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July 26, 2010

Biological control of cassava pests tried in Thailand

by Ker Than

A huge swarm of parasitic wasps has been unleashed in Thailand as a last-ditch effort to control a devastating pest outbreak, scientists say.

More than a quarter million wasps filled the skies of the northeastern province of Khon Kaen recently. The hope is that the wasps will attack mealybugs, which have been infesting the country's valuable cassava crop.

The pill-shaped mealybugs suck sap from the crops—a main ingredient of tapioca—causing them to shrivel and die.

A Phenacoccus manihoti mealybug crawls on the end of a cassava stem on a Thai plantation last year. Photograph courtesy Neil Palmer, CIAT

But that's nothing compared with the killing technique of female Anagyrus lopezi wasps, which inject their eggs directly into the mealybugs' bodies. When the eggs hatch, the larvae eat their way out of their hosts, killing them.

An Anagyrus lopezi wasp. Photograph courtesy Georg Goergen, IITA.

Like the wasp and cassava, the Phenacoccus manihoti mealybug is native to the South American country of Paraguay. But the mealybugs "got into Thailand without their natural enemies, and the population has increased very rapidly over the past couple of years," said Tony Bellotti, an entomologist at the International Center for Tropical Agriculture (CIAT), a Colombia-based nonprofit research institute that is helping organize the effort. The Thai Department of Agriculture and the Thai Tapioca Development Institute are also involved in the project.

Mealybugs were probably accidentally imported to Thailand from stem cuttings from Africa sometime during the past few years, Bellotti said.

In 2009 mealybugs had spread to more than 700 square miles (200,000 hectares) of eastern and northeastern Thailand, where the pests are known to kill up to half of the plants in a given field, experts say.

Past experience shows the Thai wasp army should work, Bellotti said: The parasites were used to great effect throughout Africa in the 1980s to help control mealybug infestations of cassava.

Scientists consider the parasitic wasps a classic example of "biological control," in which the natural enemies of a pest are imported from its native country to curb the pest's spread.

In Africa "yield losses were estimated at the time to be as high as 80 percent ... I doubt it's even 10 percent now," Bellotti said.

Anthony Shelton, an entomologist at Cornell University in New York State, said the Thai wasp attack is "very logical."

Not all biological-control experiments work out as scientists intend, but those that do—such as the 1980s African intervention—tend to be "spectacular successes," said Shelton, who was not involved in the project. If the wasps work well in Thailand, they should not negatively impact other species, he said.

Unlike most predators—which often feed on many types of prey—some parasites, such as the Anagyrus lopezi wasp, attack only one or a few species, Shelton added. "As the pests spread around, it's also important to spread around their natural enemies"—the wasps, he said.

In time, the wasp and the mealybug should reach an equilibrium in Thailand, CIAT's Bellotti said. "The parasite can't eliminate the pest [completely] because it would eliminate itself," he said. "But you hope that the equilibrium will be at a low enough level that it's no longer damaging to the crops."

CIAT's Bellotti said it shouldn't take long for scientists to know whether unleashing the wasp swarm was a good idea.

"The big test will come during December, January, and February, which is Thailand's dry season," Bellotti said. "The mealybug is a dry-season pest."

If the Thai experiment works, CIAT may deploy the wasps in Cambodia, Laos, and other Southeast Asian countries where the cassava mealybug has been reported."We don't see this as just a Thai problem," Bellotti said. "We see it as a regional problem in Southeast Asia."

National Geographic

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