The find in Northern California is the fourth nationwide involving a Medfly, which a poses a serious threat to most of California's agricultural crops.

Agricultural officials in Inland Southern California began sampling cases of the tangerines pulled from store shelves last week. So far, only a couple of dead Medfly larvae were discovered.

"We are still in the process of collecting them," said Dustin Wiley, deputy agricultural commissioner in Riverside County.

He estimated that the fruit fly is a threat to Riverside County crops worth more than $350 million a year.

San Bernardino County has asked markets to put their Spanish tangerines in cold storage until the state tells them what to do.

The Agriculture Department initially suspended imports Nov. 30 after larvae were found in clementines in Maryland and North Carolina.

But following subsequent finds in Louisiana and California, imports of clementines were suspended indefinitely, said Larry Hawkins, spokesman for the U.S. Department of Agriculture.

The small oranges, which are sold in small wooden boxes, also were ordered pulled from store shelves in 17 Southern and Western states, including California, Arizona and Nevada.

Hawkins estimated that 60,000 cases of the suspected fruit have already been seized in California.

The fruit must now be destroyed by prescribed USDA methods or shipped to states not included in the ban.

"It can't be sold in these high-risk states," he said.

Medfly infestations are common in Spain where the tangerines are grown. But the fruit is allowed to enter the United States after undergoing a cold treatment that is designed to kill larvae.

Recent discoveries of live insects, however, has prompted officials to re-examine the effectiveness of cold treatment.

"It would indicate some failure along the line somewhere," said Hawkins. "These treatments are not 100 percent foolproof."

Since the 1980s, California has spent an estimated $300 million to battle the Medfly, which feeds on more than 200 different plants.

Reach Don McAuliffe at (909) 587-3127 or dmcauliffe@pe.com