Migrant workers are getting free heath care, schooling for their kids, etc. Add all of those to the cost of your cheap head of lettuce, and all of a sudden, it's not so cheap, is it?
That's true. I read today of a robotic tomato picker, which could probably do peppers, eggplant, etc, and with modification most fruits. Introduction of such technology would dry up the market for cheap labor, and with a guest worker program, lead to fewer guest workers. It might not however stop a flood of illegal immigrants, at least not as quickly, since the coyotes would still be telling them there was work (or freebies, take your pick) in America. In the part of the country that I'm currently visiting, the illegals don't do much farm work anyway, that's all done by machines, but they do work in such industries as meat packing. But then so do many formerly inner city blacks.
My basic point is still that a guest worker program would allow much tighter regulation than the free for all we currently have. This could include making the "Guests" pay for their own and their childrens schooling and health care, at least in part. Of course many American in the inner cities don't pay for their health care or even housing, either, and they don't work a lick.
The US border is so long, and in places quite rugged, that it's virtually unblock-able, at least as far as small group of "infiltratiors" is concerned. That includes the northern one of course, which seems to be the preferred route for those from somewhat farther away than Mexico or points south. You know, the sort who may secretly be working to blow you up.
To paraphrase:
Necessity is the mother of invention.
Illegal immigration is the abortionist of invention! ;-)
If the cost of harvesting crops were substantially higher, non-illegals would be willing to work in the industry. When the costs become to high to remain competitive with prices elsewhere, the market would force new methods. New machines, and methods would be implemented, it is the American way.