All are not working --- the overall welfare program use is 35-42% depending on where you get your statistics, and many of the working brought in children and elderly who aren't working. There are only about 2 million jobs in agriculture. That doesn't really provide them that many jobs.
"Number of people age 16 and over in the nation's labor force as of July 2003. Among the nation's workers are 78.2 million men and 68.4 million women.These men and women represent 66 percent of the adult population. http://www.bls.gov"
Even if your statistic is correct, the percentage of Mexican nationals and Americans is quite closs. If one corrects for illegal aliens being employed but not paying payroll taxes (no social security number), most likely, the percentage of Mexican illegals working would exceed the percentage of Americans.