After 60 years of successful eradication, New World screwworm (NWS) has been detected in Texas. According to USDA, NWS was eradicated from the U.S. in 1966. There have been reports of cases in Texas in the early ‘70s and in 2016 in the Florida Keys — these incidents did not end in the U.S. being considered “infested.”
NWS plagued American ranchers for at least a century before scientists gave it a name. By the 1930s, USDA researchers Cushing and Patton had confirmed it as a species distinct from the common blowfly — a prerequisite for any targeted control effort. Annual losses to the U.S. livestock industry were running $5 million to $10 million, and a federally funded Screwworm Educational Program was launched in 1935 to help producers manage wounds and isolate infested animals.
Reviewing history and talking to South Texas ranchers, for livestock producers in the southern U.S. the “screwworm season” wasn’t just a date on the calendar — it was a grueling daily battle. Before eradication, the daily ranch routine was dictated by the pest that could kill a healthy calf in less than a week.
According to James Novy, former USDA assistant chief of program evaluations and planning of veterinary services, prior to its eradication in the U.S., NWS existed in tropical to subtropical environments, with populations surviving the winters in southern Florida, Texas, California and Arizona. Variations in the weather from year to year determined the degree of population reduction each year in overwintering areas. During the spring, summer and autumn the NWS population increased with a commensurate increase in territory occupied. He reports screwworms were commonly reported in the central U.S. with occasional cases as far north as the border with Canada.
The Era of “Horseback Husbandry”
Before the eradication programs began, management was defensive. Producers had to time branding, dehorning and castrating to the coldest months to avoid infestations. As Novy notes, “Since the navels of newborn animals were common sites for screwworm infestation, breeding of livestock was timed so that most newborn animals were born during the cold months.”
On the vast rangelands of the Southwest, this meant endless hours in the saddle. Infested animals would often seek shade and hide, making them difficult to find until it was too late.
Novy recalls the toll this took: “Annual losses of 10% were estimated on Texas ranches. Even after treatment, many of the animals that recovered would have damaged hides or other disfigurements, and the sale value of the animals would be reduced.”
He explains larvicides were applied to wounds to kill the NWS when present, and wounds were prophylactically treated. The effect of intensive animal inspection and wound treatment was demonstrated in Florida from 1935 to 1938, when the U.S. Bureau of Entomology and Plant Quarantine, in cooperation with the southeastern states, conducted a screwworm education and control program. Using benzol and pine tar oil to treat infested wounds, screwworm infestations were reduced within one and a half years to such a low level that there were no losses of livestock. Novy reports the program could not be sustained and NWS could not be eradicated because wildlife and non-attended domestic animals served as reservoirs.
The Turning Point: The Sterile-Male Technique
The tide turned when USDA researchers discovered a biological “glitch” in the fly: the female mates only once. USDA entomologist Edward Knipling had been developing the concept of “autocidal control” since the late 1930s: If you could flood a wild insect population with sterile males, females would produce no viable offspring. The population would collapse from within.
His colleague Raymond Bushland made the theory practical. As director of the USDA’s Kerrville, Texas, laboratory, Bushland developed a method for mass-rearing screwworm flies in a controlled setting. In 1950, after reading a Nobel Prize-winning paper by geneticist Hermann Muller on radiation sterilization of fruit flies, Knipling wrote to Muller — and Muller’s reply encouraged the team to try gamma irradiation on screwworms. Bushland tested it using X-ray equipment from a nearby Army hospital, confirming that flies could be sterilized at certain doses without losing the ability to compete for mates. The Sterile Insect Technique (SIT) was born.
In 1954, the technique was field-tested on the island of Curaçao off the coast of Venezuela. NWS was eliminated in seven weeks. Three years later, USDA launched a full SIT program across the southeastern U.S. Florida was declared screwworm-free by 1958. In 1962, a massive effort began in Texas. Novy says producers were essential to this phase, submitting larval samples to help officials track the enemy. He describes the sudden relief of 1963: “Subsequently during 1963 many livestock owners found no screwworm-infested animals on their ranches for the first time in modern history.”
The U.S. was officially declared free of NWS in 1966. Learn more about the screwworm eradication process.
The Danger of Complacency: The 1972 Resurgence
The victory was real but fragile. Re-infestation from Mexico remained an annual concern. By the early 1970s, the program was so successful that many producers had reduced their labor and stopped routine inspections.
In 1972, a combination of a warm winter, a surge in the Gulf Coast ear tick (which provided more wounds for flies to strike), and animal movements led to a disastrous outbreak. Novy explains: “By mid-May, the screwworm situation was completely out of hand. The population of screwworm flies migrating north from northeastern Mexico was too large to be sufficiently outnumbered by the production and distribution of sterile flies.”
That year, Texas had more than 90,000 confirmed cases. It served as a stark reminder that as long as the pest existed across the border, the U.S. livestock industry was at risk.
The crisis led to the creation of a joint Mexico-U.S. Screwworm Eradication Commission and a push to move the containment barrier south. Knipling and Bushland received the World Food Prize in 1992; their technique was called “the greatest entomological achievement of the 20th century.”
Over the following decades, the barrier was walked steadily southward through Mexico, Central America and ultimately to the Darién Gap in Panama. Panama was declared NWS-free in 2006.
Florida Key Deer Infestation
On Oct. 3, 2016, USDA confirmed the presence of NWS in Key deer from National Key Deer Refuge in Big Pine Key, Flo. According to USDA APHIS this was the first local infestation in the U.S. in more than 30 years and the first infestation in Florida in 50 years. March 23, 2017, USDA APHIS announced the successful eradication from the Florida Keys.
Dr. Jack Shere, USDA Chief Veterinarian, reported close to 154 million sterile flies were released, 16,902 animals inspected at checkpoints, and almost 430 hours of active surveillance in the Keys and 250 hours of active surveillance on the mainland were completed.
According to the press release, APHIS considers an area to be screwworm-free through surveillance which includes trapping flies and visually inspecting animals for signs and symptoms of NWS infestation. No new cases of NWS had been reported in Florida since Jan. 10, 2017. Science shows that, when sterile flies are released, elimination of NWS is achieved three life cycles after the last detection. The flies have on average, a 21-day life cycle, and they continue to circulate in the area for three weeks beyond each release. In the Keys, APHIS will complete five life cycles beyond the last positive screwworm detection. Out of an abundance of caution, APHIS also released flies in the Homestead, Fla., area for three completed life cycles.
Crossing the Border
In 2023, an outbreak in Panama broke the barrier, moving through Central America and reaching Mexico in November 2024. The exact cause of this breach is unclear but is most likely due to multiple factors, including interruptions in sterile fly production due to the COVID-19 pandemic and illegal cattle imports, as well as the challenges involved in surveillance of the gap’s difficult geography.
Live cattle trade to the U.S. from Mexico was first halted on Nov. 22, 2024, after a cow in the southern Mexican state of Chiapas was found to have NWS myiasis. On Feb. 1, 2025, APHIS announced cattle and bison imports from Mexico would resume utilizing new preventative measures. Imports are scheduled to begin in the next several days. The border was closed again on May 11. There was a scheduled phase re-opening starting July 7, which was canceled July 9 and has remain closed since that announcement.
While it was predicted NWS would cross the U.S.-Mexico border by the summer of 2025.
USDA’s plan to prevent the spread of NWS included investments of up to $100 million for innovations to find new ways to combat the insect and $750 million to create a facility in South Texas to sterilize flies and combat the spread.
The first U.S. case was confirmed June 3 in South Texas near La Pryor. After confirming an infestation in a 3-week-old beef calf, USDA and Texas Animal Health Commission (TAHC) warn animal movement — not fly travel — spreads the pest, and outline inspection, treatment and cattle movement steps for producers.
U.S. Secretary of Agriculture Brooke Rollins says, “We have been tracking this pest for a long time, and we have fought before, and we will do so again.”
Calling NWS “an ever-evolving and dynamic situation,” Rollins details a rapid build‑out of infrastructure aimed at keeping the pest in check. At a press conference in the new Knipling-Bushland U.S. Livestock Insects Research Laboratory along with Gov. Greg Abbott, USDA officials, Texas A&M leadership and rancher representatives, she stresses this is not a replay of the 1950s and 1960s. Instead, she frames it as a high‑stakes test of a system that has been quietly preparing for more than a year.
“Every model showed that the New World screwworm would be here in Texas by early last summer, so we bought ourselves an additional year to prepare for this moment,” she stresses.
The goal is to keep the pest boxed in and pushed back before next summer’s fly season — a timeline Rollins acknowledges is ambitious but achievable. Check the status of confirmed NWS detections on the USDA dashboard. Producers can also find current zone maps at tahc.texas.gov.
T.R. Lansford III, Texas Animal Health Commission deputy executive director and assistant state veterinarian, says hesitation is exactly what turns a manageable outbreak into a catastrophe.
He reminds producers:
- NWS is not a food safety issue. It’s not going to be in your beef. A treated, recovered animal is perfectly fine.
- It is not a swarm. The fly is reclusive; it hides in vegetation, it doesn’t come into barns or houses, and the odds of actually seeing one in your environment are very low.
- NWS is treatable and it is preventable. Find it early, treat it and you’ve not only helped that animal, you’ve broken the fly cycle. That’s what protects your neighbor. That’s what protects trade. That’s what keeps this from becoming something much larger than it needs to be.
The history of NWS proves that eradication is not a one-time event, but a state of constant vigilance. Rollins summarizes, “We’ve defeated this before. With President Trump’s leadership, we will push it back again — and protect American ranchers.”
