Not sure about screwworm being imported, it’s been in the area for a long time.
But... there’s another problem called “Pasture Dieback”, caused by an insect called the Pasture Mealybug. I have talked with USDA and Texas Ag folks that agree (in private) that the mealybug may have been imported as agri-warfare. It’s widespread in Australia, but until a few years ago was unheard of in America.
https://agrilifeextension.tamu.edu/pasture-mealybug/
All it would take is a few eggs or larvae dropped along teh highways or from a small plane............