The USDA said the released flies will be marked with a fluorescent dye so that specialists can distinguish them from wild insects during surveillance and when evaluating the effectiveness of the program. The agency currently produces around 100 million sterile flies per week at a facility in Panama, but experts say this number may be insufficient to eradicate the parasite in Mexico and to halt its northward advance.
The USDA’s decision is directly linked to about 20 confirmed cases of screwworm infestation in animals in the Mexican state of Tamaulipas, located roughly 200 miles (about 320 kilometers) from the Texas border. The parasite is known for laying its eggs in open wounds of mammals; the larvae then feed on living tissue, causing painful and extensive injuries, weakness, and—if left untreated—even death in affected livestock.
The USDA has already suspended imports of live cattle from Mexico in an effort to reduce the risk of introducing the parasite into the United States. These measures have contributed to tighter cattle supplies in the U.S., which in turn have helped keep beef prices high on the American market. The fluorescent flies and other control measures are intended to create a “buffer zone” in southern Texas to slow and ultimately stop the parasite’s movement northward.
According to the latest epidemiological data, the screwworm is not currently present in wild fly populations in the United States, and no local livestock cases have been confirmed within the country, the CDC says. However, active outbreaks continue in Mexico and Central America, with infestations reported in both animals and humans. To date, the region has recorded hundreds of thousands of animal infections and more than a thousand human cases linked to the screwworm outbreak that began in 2023 and has spread across multiple countries.
The parasite was previously eradicated in the United States in the 1960s through the release of sterile flies, a technique that successfully eliminated local populations. However, shifts in its geographic range and the weakening of biological barriers now mean that the risk of its return is real—especially in southern, cattle-producing states. The USDA is working with Mexican authorities and regional organizations, including through large-scale sterile fly release programs, to contain and stop the spread of the parasite.