A flesh-eating parasitic fly has returned to the United States after 60 years. The US Department of Agriculture confirmed the presence of New World screwworms in a calf in southern Texas. To combat this threat, officials are deploying sterile male screwworm flies, which mate with wild females and produce no offspring.

In the 1930s and 1940s, researchers found that sterilising male flies could break the life cycle of New World screwworms. By 2006, this technique had successfully eradicated the pest in Panama. Now, with the threat looming over South Texas, officials are releasing around 4 million sterile flies per week from trucks and air-dropping an additional 100 million each week to focus on areas along the US-Mexico border.

Despite these efforts, the USDA needs around 400 million flies per week to eliminate the screwworm. Currently, they can produce only about 100 million at a facility in Panama and are renovating an existing fruit fly facility in Mexico that will be operational by summer.

The agency is also constructing a new sterile fly facility near the border in Texas which won't be ready until November 2027. While humans might not be directly affected, there have been at least 2,070 human cases of screwworm in Mexico and Central America since 2023.