The Times has an article a a California farmer moving to Mexico because of problems getting sufficient labor. He's a big operator, lots of acres, lots of employees. This may be the odd case, or it may be part of a trend. I suppose there's not much difference in travel cost or time between shipping lettuce from California or Arizona and Mexico, particularly with the border becoming more open to trucking.
A couple things struck me--instead of paying $9 an hour he's now paying $12 a day. He claims to be following the same sanitary procedures as he would in the States, and I suspect it's to his self-interest to do so. The other thing--his workers don't work as productively (i.e., hard). I find that interesting. I think it's part of the advantage of emigrating, at least for work. You leave lots of distractions behind and you've put yourself at risk, so you work harder.