It isn't low cost labor. It is publicly subsidized labor. The fruit farmer is underpaying these people, and throwing their social and large parts of their basic costs on the taxpayer to support in whole.
Traditionally, these jobs were filled by young people, teens not yet on their own who depended on their families for shelter, clothing, education and medical support. Now, these teens have been undercut by social services leeches, who get those supports from the U.S. taxpayer.
But the costs are still there, even if they aren't a line item in the fruit farmer's ledger book.
I live 35 miles from Woodland. The illegals are here, trust me. They don't do agriculture anymore. The latest survey in the local paper indicated only 3% of illegals work in ag. The rest who work are in construction (49%) and services (48%). The illegals have found better wages out of the fields by undercutting U.S. workers in all blue collar job fields. This has nothing to do with a tighter border.
Try driving to central California around Modesto. The ATM machines are in Spanish first - you have to press a key for English. You can't get a job at an auto supply house unless you are bilingual.
This is all about illegals sinking their teeth deeper into the U.S. job market. This has nothing to do about the number of illegals.