Napa Valley’s newest grapevine pest is on the march.

The number of adult European Grapevine Moths found in Napa County has risen to eight since it was first spotted last month. Agricultural officials have found 29 larval specimens, as well.

The moth has been trapped on Third Avenue, east of Napa and on Zinfandel Lane south of St. Helena, but primarily the pest has been trapped in the Oakville-Rutherford area. It is estimated the pest is confined to about a 200-acre area.

Greg Clark, Napa County’s assistant agricultural commissioner, offered an update on the moth before about 75 grapegrowers and vineyard managers at a meeting Tuesday.

“We see this as an ongoing pest issue,” Clark said.

A previously unidentified moth found in 2008 has recently been confirmed as being a European Grapevine Moth. It was trapped on Money Road near Oakville, Clark said.

The European moth bores into the grape berry, causing botrytis — a fungus that can ruin grape berries. It is a different pest than the Australian Light Brown Apple Moth detected in Napa and elsewhere around the state in recent years.

The first European Grapevine Moth was detected on Sept. 29 in the Oakville area. Since then, 248 traps have been set out, or about five traps per square mile.

Those traps were pulled in late October, because the pest is going into its dormant stage for the winter. New traps will be set out in the spring.

The life cycle of the moth starts in May with the first generation, followed by a second in mid July and August and the third generation is in September.

“There is no regulatory program in place for dealing with this pest, since it has previously never been found in the United States. We are working with the United States Department of Agriculture to determine where exactly this pest is. We will be ready for it in the spring,” Clark said.

County agricultural officials and the U.S. Department of Agriculture are also working to try and figure out how the European Grapevine Moth landed here.

Clark added that a technical working group is being formed that will include experts from around the world. He encouraged growers to get unwanted fruit off the vines so the moth can’t take advantage of the decaying fruit clusters. The unwanted clusters should then be incorporated into the soil, he said.

To help growers identify the moth, Lucia Varela, with University of California Cooperative Extension in Sonoma County, said the moth is about a quarter-inch long and the female is always slightly larger than the male. Varela described its markings as brown, tannish and black.

She said the moth prefers temperatures in the mid-80s and nighttime temperatures in the low 50s.

The European Grapevine Moth has been found in Chile, Italy, Germany, France, Spain, northern Africa, Israel and southern Russia, according to Varela.