FRESNO, Calif. -- At first, Larry Ladd just let it go. But after the farmer caught six thieves plundering his walnut orchard in less than a day, he knew he had a problem.

He's not the only one.

As prices for almonds and walnuts rise with demand, a growing black market has emboldened nutnappers to cut holes in fences, sneak into distribution centers and drive off with truckloads of nuts.

"At first, I'd just ask the deputy to impress upon them that this is the wrong thing to do," said Ladd. "But then it got ridiculous."

California farmers like Ladd have reason to be vigilant: Growers here produce about 80 percent of the world's almonds and 99 percent of the nation's walnuts.

The state's walnuts earned nearly $4.4 million in 2004, according to the latest figures available from the California Farm Bureau Federation.

Savvy scoundrels target both walnuts and the more valuable almond.

Last month, a Fresno County task force that tackles rural crimes recovered 44,000 pounds of processed almonds taken from a distribution center.

The recovery was a rare break in a series of thefts that has cost California farmers at least $1.5 million in stolen almonds this year, according to the Agricultural Crime Technology Information and Operations Network.

There are typically one or two almond thefts every fall, but in the past 18 months, there have been at least a dozen reports of larger looting, said Marsha Venable, spokeswoman for the Almond Board of California, a marketing group.

Almonds sell for about $3 a pound wholesale and twice that much in stores. The nuts are worth so much that thieves who drive away whole truckloads have been known to abandon the vehicles and keep just the almonds.

"Almonds have become such a huge crop," Venable said. "It's attracting anyone."

Acreage devoted to almonds has increased 13 percent over the last five years as growers abandon other crops. Almonds were a $2.2 billion commodity in California in 2004, according to the farm bureau.

This year, growers expect to harvest just over 1 billion pounds of almonds, a figure that's expected to rise to 1.5 billion pounds over the next four years.

Farmers, processors and trucking centers have added cameras and security guards, though monitoring grounds can be difficult and expensive.

"My farm's right on a busy road. I can't install motion detectors and I can't afford to pay someone to watch the grounds around the clock," Ladd said.

The recent nut heists appear well-organized and sophisticated.

The criminals use computers to track shipments and seem to know the market. Fall is the time when demand and prices are high and supply is still low before the remaining harvest, said deputy Royjindar Singh, a spokesman for the Stanislaus County Sheriff's Department.

"It has to be someone who knows the market well and has a buyer lined up," he said.

About 70 percent of almonds grown in California go to other countries, and that's where rural crime investigators think thieves are selling them. Paperwork and a well-linked industry in the United States would make domestic sales of big stolen almond loads difficult, Singh said.

Farmers have kept each other abreast of the thefts by sending e-mails and faxes about each case.

"Even though we're competitors, we talk," said Jeannine Campos, a spokeswoman for Campos Brothers Farm, a family-owned company that has grown almonds in Fresno County since 1981.

Authorities arrested six walnut thieves who raided Ladd's farm but later released them without charges. No arrests have been made in the almond thefts.