google, Center for Immigration Studies lettuce. There are discussions about the after effects of doing away with the Bracero guest worker program. For example the price of tomatoes went down as machines took over. I've watched those machines load trucks as conveyor belts full of tomatoes transport tomatoes from rows of plants directly into the truck. One machine operator, one truck driver. Twenty-four hours a day around Sacramento (Sack-of-tomatoes).
CIS, et al studies show that lettuce could increase by five cents a head. There may still be hundreds of farm workers around Salinas like there were when I was there. But maybe they have machines to harvest the lettuce now. I don't know.
I do know that I've driven through farm country almost daily around Sack-of-tomatoes all this year. I've seen lots of weird machines and very few farm workers in the fields -- and many of them were women.