Wasp May Lay Foe to Waste

A vine-sucking insect that’s been devastating Southern California vineyards is about to meet the enemy: a parasitic wasp from Minnesota.

Release of the wasp in Riverside, Kern and Ventura counties is expected to be approved this week by the Department of Agriculture as part of an all-out war on the pesky glassy-winged sharpshooter, a voracious leafhopper that threatens the state’s $45-billion wine grape industry.

The wasp, known as Anagrus epos, is tiny -- just one-thirty-second of an inch long. But it is deadly to the sharpshooter, which transmits Pierce’s disease. There is no known treatment for Pierce’s, which attacks wine, table and raisin grapes as well as almond and citrus trees and oleanders.

Pierce’s disease decimated half of the Temecula area’s 3,000 acres of grapevines in 1999 and 2000. Today, 14 counties, primarily Riverside, Kern, Ventura and Tulare, are infested.

“There is no silver bullet either for the bug or for the disease,” said Jay Van Rein, spokesman for the state Department of Food and Agriculture. “We just need to find a solution to one or the other.”

The introduction of the minute wasp is the latest in a series of pest-control efforts that use one insect to kill another. Biological control, as the practice is known, has a 100-year history in California, said David Morgan, an environmental research scientist with the state agriculture department.

“This particular wasp has never been used to target the sharpshooter before,” Morgan said.

The sharpshooter, native to the southeastern U.S. and northeastern Mexico, arrived in California in 1989, most likely hitchhiking on imported nursery plants, and within a decade had proliferated throughout Southern California.

The half-inch sharpshooter extracts fluid from host plants with its needlelike mouth, depositing a lethal bacterium that causes Pierce’s disease in the process. The disease kills grapevines by clogging their water-carrying vessels.

The sharpshooter didn’t bring the disease with it, but rather tapped into plants already infected when it arrived. Pierce’s disease can be traced to an 1880s outbreak in the Los Angeles basin that wiped out 40,000 acres of wine grapes, Morgan said. Since then, the disease had been regarded as a manageable nuisance.

Then the voracious sharpshooter arrived.

“It’s like putting a match and gasoline together,” said Kevin Andrew, chairman of the state agriculture department’s Pierce’s Disease Advisory Task Force.

Once a plant is infected, it becomes a reservoir for the disease, spreading it to other feeding sharpshooters, which in turn pass it on to other plants.

The sharpshooter is creeping northward. Pocket infestations have cropped up in Sacramento County and other Northern California locations; egg clusters were discovered in Napa and Sonoma counties last month.

State agriculture officials established the Pierce’s disease task force in 1999. About $166 million in state and federal funds has been spent in the last five years to pay for research and eradication efforts.

Grape growers have contributed to the effort with a self-imposed annual assessment of up to $3 per $1,000 of crop value, which has generated about $5 million a year for the last four years. This month, growers will vote on whether to continue the fees for another five years.

Aggressive inspections of Southern California nursery plants destined for Northern California have helped curb the sharpshooters’ spread, along with pesticide treatments and statewide release of more than 1 million parasitic wasps of other species.

But the newest hope lies in Anagrus epos.

The hearty predator, which doesn’t have a common name, is hoped to be a more effective combatant than the four previously released species because of its ability to survive winters -- it’s from Minnesota, after all -- and because it can produce more than 10 offspring per sharpshooter egg.

Sensitive receptors on its antennae help the wasp find sharpshooter egg clusters. The wasp lays its tiny eggs inside the larger eggs of the sharpshooter. When the young wasps develop inside the sharpshooter egg, they kill and eat the developing sharpshooter nymph inside the host egg.

The wasps mate almost immediately after emerging from the eggs and disperse to look for more sharpshooter eggs to infest. The hope is that the wasp will help keep sharpshooter populations relatively low and slow their spread.

“It’s not an eradication measure,” Van Rein said. “It’s more like preventive medicine.”

The wasp has been closely studied for the last year while in quarantine at UC Riverside. It isn’t expected to harm any native insects or plants or pose unpredictable environmental consequences, Morgan said.

Morgan and others expect the Department of Agriculture to approve the use of the wasp. If approved, the first release would be on a small scale, about 50 to 100 insects, at sites in Ventura, Kern and Riverside counties this week. A release of about 1,000 insects would follow in two weeks.

Wasps are a preferred means of biological control because they tend to subsist on one particular species of animal, Morgan said.

Other parasitic wasps that have been introduced in California have battled the ash white fly that was destroying pomegranate trees and broccoli crops, the giant white fly that was infesting hibiscus plants and fig trees, and longhorn borer beetles that were attacking eucalyptus trees.

Researchers hope the Minnesota natives will soon join the list.

“The big question is are they going to be able survive the heat of Southern California,” Morgan said. “Nothing’s ever sure in nature or science.”