On a recent afternoon, many stools stood empty beside a conveyor belt where workers examined a stream of green tomatoes and tossed aside those with holes and cracks at a South Florida packing house.

Paul J. DiMare, one of the country’s leading producers of fresh tomatoes, recalls the days when hundreds of foreign workers lined up to labor in the fields or box the produce at his facility in Florida City. The machines roared for more than 12 hours a day as truckloads of tomatoes rolled in to be washed, inspected and packaged. Now, they run for only four.