“It didn’t take me six hours to realize I’d made a heck of a mistake,” Mr. Harold said, standing in his onion field on a recent afternoon as a crew of workers from Mexico cut the tops off yellow onions and bagged them.

Six hours was enough, between the 6 a.m. start time and noon lunch break, for the first wave of local workers to quit. Some simply never came back and gave no reason. Twenty-five of them said specifically, according to farm records, that the work was too hard. On the Harold farm, pickers walk the rows alongside a huge harvest vehicle called a mule train, plucking ears of corn and handing them up to workers on the mule who box them and lift the crates, each weighing 45 to 50 pounds.

“It is not an easy job,” said Kerry Mattics, 49, another H-2A farmer here in Olathe, who brought in only a third of his usual Mexican crew of 12 workers for his 50-acre fruit and vegetable farm, then struggled to make it through the season. “It’s outside, so if it’s wet, you’re wet, and if it’s hot you’re hot,” he said.

Still, Mr. Mattics said, he can’t help feeling that people have gotten soft.

“They wanted that $10.50 an hour without doing very much,” he said. “I know people with college degrees, working for the school system and only making 11 bucks.”