No es it has not been shown that US citizens won't to the heavy and miserable manual labor required to grow and pick our vegetables, fruits, and nuts.  This is BS.  What has been shown is that the law of supply and demand works.  Wages for such work sink when desperate labor is available and US citizens are not desperate.  Without illegals, wages would rise dramatically until American citizens found the work acceptable.  Prices would rise as well.  Exactly the same as will happen when mandatory minimum wages are imposed.