There is a reason that American teenagers aren't working in orchards... if growers paid enough to get teens to take the jobs, nobody would be able to afford fruit.
No. The reason is that the laws (child labor, working conditions) make it impossible for them to use teenagers any more.
Meanwhile the illegals can't complain about working conditions - and will work for less than minimum wage in (those occupations where it applies.)
US citizens needn't apply because they can't compete. (Even if they were willing to work for sub-legal prices and/or in sub-legal conditions, the employer can't risk that they might turn around and demand the missing money or compensation for the conditions.) The illegals, meanwhile, can afford to work that cheaply because social programs can pay for much of the support of them and their families - turning programs intended to help the poor into subsidies for their employers.
Meanwhile, the government's non-enforcement of the laws against the illegals working means that, in highly competitive markets (such as construction contracting), employers are left with a Hobson's choice: Use illegal labor and be competitive, or try to use legal labor and go out of business.