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.

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