This is a complete WAG, but if I were to take a stab in the dark, I suspect the makeup of Wisconsin's agriculture industry is rather different than California's.
It looks like Wisconsin's industry is geared largely toward livestock and dairy, with fruit and vegetable crops largely composed of things like cranberries, corn, and potatoes:
It's my understanding that migrant workers in CA are largely employed picking fruits, vegetables, etc.
Again, I'm by no means speaking authoritatively here, but there's certainly more to this than simply comparing the dollar-value of each state's combined agricultural output.
There is no doubt the makeup of Wisconsin's agriculture industry is different. The climate alone is very different between these two states, not counting other things like availability of irrigated land, energy, transportation etc.
Edit: I was just trying to make a point that being in the country illegally does not seem like a necessary condition of food production and WI is a fine example of it.
I also have no doubt that if WI had more illegal immigrants available they could be employed in the agriculture just as much as they are in CA. It just happens the closest border to WI is with Canada and Canadians don't seem to be very enthusiastic about working illegally in the America's Dairyland.
Well, part of what I was getting at is that it appears, from my layman's point of view, that WI's agriculture business is geared more toward crops that are easily industrialized and would require fewer workers in general.
e.g., a single worker operating a piece of machinery can harvest hundreds of acres of corn. A tomato, on the other hand, has to be hand-picked so that the fruit isn't damaged in the process. That requires substantially more workers.
It looks like Wisconsin's industry is geared largely toward livestock and dairy, with fruit and vegetable crops largely composed of things like cranberries, corn, and potatoes:
http://www.aae.wisc.edu/pubs/status/docs/status2012.pdf
California, on the other hand, apparently produces "nearly half of US-grown fruits, nuts and vegetables."
http://www.cdfa.ca.gov/statistics/
It's my understanding that migrant workers in CA are largely employed picking fruits, vegetables, etc.
Again, I'm by no means speaking authoritatively here, but there's certainly more to this than simply comparing the dollar-value of each state's combined agricultural output.