US, Mexico open sterile fly plant to fight screwworm threat

The U.S. and Mexico took a major step in the battle against New World screwworm, opening a facility to produce sterile flies that authorities believe is the best method for stopping the flesh-eating parasite from spreading through cattle herds.

The production facility in Metapa, near Mexico's southern border, is expected to produce up to 100 million sterile flies a week, becoming only the second such plant in the Americas. Combined with the roughly 100 million flies produced at an existing facility in Panama, the new capacity comes closer to the 500 million a week figure that helped eradicate the pest from North America decades ago.

"Our countries have beaten this before," U.S. Agriculture Secretary Brooke Rollins said on Saturday at the facility. "We will beat the New World screwworm again, sooner than anyone would have thought."

The opening comes as the screwworm has reemerged as a major threat to the U.S. cattle industry. The parasite is actually a fly whose larvae infest the wounds of warm-blooded animals. The pest has infected thousands of cattle in Mexico since 2024, prompting the U.S. to halt livestock imports from the country.

The first infections in U.S. livestock in roughly half a century were confirmed in Texas earlier this month, creating panic across a cattle industry already struggling with a national herd that is at a 75-year low.

U.S. Ambassador Ronald Johnson on Saturday pledged an additional $84 million to stop the screwworm's spread, and in a statement warned that a major outbreak could cost the U.S. agriculture sector more than $700 million each year.

The Metapa plant, a fruit fly facility the U.S. Department of Agriculture contributed $21.9 million to renovate, is expected to begin producing 30 million sterile pupae a week in July, before ramping up to 60 million in August and then 100 million by November.

The facility will send the pupae in cold containers by plane for distribution in both the U.S. and Mexico. Johnson said the flies will be spread "in the most strategic manner possible, to impact this as quickly as possible and push it back south, out of the United States, out of Mexico," with the goal of eradicating screwworm completely.

"It's very opportune that the Metapa plant is opening," said Phillip Kaufman, the head of entomology at Texas A&M University in College Station, Texas, though it will "take them a few months to get up to full production."

The facility will sterilize screwworm pupae with radiation, and the sterilized flies are then released to mate with wild insects. Since females typically mate only once in their life cycle, the sterile males prevent new flies from being born.

The strategy is believed to be America's best weapon against the deadly parasite, proving effective in the previous outbreak. Still, it will take time to reach the 500 million flies a week needed to mount an effective response. The U.S. is working on its own facility in Texas, but that isn't slated to open until November 2027. Its initial production is aimed at 100 million, later ramping up to the full capacity of 300 million flies a week.

The concern is over how far screwworm will spread before the 500 million goal is reached. In the meantime, the U.S. is relying on measures including quarantines and the distribution of medications, while asking ranchers to closely monitor their animals.

The U.S. screwworm outbreak is the first in the nation's livestock since infestations in the 1960s and 1970s caused significant damage to herds. Roughly 26 infections have been detected in the U.S. over the past month, including cases involving sheep and goats, and even a dog. While the spread has been gradual, the 185,000 cases reported in animals across Mexico and Central America as of June 22 have led to concerns of a wider outbreak.

The efforts to combat infections have emerged as Mexico has sent record exports to the U.S., with the countries heading to a deadline in early July to renew the deal that governs free trade across North America. President Donald Trump has criticized it, and suggested his administration would not approve a 16-year extension, though the pact will remain in effect as the countries continue to negotiate.

Mexico's President Claudia Sheinbaum said on Saturday that the facility was a sign of how the countries could collaborate on technical issues, while respecting one another.

"Mexico and the U.S. share one of the most dynamic borders in the world," she said. "Cooperation between sovereign countries will always be more powerful than confrontation."

The USDA in January said the zone for dispersing sterile flies would extend 50 miles into Texas, along the U.S. border with Tamaulipas state. The sterile pupae can at times be dyed, giving the adult flies a fluorescent glow under ultraviolet light that allows health officials to distinguish them from wild populations.

As of June 25, Mexico reported under 2,000 active cases of infected animals, including cattle, dogs and horses. The first specimens to be used for reproduction at the recently opened plant will be imported from Panama.

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(With assistance from Erin Ailworth.)

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This story was originally published June 27, 2026 at 6:57 PM.