The US Department of Agriculture (USDA) has announced that it will release sterile screw-worm flies with fluorescent markers in northern Mexico and southern Texas border areas to prevent the spread of the New World Screwworm (NWS) insect pest from Mexico to the United States.
Screw-worm flies are a type of parasitic fly, similar in size to houseflies or slightly larger, with a metallic green or blue body color, orange eyes, and three dark stripes on their back. Female flies lay eggs in wounds of warm-blooded animals, usually livestock. After hatching, large numbers of larvae burrow into living tissue with sharp mouthparts. Without timely treatment, this could lead to the death of the host.
While the US successfully eradicated screw-worm flies in the 1960s, the pest has recently spread northward from Central and South America.
In response to this initiative, entomologist Sonja Swiger from Texas A&M University stated on Monday, February 2, that “to protect Texas and even the whole United States, I believe this is absolutely necessary.”
In the past, USDA has mainly released sterile screw-worm flies in Mexico to suppress pests, but experts believe the scale is still insufficient. The recent action originates from Tamaulipas, a state bordering southern Texas, following 20 cases of animals infected with screw-worm flies at the end of 2025.
Swiger stated that although screw-worm flies have limited flying distances, they can attach to hosts and travel longer distances inside them. To halt the spread, an increased release of sterilized flies is necessary to prevent further infestation.
The USDA announced at the end of last month that it would invest $100 million to increase the production of sterile flies and related control programs, with plans to establish a new factory in Texas.
The strategy involves releasing a large population of sterilized male flies near the affected areas. If wild female flies mate with the sterile males, the eggs laid will not hatch. This interrupts the reproductive cycle and helps control the pest population.
As the New World Screwworm continues to spread north from Mexico, approaching the southern border of the United States, Texas declared a state of disaster on January 29 to prevent their invasion. Governor Greg Abbott announced collaboration with USDA to establish a $750 million “Sterile New World Screwworm Production Facility” near Edinburg, Texas.
According to USDA’s statement at the end of January, the release of sterile flies will be expanded northward along the border of Tamaulipas, establishing a “buffer zone” extending about 50 miles into Texas.
The Mexican government expressed agreement with the US decision to release sterile flies along the border, aligning with solutions proposed since November 2025.
To aid animal health officials in distinguishing between wild and released flies, the sterile flies will be dyed during the pupa stage. The dye, visible under UV light, will transfer to the sterile adult flies after the larvae hatch, enabling visual identification.
USDA plans to continue deploying intensive measures, including implementing sterile fly import agreements, border surveillance trapping, and investing in national wildlife conservation programs to address the harm caused by screw-worm infestations.
Authorities advise immediate medical attention if suspicious lesions are discovered on the body or if there is a concern about screw-worm infection, despite screw-worm infestations being uncommon in humans.
In the 20th century, the US successfully eradicated wild screw-worm populations by aerially releasing large numbers of sterile flies. However, experts warn that the increasing cases in Tamaulipas suggest the infestations may have established, with pests potentially spreading when livestock is moved from infested areas to new regions.
Furthermore, the US government has ordered a temporary suspension of livestock imports from Mexico to prevent the spread of these pests. This measure may exacerbate the shortage of US beef supply, leading to increased beef prices, impacting some beef suppliers.
Devin Cole, the Chief Operating Officer of Tyson Foods, stated during a financial conference call on February 2 that due to the tight supply of cattle raising costs, the beef business of the company is facing severe losses. He mentioned, “Currently, we have no information on when the government will reopen the border.”
(Adapted from Reuters reporting)