A sizable proportion (half?) of the seasonal agricultural workers in America are undocumented immigrants and have been for 2 generations.
It used to be that they would migrate seasonally and go back home during the off season. But since the 1980s it has gotten increasingly expensive to cross the border, which means that migrant workers must stay in the US longer to pay for the crossing, which means that during the off-season those migrants find other jobs (in factories, restaurants, as janitors, gardening, ....) which they then keep instead of going back to agricultural work the next season. Agriculture then needs to recruit a fresh batch of immigrant workers every season.
Ironically, making the border harder to cross thus works as a kind of ratchet which prevents seasonal workers from moving back and forth, and ends up increasing the total number of undocumented immigrants in the country.