According to Census data, there are only six civilian occupations in America that are majority immigrant (and even then, just barely) and there are no occupations in America that are majority illegal immigrant. Furthermore, only 5% of illegals even work in agriculture.
http://cis.org/sites/cis.org/files/occupations-5-1.pdf
Native-born Americans of all races are willing to do jobs that are far filthier and more dangerous than picking fruit. When ing coal-mining has an overwhelming majority of native-born employees, there really is no job Americans won't do.