The population of American fields by a foreign workforce is nothing new. Between the 17th and 19th centuries tens of thousands of African slaves were forced into labor, followed by immigrants from Europe and Asia who found themselves in positions of indentured servitude. It is currently estimated that Mexicans make up about half of all unauthorized immigrants.
Is it this immigrant workforce that is preventing Americans from gaining employment? When the United Farm Workers union in America launched a campaign offering to connect unemployed people to farm jobs, of a thousand inquiries only three people accepted positions. This was the reaction during a period when unemployment was hovering around 10%. [link]