The US Department of Agriculture will disperse glow-in-the dark, sterile flies in Mexico, closer to the US border, and in southern Texas, as officials race to keep flesh-eating New World screwworm pests from spreading in Mexico, the agency said.

Screwworms are parasitic flies whose females lay eggs in wounds on warm-blooded animals, often livestock. Once the eggs hatch, hundreds of screwworm larvae use their sharp mouths to burrow through living flesh, eventually killing their host if left untreated.

The USDA has halted US imports of Mexican livestock to keep out the pest, worsening a cattle shortage that has pushed beef prices to record highs for consumers.

The agency also produces 100 million sterile flies per week at a facility in Panama and disperses them in Mexico to prevent wild screwworm flies from reproducing. Now, the agency says, it is going to take the same flies further north near the border.

The shift came after the USDA has reported 20 screwworm infestations since December 26 in animals in the Mexican state of Tamaulipas, which borders southern Texas.

“Given that we need to protect Texas and the US, I think it’s definitely something that had to be done,” Sonja Swiger, an entomologist at Texas A&M University, said on Monday.

More sterile flies needed

Experts said the USDA needs well more than 100 million sterile flies per week to eradicate the pest in Mexico.

The agency announced last year that it intended to build a production facility in Texas, but that could still be at least a year away from opening. Last month, the agency said it would spend up to $100 million on other projects that aim to boost sterile fly production and help fight screwworm.

For now, the USDA will start releasing sterile flies north of where Mexico has reported active cases to attempt to create a “buffer zone” to halt the pest if it continues moving north, according to a statement issued late on Friday. The new dispersal area will include operations about 50 miles into Texas, along the US border with the state of Tamaulipas, according to the USDA.

The northernmost active case in Mexico was about 200 miles away from the US border, and cases have continued to spread in Tamaulipas and further south in Mexico, said Dudley Hoskins, a USDA under secretary.

A hospital in Spain’s Barcelona said Monday that it had performed a pioneering facial transplant.

“Our highest priority is protecting the United States from screwworm,” he said in the agency’s statement.

Glowing flies

The USDA said it would apply fluorescent dye to sterile flies before they hatch so officials can distinguish them from wild flies that pose a threat. The sterile flies will glow under ultraviolet light and may also be visible to the naked eye, the agency said.

The US decision to direct the release of sterile flies toward the border region was consistent with technical proposals Mexico has been making since November 2025, Mexico’s government said in a statement.

The US eliminated screwworms in the 20th century by flying planes over hotspots to drop boxes packed with sterile flies.

“They’re pretty effective as long as you can outnumber the population,” said Max Scott, an entomology professor at North Carolina State University.

Screwworm can be detected in new areas when livestock are transported from an infested region. However, rising cases in Tamaulipas signal a local population of flies has likely started to establish there, experts said.

Unseasonably cold weather in Texas could temporarily help prevent the pest’s movement, Tyson Foods COO Devin Cole said on an earnings call. The meatpacker’s beef business has bled money as tight cattle supplies have raised costs.

“We don’t really have anything that would give us any insight as to when the government would open the border,” Cole said.