In the 1960s, the US bred up to 150 million sterile screwworm flies a week in Texas and dropped them from planes to protect livestock from a flesh-eating parasite

In June 2026, U.S. officials confirmed the first case of New World screwworm in Texas since 1966, according to the USDA Animal and Plant Health Inspection Service. That news sent a lot of people looking for how America beat this parasite the first time around. The entire story, one of the strangest and most successful pest-control campaigns in U.S. history, has been preserved by the USDA National Agricultural Library. It includes a Texas fly factory, hospital X-ray machines, and planes dropping millions of insects.

What was actually crawling into the wounds
The New World screwworm is not a worm. It is the larval stage of a fly called Cochliomyia hominivorax, a name that roughly translates to “man-eater.” As stated in the USDA National Agricultural Library, adult female flies deposit eggs at the edge of open wounds of warm-blooded animals. Larvae that hatch burrow into living flesh and eat it, enlarging the wound and attracting more egg-laying females. Secondary infection in many cases kills untreated infected animals. The same archive reports that in 1935 alone, screwworms killed 180,000 livestock in fewer than half of Texas's counties.

A discovery, a theory, and a way to raise millions of flies
Scientists didn’t even know they were dealing with a separate species until 1933, when USDA entomologist Emory Cushing, working with a University of Liverpool researcher, proved that the screwworm fly was different from the common blowfly, the USDA archive notes. That discovery paved the way for a strange but brilliant idea by USDA scientist Edward Knipling: since female screwworm flies mate only once in their lives, flooding the wild population with sterile males would eventually cause reproduction to collapse. By 1936, fellow USDA researcher Raymond Bushland had worked out how to mass-rear screwworms in a lab, feeding the larvae a combination of ground meat, beef blood and water, according to the archive. The last piece of the puzzle was a way to sterilize flies by the millions.

The radiation breakthrough, and a test that half-failed
That came together after World War II. In 1950, a colleague told Knipling about an article by the Nobel Prize-winning geneticist Hermann Muller about sterilizing insects with radiation, and Bushland checked with hospital X-ray equipment that the technique worked on screwworms as well, the USDA archive says. Later, Oak Ridge National Laboratory’s Cobalt-60 made large-scale fly sterilization practical.

The first field test was in Sanibel Island, Florida, in 1953. It confirmed the technique worked, sterile flies suppressed the local population, but the island wasn't isolated enough: new fertile flies kept arriving from the mainland and undid the progress, according to a 2016 review titled ‘SIT 2.0: 21st Century genetic technology for the screwworm sterile-insect program’ published in BMC Biology. A better location was found on the more isolated island of Curaçao in 1954. That sterile-fly technique wiped out the entire local population there, and eradication was declared after about five months, the same review said.

Texas builds one of the world's strangest fly factories
The USDA archive shows that Florida had successfully used the same strategy by 1959. Then Texas. In 1962, the USDA began building a huge rearing plant on the site of a former Air Force base near Mission, Texas. Larvae grew in trays of meat and blood that moved along a conveyor system and were then irradiated and packed into cartons to be dropped by small planes over South Texas and the border area. The original design called for about 50 million sterile flies to be produced each week, but with added capacity and improved rearing methods, the facility was producing as many as 150 million flies a week by 1966, The Texas Tribune reported, citing USDA’s own archival records. That same year, the USDA declared the United States entirely free of native screwworm populations, a milestone recorded in the USDA archive itself.

Why the story from the 1960s is back in the headlines
For about six decades, screwworms were kept out of the U.S. by an active sterile-fly barrier near Panama. But cases began to rise again in Central America and Mexico, and in November 2024 a confirmed case in Mexico led the U.S. to close the border to Mexican cattle imports, tightening the cattle supply in Texas, reports The Texas Tribune.

By June 2026, according to the USDA, the parasite reached a calf in South Texas, the first confirmed U.S. case since 1966. USDA is releasing sterile flies near the border again, so the old playbook is already in motion. This is a strategy that has a recent successful track record. The 2018 study, ‘Screwworm (Diptera: Calliphoridae) in the United States: Response to and Elimination of the 2016–2017 Outbreak in Florida,’ in the Journal of Medical Entomology documented a smaller screwworm outbreak in the Florida Keys in 2016 and 2017, after about 188 million sterile flies were released, before eradication was declared in March 2017.

The takeaway
This is no longer ancient history. The USDA estimates that the original eradication still saves the U.S. livestock industry more than $900 million annually, according to the USDA archive. Now, the fate of that number depends on how the current outbreak unfolds. Either way, the tale of Texas mass-producing flies and dropping them from airplanes is more than just a strange footnote from the 1960s; it's the very same tactic the country is relying on today.