It looks like that data comes from the Bureau of Labor Statistics. I'm going to go ahead and guess that they don't include undocumented workers.

A lot of those construction jobs are probably only looking at union work. The author says as much below. I don't know what it's like for electricians or carpenters, but the concrete business has a lot of undocumented workers, especially on non-government jobs. And even when they are somewhat legit they don't typically join the unions or get tagged as 'cement masons' (finishers) even though a lot of the time they do finishing work as well as the general labor.

Who holds that theory? As a farmer myself, I'm not really surprised at all and it was my immediate thought upon reading the headline. Most farm work is highly skilled, which: a) pushes for higher pay, allowing locals to compete. b) requires skills that will often not be easily obtained by those coming from poorer countries. The jobs that are easily handed to migrant workers are a relatively small segment of the industry, and I would expect a dwindling segment at that.

I'll add that it is striking how non-diverse rural areas tend to be. I can probably count on one hand the number of non-white people living in my rural community. That doesn't help with the numbers when hiring from the local worker pool.

if they're illegal, they pertinent aren't counted in the BLS statistics.

They may count hispanics as white.