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Fifty years ago most of the illegals might have been working as migrants, but it is just a pimple on the real problem today.
Twenty million illegals aren't working on farms. They are in virtually every city of the country, doing jobs most people wouldn't believe. And the companies practice a kind of “don't ask don't tell” using beards to keep themselves safe.
We have a large company here that has nothing but illegal groundskeepers. Of course they have a “policy” that they will only do business with companies that “prove” their employees are legit. Of course, nothing is said about their “subcontractors.”
It's going on everywhere and in every occupation. And having relatives that are farmers, I would hate to see the farmer caught up in this. But the fact remains - until employers stop looking for illegals, illegals won't stop showing up.