In Focus

Can Parasitic Wasps Help Save America's Citrus?

Scientists are trying to spare California from a disease that devastated Florida’s citrus crop.

In a lemon grove in Riverside, California, a half dozen beige mesh tents rise above the fruit trees. Within each tent a solitary lemon tree bustles with invaders from another continent: small insects called Asian citrus psyllids. They're potential carriers of a plague that could decimate California's citrus groves. But scientists here are deliberately breeding them—in order to breed an antidote.

"Warning," read signs posted on the tents in English and Spanish. "Wasps inside."

Tamarixia radiata is a flea-size parasitic wasp from Pakistan that attacks Asian citrus psyllids. Hundreds of thousands of the tiny wasps have already been released in California. Last month the U.S. Department of Agriculture promised the state $1.5 million to ramp up the program to around a million wasps a year.

A lot is riding on the little bugs.

Citrus greening, as the citrus disease is called, has already destroyed 90,000 acres of trees in Florida groves. Most kinds of citrus, from lemons to oranges to grapefruit, are susceptible. California's two-billion-dollar industry, which grows 80 percent of the citrus sold as whole fruit in the U.S., is desperate to avoid Florida's fate.

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Adult Asian citrus psyllids—this one glued to a needle for the photographer's sake—spread citrus greening as they move from tree to tree feeding on leaves. Photograph by Sam Droege, USGS Patuxent Wildlife Research Center

Alarming Progress

Citrus greening, also known as huanglongbing or yellow dragon disease, is caused by a bacterial infection that progressively blocks a tree's ability to transport fluid. Partially green fruit are just one of the symptoms that emerge as the tree slowly dies.

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These tiny parasitic wasps, Tamarixia radiata, feed on and kill Asian citrus psyllids. Researchers with California's Department of Food and Agriculture plan to raise more than four million of the wasps this year and release them around the state. Photograph by David Liittschwager

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Psyllid nymphs that are being raised to feed wasps release a sugary compound as they feed on a leaf.Photograph by David Liittschwager

The disease is spread by Asian citrus psyllids, which suck up the bacteria as they feed on leaves, then deposit them again as they move from tree to tree. There's no cure yet.

Asian citrus psyllids arrived in Florida in 1998. Citrus greening hit Florida in 2005. Since then it has spread to Texas, Georgia, and Louisiana. It's also killing trees in Central and South America and Africa as well as in Asia.

The psyllids have already migrated to California, invading about 47,000 square miles from Santa Barbara to the Imperial Valley. But so far agricultural inspectors have found only one instance of citrus greening, southeast of Los Angeles in Hacienda Heights at a house with an orange tree that had been illegally imported from China.

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Oranges infected with citrus greening, like these in a Clewiston, Florida, grove, are hard and bitter and can't be sold. Photograph by Richard Perry, New York Times/Redux

That could change in an instant, though. "It's like if you have a hundred mosquitoes without malaria, but then the hundred and first one has it," said David Morgan, a British entomologist who runs the wasp-rearing program for the California Department of Food and Agriculture.

It can take up to two years for an infected tree to show symptoms, he says, so it's entirely possible much more of California is already infested. For now, the agency is attacking the problem by trying to kill off as many Asian citrus psyllids as possible.

In commercial citrus groves, farmers can "look after themselves," as Morgan puts it, by spraying insecticide. But backyard trees make up the bulk of the state's citrus population; in southern California there are more than two citrus trees per house. Spraying insecticide is impractical, inefficient, and unappealing to many homeowners. The danger is that the disease will gain a foothold in the suburbs, then jump to commercial groves in central California.

Morgan and his colleagues are hoping to spread their parasitic wasps to many of those backyard trees. The whole plan sounds ominous in a sci-fi sort of way. But these are nothing like the menacing and much larger wasps that build hives under eaves and deliver painful stings. Tamarixia are less than a millimeter (4/100 inch) long, barely bigger than a period. And while they do have stingers, Morgan insists they're too small to pierce human skin.

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Some 20,000 parasitic wasps are being raised in this tent in a lemon grove in Riverside, California. Photograph by David Liittschwager

Within a Budding Grove

In the Riverside lemon grove, Raju Pandey, an entomologist with the Citrus Research Board, an industry organization, is breeding wasps that Morgan's team will ultimately release. One tent can produce 23,000 wasps. In greenhouses nearby, Morgan is breeding wasps of his own, in dozens of white tents containing small potted citrus plants.

He and his colleagues begin by installing nine plants in a tent and then adding around 300 Asian citrus psyllids. Each plant supports about a thousand nymphs. Once the population is in full swing, the researchers add wasps.

About a third the size of the psyllids, the wasps lay eggs on the psyllids' bellies. Two weeks later the new generation emerges. The scientists vacuum them up with small metal tubes, store them for a few days at cool temperatures to prepare them for the real world, then release them in someone's yard on an orange or lemon tree.

Since 2012 Morgan and his team have been methodically releasing small vials of the wasps—200 to 600 per tree—in eight counties across southern California, using a system of five-mile grids and a mathematical model developed by the USDA to predict the highest-risk areas.

The insects are expensive to rear, so the state is releasing small numbers of wasps and hoping they'll establish themselves over a larger area. Scientists have found Tamarixia living as far as eight miles from where they were released.

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NG Staff. Source: Rick Dunn, Citrus Research Board

Gifts from Pakistan

The wasp was first collected in Pakistan by Mark Hoddle, a University of California, Riverside, researcher who was on a mission to find psyllid predators in a part of their native range with a climate similar to southern California's. Before being approved for biocontrol use, Tamarixia underwent a lengthy USDA evaluation intended to ensure that it wouldn't harm any native psyllids.

Any potential biocontrol agent "needs a life history that matches our goals," says Morgan. "It has to reproduce more rapidly than the pest, live in the same environment, and eat it." Another wasp Hoddle collected in Pakistan, Diaphorencyrtus aligarhensis, is currently in quarantine and may be ready for psyllid-killing duty soon.

Meanwhile, Pandey may have inadvertently stumbled on another defense against the psyllids. Recently, he was preparing to release wasps into three of his field cages, all of which were brimming with Asian citrus psyllids. But two days after confirming that the insect populations were thriving, he found nearly all of them dead.

He sent dead psyllids and tree cuttings to two groups of researchers, who are now scrambling to determine what killed the bugs. If the culprit turns out to be fungi or bacteria, it could lead to a whole new way to fight citrus greening.

Other researchers are trying to breed Asian citrus psyllids that can't carry the disease and genetically modified citrus plants that are resistant to it. Many lines of attack are needed, because it's already clear that Tamarixia alone can't ward off huanglongbing forever. At best the wasps slash the psyllid population only by about a third.

"It's better than not controlling them at all," says Morgan. "But it's not a viable option as a stand-alone control strategy."

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Researchers place 30 psyllids and 30 wasps in each small enclosure. The psyllids feed on the leaves, the wasps feed on the psyllids, and a month later there are a thousand wasps. Photograph by David Liittschwager

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Sean Gonzalez collects wasps, which have stingers too small to pierce his skin, with an aspirator.Photograph by David Liittschwager