From his cramped trailer office along a dusty road lined with grape vines near Selma, Guillermo Zamora hires farmworkers by the dozens and dispatches them across Fresno County to prune and pick crops. He's a Mexican immigrant farmworker turned farm-labor contractor — the go-to guy for laborers who need jobs and growers who need workers.

Most days, he rumbles along country roads in his Chevy Silverado pickup with orange flame decals. The American flag on the dashboard lets people know where his allegiance lies.

"You ain't going to find a Mexican more proud to be an American," said Zamora, a legal U.S. resident, on a recent summer morning. "This is a beautiful country."

The truth is, though, Zamora doesn't hire Americans. Farm labor contractors and most other employers in the Valley's multibillion-dollar agriculture industry rely almost exclusively on immigrants — mostly illegal immigrants.

Employers hire them as long as they have a forged Social Security card and a green card, which can be bought for less than $100 through a vast underground industry of fake-document vendors in the Central Valley.

The system works well for farmers and farmworkers — as well as many restaurants, hotels and construction companies. But many innocent legal residents are hurt because document counterfeiters often hijack their Social Security numbers.

Agriculture employers often say they can't tell whether the cards are real — but hardly any use a voluntary government online program that helps detect fakes. They say that if they did, they would go out of business. Says Zamora: "I'd end up with no people."

The scheme keeps the Central Valley's economy running on a simple, unspoken rule: Don't ask, don't tell. Employees pretend they're legal residents; employers pretend they don't know any better.

"It's a game — a big game," said Joseph Riofrio, a city councilman in the western Fresno County farmworker town of Mendota, where perhaps a third of the residents are illegal immigrants. "But it's a necessary game. If this game doesn't continue, then the fruit isn't picked, the vegetables aren't picked, and the vibrant agriculture industry stops."

Lidia's grandmother didn't want her to cross the border. She had heard too many stories of children dying on the journey. Lidia's mother felt differently. One day when Lidia was 3, her mother took the girl on what she said was a shopping trip. They never came back.

On their first attempt at crossing near Tijuana, a border agent chased them down. They were released the next day. Lidia's mother said they would try a second time. Lidia cried until she fell asleep in her mother's arms.

The next thing Lidia remembers, she and her mom were under a freeway on the other side of the border listening to traffic rushing overhead. A car stopped and honked its horn. They got in.

That's how Lidia got to Selma.

After high school, Lidia needed a job. First, though, she had to get documents that would let her work. She asked her friends what to do. They took her to a woman who sold her a Social Security card and a green card for $120. The cards didn't look real, but she hoped they would do the trick.

A friend told her she should apply at a packinghouse that was known to hire illegal immigrants.

"I asked her if it was OK to apply even though I was undocumented and she said, 'Oh yeah, it's fine because everyone is undocumented there so it won't make a big difference if your papers are strange-looking or something,' " Lidia said.