Immigrant farm labor is needed and legal immigrants fill that need. Illegal immigrants drag wages down. Cesar Chevez was against illegal immigration because he knew it was bad for legal union farm workers he represented. The USA needs to grant more farm worker visas.
The problem is the visas run out and the immigrant stays and becomes illegal. Once an illegal is in the USA, how do you enforce immigration laws when the federal government has a problem with Arizona asking suspected illegals for papers establishing citizenship ?
Immigrants discover they can work in other low skill better paying jobs like construction, landscaping, home cleaning which then depresses those wages for real American citizens, while at the same time increasing society costs for law enforcement, schools, housing, etc. They stay and have babies that become American citizens, overwhelm our public schools, hospital emergency rooms, apply for and get General Assistance (welfare) and food stamps and subsidized public housing for those 'anchor babies'. Quasi non-profit organizations reward and encourage illegal immigrants and assist them in obtaining tax payer funded benefits.