first documented case of an intentional introduction of a plant pest into the United States. But there's just one problem.

CORBYN: You have to remember that the evidence is only circumstantial. Nobody's come forward and put their hand up and said, I did this.

RAZ: In 2010, Timonthy Paine published his findings in the Journal of Economic Entomology, a peer-reviewed academic journal. It was met with some skepticism among other researchers. And here's what they told Zoe Corbyn.

CORBYN: There's many more flights from Australia to L.A. than there was in the past, and it's very possible that something could've come in that way, you know, crawled on board and crawled out the other side.

PAINE: Yeah. You can find alternative explanations for all of this, no question about it. This is something that we think is likely to have happened, but we don't have a smoking gun.

RAZ: And without that smoking gun, Timothy Paine is out of luck. But he says for him, it's not just about the eucalyptus, it's about the possibility of biological terrorism that really scares him.

PAINE: Eucalyptus are important for this state, but they're not life or death. But if you're talking about a major food crop, or a disease organism, the prospects are very, very disturbing.

RAZ: For now, Timothy Paine is moving on with his work. He's found natural enemies to fight four of the insects. The other twelve, though, are still doing their best to annihilate the eucalyptus trees of California.

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