If citizens and legal residents wanted the jobs, they would have them.
Illegal immigrants don't create the shadow labor market you are crying about, minimum wage law does. The illegals are only here because of the opportunity created by minimum wage... without that, employers could pay legal citizens whatever they wanted, and those citizens would displace illegals and remove the incentive for illegals to come here
What you and other "anti-immigration" idiots don't seem to understand is you can't stop people from migrating from one place to another, especially in search of opportunity. You can't really stop anyone from doing anything. So just decrying illegal immigration without understanding what drives it is stupid. Taking a hard line on illegal immigration is like taking a hard line on drugs- the only way to ACTUALLY stop people from doing drugs is to address the factors that cause people to do them. It's no different with illegal immigration.