The heart of Napa's grape growing region was placed under quarantine Tuesday to stop the spread of the European grapevine moth.

The quarantine boundaries extend to a sliver of Sonoma County that has a small olive orchard and almost 50 acres of grapes.

The state's action, which was expected, is likely to expand because more moths already have shown up in traps in Napa County. State rules call for all lands within three miles of an infestation to fall under quarantine.

"There's going to be more growers affected when they figure out the boundaries," said Sonoma County Agricultural Commissioner Cathy Neville.

Farmers and agriculture officials consider the European grapevine moth a serious threat to the state's grape industry. The moth was first detected in the U.S. last September near Oakville, where the pest destroyed the grapes in one vineyard at harvest.

The larger infestation extends from St. Helena to Napa and in some places spreads more than 10 miles from east to west across the valley.

Another infestation area east of Napa also also came under quarantine Tuesday, including a small portion of Solano County.

The two quarantine areas encompass 162 square miles. The sliver of Sonoma County is located near Trinity Road east of Glen Ellen.

Farmers in the quarantine areas still can move crops to market, but they must demonstrate that they aren't helping the moth expand outside the boundaries.

"Grapes are our state's top crop," said A.G. Kawamura, secretary of the California Department of Food and Agriculture. "This quarantine will help us ensure that the infestation doesn't have an opportunity to spread."

Agricultural inspectors have set out traps around the state in an effort to determine if the pest exists in other counties. Sonoma County has hired 10 workers and is placing about 4,000 traps.

The state is putting out 50,000 traps statewide. To date no moth has been found outside Napa County, said Steve Lyle, a spokesman for the agriculture department.

The trapping effort is aimed at understanding the scope of the moth infestation so that state and federal scientist can devise the best plan for eradicating it, said Dave Whitmer, Napa County's agricultural commissioner.

Experts familiar with the moth already have urged farmers and agriculture officials to do everything possible to keep the pest from becoming established.

"I think," said Whitmer, "they're saying, &‘This is a bad one folks.'"