Bayourod, if the illegals weren't in Houston or Phoenix or Chicago then just as in upstate NY the citizens would do the jobs as they always have. The illegals are driving down wages and benefits, that's why employers prefer them over Americans. You don't need a Master's degree to figure it out.
I'm familiar with the theory of supply and demand, but I reject the idea that illegals are "driving down wages". I view them as relieving pressures that force wages up beyond that of a normal market.
Remember that the reason we originally set limits on immigration, and the reason they are so low today is because the labor unions dominate the Democrat Party. Unions use both the federal and state governments to artificially create shortages of labor in order to extort higher wages. The results are rust belt states and declining industrial sectors such as steel.