By Cassandra Garrison TAPACHULA, Mexico (Reuters) -On a scorching afternoon in the rural heartland outside Mexico's southern city of Tapachula near the Guatemala border, rancher Julio Herrera calls his herd back from an afternoon of grazing.

Freddy Nieto is the manager at El Sauz Ranch in South Texas, which runs cattle but also offers deep-pocketed clientele the opportunity to hunt wildlife, from whitetail deer and wild hogs to exotic animals. “This might be the worst biological outbreak that we're facing in our lifetime,” he said.

The U.S. plans to build a $750 million factory in southern Texas to breed billions of sterile flies to ramp up its efforts to keep flesh-eating maggots now in Mexico from damaging the American cattle