Some academics say warnings of a farm labor debacle are exaggerated. “By and large the most dire predictions don’t come true,” said Philip Martin, an agricultural economist at the University of California, Davis. “There is no doubt that some people can’t count on workers showing up as much as they used to,” he said. “But most of the places that are crying the loudest are exceptional cases.”

But some recent studies suggest that strains on the farm labor supply are real. Steve Levy, an economist at the Center for Continuing Study of the California Economy in Palo Alto, compared unemployed Americans with illegal immigrant workers in the labor market. “The bottom line,” he concluded, “is that most unemployed workers are not available to replace fired unauthorized immigrant workers,” in part because very few of the unemployed are in farm work.

Mr. Scaroni said he started growing in Mexico reluctantly, after seeing risks to his American operations. At peak season his California company, Valley Harvesting and Packing, employs more than a thousand immigrants, and all have filled out the required federal form, known as an I-9, with Social Security numbers and other identity information.

“From my perspective everyone that works for me is legal,” he said. But based on farm labor statistics, he surmises that many of his workers presented false documents.

An impatient man in perpetual motion, Mr. Scaroni marches through his fields shouting orders to Mexican crew leaders in rough Spanish while he negotiates to buy new trucks in Mexico on a walkie-talkie in one hand and to sell produce in the United States on a cellphone in the other.

Frustrated with experts who say that farmers with labor problems should mechanize, he plunges his hands into side-by-side lettuce plants, pulling out one crisp green head and one that is soggy and brown. After his company invested $1 million in research, he said, “We haven’t come up with a way to tell a machine what’s a good head and what’s a bad head.”

He also dismisses arguments that he could attract workers by raising wages, saying Americans do not take the sweaty, seasonal field jobs. “I know beyond a shadow of a doubt that if I did that I would raise my costs and I would not have a legal workforce,” Mr. Scaroni said.