Screwworm — What Sterile Flies Could Not Stop
The screwworm fly was eliminated from North America by sterile insect releases. It returned in 2024, reaching Texas by 2026. What the breach reveals about sustained containment.
In 1954, researchers released sterile male screwworm flies over Curaçao. Within seven weeks, the island was free of the pest — the first time a wild insect population had been driven to local extinction by biological means rather than chemicals. The technique worked so well that by the early 2000s, the New World screwworm (Cochliomyia hominivorax) had been eliminated from the United States, Mexico, and most of Central America.
In December 2024, the fly reappeared in southern Mexico. By June 2026, cases were confirmed in Texas.
The screwworm is not a new species. It did not evolve resistance. The biological barrier that kept it contained failed, and the monitoring programs meant to detect a breach early were no longer in place at sufficient scale. What happened is a case study in what happens when an eradication program’s gains outlast the infrastructure that produced them.
What the screwworm does
Cochliomyia hominivorax is an obligate parasitic blowfly — its larvae feed exclusively on living tissue of warm-blooded animals. Unlike common flesh flies that colonize wounds already infected or necrotic, the screwworm targets healthy tissue. A female locates a wound, surgical site, or body opening and lays 250 to 500 eggs. The larvae hatch and burrow into flesh; if disturbed, they screw deeper into the wound, which is where the common name comes from.
After three to seven days as larvae, they drop to the ground to pupate, emerging as adults about seven days later. The full lifecycle takes roughly 20 days. A single female can lay up to 3,000 eggs in her lifetime and fly up to 200 kilometers. She mates once and retains sperm for life — a biological detail that turns out to be the key vulnerability the sterile insect technique exploits.
The fly primarily affects livestock — cattle, sheep, goats, horses — but it also infests humans. Untreated infections cause severe tissue destruction, secondary bacterial infections, and sepsis. In livestock, a single infestation can destroy an animal’s economic value within days. Historically, screwworm caused billions of dollars in annual losses across the Americas before control programs began.
How sterile flies work
The sterile insect technique (SIT) was first proposed by Russian geneticist A.S. Serebrovsky in 1940 and independently developed by Raymond Bushland and Edward Knipling at the USDA during the 1950s. The concept is straightforward: mass-rear male insects, sterilize them with X-ray irradiation, and release them in overwhelming numbers into the wild. These sterile males compete with fertile males for mates. Because female screwworms mate only once, a single mating with a sterile male renders the female infertile for life. No offspring means the population declines over successive generations.
The technique does not involve genetic engineering or non-native species. It is species-specific and leaves no chemical residues. The tradeoff is that it requires sustained investment: continuous mass rearing, irradiation facilities, aircraft for aerial release, and monitoring networks to detect wild flies.
Early results were dramatic. The Curaçao test in 1954 eliminated the fly in seven weeks. By 1960, screwworm was eliminated from the southeastern and southwestern United States. Between 1962 and 1975, the U.S. and Mexico released over 94 billion sterile flies. Eradication continued southward: Guatemala and Belize (1988–1994), El Salvador (1991–1995), Honduras (1991–1995), Nicaragua (1992–1999), Costa Rica (1995–2000).
The U.S. officially eliminated the screwworm in 1982.
The biological barrier
The fly remained endemic in tropical and subtropical regions of South America, south to Uruguay and Argentina. To prevent northward spread, a biological barrier was established in Panama at the Darién Gap — the narrowest point between Central and South America. Starting in 1998, up to 50 million sterile flies were released weekly on both sides of the gap to create a buffer zone where wild populations could not sustain themselves.
The barrier worked for decades. It was an exercise in sustained negative population pressure — releasing enough sterile males that any flies migrating north from South America mated with sterile partners and produced no offspring. As long as the releases continued at sufficient scale, the barrier held.
The breach
In December 2024, screwworm cases reappeared in southern Mexico. The Panama barrier had been compromised. The exact timing and cause of the breach are not fully public, but the pattern is clear: reduced surveillance and funding for SIT operations weakened the containment infrastructure that had worked for decades.
The fly moved northward through Central America into Mexico. In September 2025, a case was confirmed in Sabinas Hidalgo, Nuevo León — less than 70 miles from the U.S. border. In May 2026, another case appeared in Coahuila, within 31 miles of the border.
The U.S. responded by halting livestock imports from Mexico in May and July 2025. These restrictions are a temporary containment measure, not a long-term solution. The fly does not respect trade policy.
On June 3, 2026, the USDA confirmed a screwworm case in Zavala County, Texas — the first confirmed presence in the continental U.S. since eradication was declared. Two days later, a second case was found in a one-month-old calf 5.6 miles from the first. Three additional cases were confirmed on June 8, 2026, including detections near La Pryor, Texas.
The USDA estimated that a sustained outbreak in Texas could cost $1.8 billion in livestock losses and trade disruptions.
Human cases
The fly infests humans less frequently than livestock, but the consequences are similar: progressive tissue destruction requiring surgical intervention. A 2019 report documented pin-site myiasis — larvae entering through medical device insertion sites — in a 77-year-old man in Colombia. In February 2025, Nicaragua confirmed 30 human cases during the Central American resurgence. In August 2025, the first human case in the United States in decades was confirmed.
The human cases are a secondary concern compared to livestock impact — screwworm is primarily an agricultural and veterinary threat. But they illustrate that containment failures have public health dimensions beyond economic ones.
What made SIT work and what made it fragile
The sterile insect technique succeeded because of three factors: the biology of the target species, sustained political will, and continuous funding. Female screworms mate once — a rare trait among insects that makes SIT exceptionally effective. Species that mate multiple times require far higher release ratios to achieve suppression.
The program also required international coordination across dozens of countries, shared rearing facilities, and harmonized monitoring protocols. The economic case was strong: direct benefits of screwworm eradication in North and Central America are estimated at over $1.5 billion per year against a half-century investment of approximately $1 billion. Mexico protects a $3 billion annual fruit export market with a $25 million yearly investment in sterile fly releases.
But the program’s success created its own vulnerability. As screworm became a memory rather than a present threat, the political and financial case for maintaining release operations weakened. The biological barrier in Panama required continuous weekly releases of tens of millions of flies into an area where no one had seen a wild screwworm in years. When funding or monitoring lapsed, the buffer thinned. Flies from South America crossed undetected.
This is not unique to screwworm. SIT has been used against fruit flies, tsetse flies, codling moths, pink bollworms, and mosquitoes across the US, Mexico, Libya, Zanzibar, Senegal, Chile, Argentina, Peru, Croatia, Israel, South Africa, Spain, Canada, Australia, Japan, and the Netherlands. Each program faces the same tension: the more successful it is, the less visible the threat becomes, and the harder it becomes to justify the ongoing cost of prevention.
The precedent for recovery
A breach is not a permanent failure. When screwworm was detected in the Florida Keys in July 2016 — the first U.S. continental presence since eradication — the response was rapid and effective. Over 150 million sterile flies were released from October 2016 through March 2017, successfully eradicating the outbreak. The operation also protected the endangered Key deer population, demonstrating that SIT can serve dual conservation and agricultural purposes.
The Texas situation is different from Florida in one important respect: Florida is an island with a natural geographic barrier. Texas shares a porous border with Mexico, where the fly has reestablished itself. Containment requires coordination across jurisdictions and sustained releases on both sides of the border — the same international cooperation that made the original eradication possible.
What the re-emergence shows
The screwworm’s return is not a failure of the sterile insect technique. SIT works — it has been proven repeatedly over seven decades. The failure was institutional: the assumption that eradication is permanent once achieved, rather than a condition maintained by continuous investment.
The biological barrier in Panama was designed to hold as long as releases continued. When they did not, flies crossed. This is the difference between elimination and eradication: elimination removes a threat from a region but leaves it present elsewhere. True global eradication would require eliminating the fly everywhere — including South America — which has proven impractical given the species’ range and the tropical ecosystems it inhabits.
The screwworm story also illustrates a broader pattern in pest management and public health: the hardest part of an eradication program is often not the initial elimination, but maintaining the infrastructure that keeps the threat away after it is no longer visible. Polio vaccination campaigns, malaria control, and invasive species barriers face similar dynamics. Success makes the threat invisible, which makes continued investment politically difficult, which makes re-emergence more likely.
The sterile insect technique remains the most effective tool available for controlling Cochliomyia hominivorax. The question is whether the programs that use it can sustain themselves through the period when their success makes them seem unnecessary.