I see this argument a lot. While it holds truth with the current situation, ultimately its logic has a big flaw. The basic law of supply and demand would create the void, and it would be filled. What I mean is, with the availability of illegal immigrants willing to work for less $, the value of the positions you refer to decrease, creating a pay grade that only illegals would accept. A self-perpetuating problem. Being a business owner, Im sure you'll understand when I say that if the pool of illegals was no longer available, these positions' pay rates would increase until the jobs were filled. People seem to think if the illegals disappeared tomorrow, hand-picked fields would no longer get worked. False thinking. Reality is farm owners, to continue the example, would simply pay what would attract non-illegals. Ultimately the price would be felt in the grocery store, by the consumer. The problem is not that non-illegals are unwilling to do the work illegals are, its that consumers like the cheaper prices of goods. But this is also a self-perpetuating problem, as you and I paying less for those groceries also helps to stifle the economy in the form of less jobs in the job pool, thanks to creating the void for illegals to fill.
Another problem the artificially cheap labor rates illegals create is less automation. Fields that would otherwise be worked by new, automated machinery, is still worked by hand due to the labor rates offered by illegals. Not all farmers would/could automate the work if illegals disappeared, but certainly come of them could. More automated equipment would mean more higher paying jobs building and maintaining these pieces of equipment. And before anyone says it, at least some of the equipment would be built domestically, certainly all maintenance would be local.
The flaw in this logic is, once again, labor rates and benefits (or the lack of which). Illegals do not get insurance (usually), their employers dont have to worry about paying unemployment (etc) nor worry nearly as much about working conditions, etc. If the farmer had to hire legal US citizens, the cost of labor would increase by more than just the $ per hour rate increase it would take to entice legal citizens to work for them. They would also be expected to have sick pay, vacation allotment, insurance, workman's comp, unemployment, etc. All this would, as I said above, eventually be paid by the consumer.
Simple supply and demand situation here guys. Im often amazed how intelligent, educated, even powerful people fall into the flawed thinking of 'Americans wouldnt do the work'. I guess fields were never worked by hand, docks were never worked by Americans, floors didnt sweep themselves and rich people all cleaned up after themselves before the huge upswing of illegal immigrants came into this country...?