The U.S. Senate is debating a revision to the nation's immigration laws, including a measure -- backed by the White House -- that would create the largest guest worker program in nearly half a century. All but the smallest farms depend on immigrants, legal and illegal? so American farmers support the controversial measure. They say without enough people to work the fields, the nation's abundant - and inexpensive - food supply is threatened.

It's a chilly morning in an orange grove just outside the tiny town of Yettem, in the shadow of the Sierra Mountains. Workers climb on tall metal ladders to pick navel oranges and load them into heavy canvas bags strapped to their shoulders.

Jon Stearns, a manager for a nearby packinghouse, says these crews are stretched thin this season, working 6 and 7 days a week so fruit doesn't rot on the trees. "Normally we have 40-50 people at this time of year," he says, "and I think our biggest crew is only about 20 or 25 people right now. So it's tough, it's real tough."

And farmers say it will get tougher as peaches and grapes ripen and the competition for workers heats up. It's gotten so fierce that workers use their cell phones to call friends working in nearby fields, and sometimes leave if they hear about a better-paying job up the road.

Economist Phil Martin at the University of California, Davis, says farmers are feeling the squeeze because of increased border enforcement. Farm workers who used to cross back and forth to Mexico each season are now building more permanent lives in the United States. "The total number of people coming illegally hasn't changed very much. By the same token, you make it harder to cross the border, and once you get in - you have a different time horizon, you're going to stay longer." Professor Martin says that makes workers more likely to seek and find non-farm jobs.

Those jobs are often in construction, where workers can sometimes earn double what they made in the fields. Margarito, 22, arrived from Mexico last year. He spent just one season picking grapes before picking up a hammer instead. "It's more stable," he explains. "In the fields, when it rains, there's no work. Sometimes you go a month or two without work. Here, there's work all the time."

But there's still work to be had in the fields and fewer hands to pick the crops. Labor contractor Steve Scaroni's crews harvest lettuce for some of the largest bagged salad producers in the United States. He wants politicians to know a farm worker labor shortage could spell a major crisis for American agriculture. "Our politicians are so disconnected from the reality of who's doing the work out here. I'm talking about who's picking the lettuce, whose washing the dishes in the fancy hotels our politicians eat and sleep in. This is done by entry-level low skill immigrants."

Scaroni says Americans don't want those jobs, and if he can't find workers to pick the crops in California, then the crops will have to go to the workers. He moved his operation to lettuce fields in Mexico this week, part of the hundreds of hectares of farmland California farmers are harvesting south of the border.

In the meantime, some farmers are plowing under their grapes and planting almonds, which require less labor. Other farmers are looking elsewhere for workers. Supervisors in the Yettem orange grove are bringing in 40 workers from Thailand this spring to pick fruit as part of a special temporary pilot program.

But a Mexican farm worker named Rodolfo says he knows of only one way to ensure that the workers already here stay in the fields. "If they want more people, they need to pay people more. If they don't pay enough, then people will go where they'll make more."

But farmers say raising wages would put the price of an orange out of reach for most American consumer - who are used to a cheap, plentiful food supply.