U.S. Cattle Industry Concerned
The USDA announced on September 21, 2025 that a case of New World Screwworm had been detected in Nuevo León, Mexico, a mere 70 miles from the U.S. border. While there are still no cases in the US, that’s some of the worst news that biocontrol advocates have heard in a long time.
Endemic to central America, Mexico, and the southern United States, screwworm was the bane of cattle producers throughout much of American history. This nasty parasitic fly lays its eggs in the wounds, ears, noses, and eyes of cattle and other mammals and birds. Ranchers used to have to routinely inspect their animals for wounds, pulling out the maggots by hand and applying medicinal salves. Left untreated, animals infected by screwworm eventually die, having been eaten alive by hundreds of the wormy pests.
By the end of the 1930s, scientists had thoroughly studied the lifestyle of the screwworm, and an entomologist named Edward F. Knipling came up with a theory that the pest could be controlled by releasing huge numbers of sterile males into the environment. Females who mated with sterile males would lay infertile eggs, which wouldn’t hatch.
In the 1950s, scientists developed an effective way to sterilize male flies without otherwise injuring them—exposing them to radioactive cobalt for 13 minutes. They began releasing millions of sterile male flies in Florida, and by 1959 screwworm had been eliminated in the southeastern United States.
Eradicating the pest from the Southwest proved to be harder, because flies don’t respect international borders and kept coming up from Mexico. This led to international cooperation with the Mexican and Central American governments to eradicate screwworm in their countries as well. By 2000, screwworm had been eradicated everywhere north of the Darien Gap between Columbia and Panama.
The world’s last remaining sterile male production plant, located in Panama, successfully kept screwworms from migrating back north from Columbia for 20 years. Unfortunately, government restrictions and supply chain issues during the COVID-19 pandemic disrupted the cattle inspection and fly release program, which is how some flies snuck through and started heading back north.
Seventy years after the development of the sterile male technique, it’s still the only known way to combat screwworm. That’s why the U.S. is partnering with Mexico to release sterile flies and is building a new sterile fly production facility in Texas, which is supposed to be complete by the end of 2025.
Eco-ag farmers aren’t immune to screwworm; it was historically the scourge of all cattle farmers in the southern U.S. Like most parasites, screwworm attacks are worst on unhealthy animals, and careful monitoring and care by the farmer can help keep infestations from getting out of control. But everyone in the Southwest who has cattle should support the international screwworm eradication campaign, which has long been considered one of biological control’s best success stories.
