First off, this is a very incorrect broad brush. In many areas, the folks doing these jobs are VERY well paid regardless of their origin.
However, let's take your assertion at face value and say that these jobs can be filled with higher wage Americans. Where do those higher wages come from? And we arent talking marginally higher wages according to you...we are talking huge (again, according to you) leaps in wage. Won't the contractors and farmers have to dramatically raise the cost of their goods and services?
Furthermore, since unemployment is pretty danged low, what sector will these folks move from and what will those employers do for labor? It would seem they would have to also raise wages substantially to prevent loss of workforce.
Then all the other consumers must pay for these increased costs because they might be able to avoid construction and landscaping but they surely can't avoid food.
Isn't this precisely the kind of inflationary argument routinely made against large mandatory increases in minimum wage?