I have posted the link below numerous times from the Center for Immigration Studies. It is somewhat dated now — May 2013 data.
Are There Really Jobs Americans Wont Do? A detailed look at immigrant and native employment across occupations
http://cis.org/are-there-really-jobs-americans-wont-do
America has plenty of unskilled unemployed citizens that will work. If the price of a head of lettuce or a Big Mac is $5 so be it.