Most fly larvae eat dead flesh. The New World screwworm is the unpleasant exception. Its maggots feed on living tissue, which is why ranchers have feared it for as long as there have been ranchers.
The name comes from how they feed. The US Department of Agriculture (USDA) describes larvae burrowing into a wound like a screw being driven into wood, tearing with sharp mouth hooks. A wound as small as a tick bite can attract a female, who lays her eggs at its edge, usually hundreds at a time and up to 3,000 in a lifetime. They hatch within hours. As the wound gets bigger it attracts more flies, and they lay more eggs. An American Veterinary Medical Association expert told the Associated Press that a large bovine can be dead within two weeks.
It's back
The first US animal case of this outbreak was confirmed on 3 June 2026: a three-week-old calf in Zavala County, Texas. By late October the tracker based on USDA data listed 66 animal cases in 25 counties, 57 in Texas and 9 in New Mexico. Those figures will have changed by the time you read this. The first case in a wild animal, a female elk shot in Pecos County, was confirmed on 21 October.
There's been no locally acquired human case in the US. The CDC describes the risk to people as very low, and the only US human case it lists involved a traveller returning from El Salvador in 2025.
The strangest pest control ever attempted
In the 1950s, American researchers worked out something counterintuitive: you could defeat this fly by releasing more of it. Their approach rested on one biological quirk, which is that a female screwworm mates only once in her life.
Mate her with a sterile male and that's it. She'll lay eggs, but they'll never hatch, and she doesn't get another try. Rear enough flies, sterilise them with radiation, and scatter them over the landscape, and the wild population runs out of fertile matings.
The first proper test took place on the island of Curaçao in 1954, with 1,000 sterile flies per square mile each week, dropped from aircraft. After three weeks, 70% of the egg masses were sterile. After six, 84%. After nine, there were hardly any egg masses left, and all of those were sterile.
Then came the big version. Between 1962 and 1975 the US and Mexico released more than 94 billion sterile flies, at one point dropping them out of planes in paper cups through special chutes. The USDA's own history credits the method with clearing the fly from the US in 1966, while Science News gives 1982 as the official date. Sources differ depending on what they're counting.
What's happening now
The same idea is being used again, at a new scale. Farm Journal reported around 40 release flights a week in 2026, dropping about 100 million sterile flies. On 7 September 2026 the US government released its billionth sterile fly in Texas. A new $610 million plant at Moore Air Base in Edinburg, Texas, is aiming to begin in April 2027 at 100 million flies a week, and later to reach 300 million.
That's where the headline comes from. Those planes are real, those are real flies, and it's all deliberate.
Two footnotes
The species' Latin name, Cochliomyia hominivorax, means roughly "man-eater", and cases in people do happen in Central America and Mexico. Don't let the name inflate the risk to people in the US, which the CDC rates as very low. And one correction to the common telling: the released flies were males and females. Male-only strains are a recent development.
What we read
- CDC, New World Screwworm Outbreak, situation summary
- USDA APHIS, New World Screwworm: What you need to know
- Science News, on the spread and eradication of screwworm, 2026
- GenomicEpi, New World screwworm outbreak tracker
- Associated Press via ADN, US plans to breed billions of sterile flies, 2025
- USDA ARS, history of screwworm eradication
- Farm Journal, Operation Sterile Fly
- WEAU, Government releases billions of flies, 2026
- Construction Briefing, on the $610m sterile fly facility, 2026
