Half of Sonoma County's vineyards soon could come under quarantine due to the European grapevine moth, a pest that infests the Napa Valley and has now been found in six other counties, most recently Merced.

State officials are leaning toward a single quarantine area for Sonoma County that extends from north of Healdsburg to the Carneros region south of Sonoma.

The exact boundary lines have yet to be spelled out, but "it's pretty much one large quarantine area" rather than a series of smaller ones, county Agricultural Commissioner Cathy Neville said Wednesday.

The quarantine could take in about 30,000 acres of grapes, about half the county's vineyard land, said Stefan Parnay, chief deputy agricultural commissioner.

Neville and her staff will respond by holding a half-dozen meetings with affected grape growers once the state provides them with proposed boundaries for the quarantine.

"We're talking about hundreds of growers, so we're looking at the best way of contacting them," Parnay said.

The expansion of the quarantine is due to new moths found around Windsor and Healdsburg and to new federal rules.

Those rules, proposed by moth experts advising the U.S. Department of Agriculture, now expand the quarantine boundary from three to five miles in all directions from a moth found in an infested area.

Under the new rules, officials are predicting an overlap of the five-mile radiuses marking moths found around Sonoma, Kenwood, Calistoga, Windsor and Healdsburg.

Growers in the quarantine area can still move their crops to market, but they must do so in ways that keep the pest from spreading. That will mean cleaning tractors and other equipment before leaving a vineyard, as well as making sure grapes don't spill on the way to wineries.

"It's going to mean more work for sure," said Nick Frey, president, Sonoma County Wine Grape Commission. Nonetheless, he said, the number of moths found here "are low enough that we have a chance to eradicate the pest in our county."

Grapevine moths, native to Mediterranean Europe, last year destroyed the crop in one Oakville vineyard and damaged others of Napa Valley growers. The moth larvae burrow into and feed on the berries, often infecting the fruit with botrytis, or bunch rot.

To date, inspectors have trapped 18 moths in Sonoma County, Parnay said Wednesday. In contrast, agriculture inspectors have trapped well over 30,000 moths in Napa County, where the pest was first confirmed in the U.S. last September.

This year the moth has been found and quarantine areas have been proposed in Sonoma, Mendocino, Solano and Fresno counties. On Wednesday a federal spokesman confirmed that three grapevine moths have been found in Merced County, and a quarantine will be established there.

Growers in Napa County are treating with what experts for the University of California characterize as low impact pesticides, including some approved for organic operations. The growers have said the pesticides make it possible to protect their grape crops.

Frey said Sonoma growers soon will begin pesticide treatments in vineyards within 1,000 meters of where a grapevine moth has been trapped.

Larry Hawkins, a spokesman for the U.S. Department of Agriculture, said the advisory experts were asked how big to make the quarantine areas. In the end, they weren't comfortable having a radius smaller than five miles of a trapped moth.