I don't believe enough US citizens will work in farm labor because of the long hours in often brutal heat or cold. I also doubt enough US citizens will move their families multiple times every year in order to follow the crops. In general, I believe if capital is free to cross national borders in pursuit of greater profits, then labor should enjoy the same rights.
You can believe whatever you want, but there is no evidence of that. The only evidence is that they won't do it for the wages illegals will. There is no evidence you can't hire people to do any job as long as the market sets wages. You keep them artificially low by importing illegals to do it costing Americans jobs.
Wages are depressed by imported farm labor, even so, the work itself will likely go undone if left to the domestic market without a drastic rise in wage rate:
"In 1960, half of all the native-born men in the U.S. labor force were high school dropouts eager to take unskilled outdoor jobs in agriculture and construction.
"Today, fewer than 10 percent of the native-born men in the work force lack high school diplomas.
"But the economy still generates plenty of unskilled jobs, and most unskilled immigrants don't displace American workers. They fill niches — not just farmhand, but also chambermaid, busboy and others — that would otherwise go empty. And they support more skilled, more desirable jobs — foremen, accountants, waiters, chefs and more — at the businesses where they work and others in the surrounding community.
"Just raise the wage, you say, and an American would take the job?
"Not necessarily, and very unlikely if it's a farm job.
"Farmers have been trying that — for decades.
"They raise the wage.
"They recruit in inner cities.
"They offer housing and transport and countless other benefits.
"Still, no one shows — or stays on the job, which is outdoors and grueling and must get done, no matter how hot or cold or otherwise unpleasant the weather. And of course, at some point, there are limits to how high a wage a grower or dairy farmer can pay before he is forced out of business by a farmer who produces the same commodity in another country, where the labor actually is cheap."
Without Immigrant Labor, the Economy Would Crumble - NYTimes.com
IMHO, based on the five or six years I spent working in California citrus orchards, domestic labor will never put up with all the negatives of farm labor unless the government subsidizes their pay in some way.