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LOS ANGELES — Pesticides haven’t worked. Quarantines have been useless. Now California citrus farmers have hired an assassin to knock off the intruder threatening their orchards.

The killer-for-hire is the tamarixia radiata, a tiny parasitic wasp imported from Pakistan.

Its mission: Rub out the Asian citrus psyllid, which has helped spread a disease that turns citrus fruit lumpy and bitter before destroying the trees.

The pest is wreaking havoc in Florida’s 32 citrus-growing counties. In California, it’s been detected in nine counties, most of them south of the commercial growing areas in the Central Valley. Farmers are hoping the tamarixia can help keep it that way.

The wasp, which flew coach in a carry-on bag all the way from the Punjab region, is a parasite half the size of a chocolate sprinkle. But it kills psyllids like a horror-movie monster, drinking their blood like a vampire. The female wasp can lay an egg in the psyllid’s belly. When it hatches, it devours its host.

The wasp “is going to be our No. 1 weapon to control the Asian citrus psyllid,” said Mark Hoddle, an invasive-species expert from the University of California, Riverside, who, over several trips, brought legions of wasps to California.

“We have no other choice except to use this natural enemy or do nothing. And the ‘do nothing’ option is unacceptable.”

So far, Hoddle and his teams have released more than 75,000 wasps across Southern California to beat back the disease, known as Huanglongbing or citrus greening. The malady was first detected in California last November, in a backyard citrus tree in Hacienda Heights.

The disease can lie dormant for a few years before tests are able to detect it, so experts suspect other trees are already infected.

“We’re looking for a needle in the haystack before it sticks us,” said Joel Nelsen, president of the California Citrus Mutual, a trade group.

The U.S. Department of Agriculture has enacted quarantines in nine states, including Florida, Texas and California. The quarantines ban interstate movement of citrus trees and require labeling of nursery stocks from areas where greening has been detected.

That recent discovery raises the fear the pest is creeping into prime citrus-growing areas. It could threaten California’s $2 billion industry, which accounts for about 80 percent of the U.S. fresh-market citrus production. (Florida’s citrus is primarily processed for juice.)

Since 2010, California growers have spent about $15 million yearly to fight the psyllid. Much of that money goes toward massive detection and awareness efforts. That’s on top of millions the federal government and state Department of Food and Agriculture have kicked in.

The psyllids don’t kill citrus trees. They’re merely the agent that spreads Huanglongbing. An infected psyllid acts much like a dirty syringe flying from tree to tree, feeding and depositing a bacterium each time it unfurls its stinger.

Whether the killer wasp can bring the psyllid to heel remains to be seen.

Florida growers imported a strain of the wasp from Vietnam, but it proved ineffective. The predator never took hold, partly because there was not enough genetic diversity needed to establish a population, researchers said.

Some growers there now are looking to develop genetically modified citrus that would be immune to greening — a controversial strategy that could turn off consumers.

In California, farmers are betting on the wasp — and on Hoddle and his wife, Christina. The UC Riverside entomology experts have spent their careers helping control invasive species around the world.

Since late 2011, they have been releasing the wasp, mainly in Los Angeles County. Agriculture officials halted pesticide spraying in L.A. this year, partly because it proved too cumbersome. Six out of 10 citrus trees in the county grow in backyards, making large-scale containment efforts difficult.

“This is ground zero for our war,” Mark Hoddle said.

The goal is to reduce the psyllid population and provide a line of defense between urban areas in Southern California and the commercial growing zones.

The Hoddles and teams they’ve trained have been going neighborhood by neighborhood releasing thousands of wasps and tracking the parasites’ success.

The couple conducted extensive testing beforehand to make sure the wasp wouldn’t disrupt California’s ecosystem, considering past efforts at introducing nonnative species have gone awry.

To satisfy the federal government’s concerns, the Hoddles quarantined the wasp for 18 months and performed several experiments to see if it would attack other, native species. Time after time, the wasp attacked only the Asian citrus psyllid.

After the researchers submitted a 60-page report to the Department of Agriculture, the release program was greenlighted. The wasp poses no danger to humans or pets, Mark Hoddle said.