Alexandra, a 55-year-old undocumented immigrant, was on her way to work at a watermelon farm in the border city of Edinburg, Texas, recently when her oldest son stopped her before she stepped out of her aging trailer.

“Please don’t go. You are going to get deported,” he told Alexandra, who asked that her last name not be used because she did not want to attract attention from federal immigration agents. Her son then showed her graphic videos of federal agents chasing and handcuffing migrants seemingly all over the Rio Grande Valley of Texas. “That could be you,” he said.

President Trump’s conflicting orders to exempt, then target, then again exempt farm workers from his aggressive immigration sweeps of work sites have caused havoc in agricultural industries across the country, where about 42 percent of farm workers are undocumented, according to the Agriculture Department.

But perhaps nowhere is fear among farm workers more palpable than on the farms and ranches along the southwestern U.S.-Mexico border, where for centuries workers have considered the frontier as being more porous than prohibitive.

Administration officials have vowed to make good on a once-popular campaign promise from Mr. Trump to deport millions of undocumented workers, in what he has said will be the largest mass deportation in U.S. history.

As workplace raids have eroded that popularity and sparked angry protests across the country, the border region has been eerily quiet.