On the Fly: Lessons From a Screwworm
- Heather McSharry, PhD
- Jul 1
- 18 min read
Updated: Jul 13
Summary
New World screwworm, Cochliomyia hominivorax, once made ranching in the southern United States a battle against a parasite that ate animals alive. In the 1950s and 1960s, a radical idea from entomologist Edward Knipling—releasing sterile male flies into the wild—transformed that reality and became one of the most successful biological control campaigns in history. Decades later, New World screwworm has returned to the headlines, with confirmed detections in Texas in 2026 and a renewed push to expand sterile fly production, surveillance, and international cooperation. This episode and article trace the arc from eradication to maintenance to today’s outbreak, using screwworm as a case study in how understanding biology, building infrastructure, and sustaining public health systems can change the course of history—and why those systems matter long after the initial crisis seems to be over.
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Full Episode
In 1966, the United States declared victory over one of the most destructive parasites it had ever faced. Ranchers had spent generations watching livestock being eaten alive. Veterinarians knew the smell before they saw the wound. Entire economies were built around fighting a fly. Then...we defeated it, not through poison, but because someone proposed an idea that sounded completely absurd. What if we released millions...of flies? This month we're exploring turning points in infectious disease history. And this may be one of the strangest—and most successful—public health victories you've never heard about.
This is On the Fly: Lessons From a Screwworm
The Monster We Feared
The New World screwworm, Cochliomyia hominivorax, is a parasitic fly native to the Americas. Its larvae are what make it infamous. Larvae are the immature stage of the fly...or what we call maggots for most flies. Unlike many maggots that feed on and clean up dead tissue, screwworm larvae are obligate parasites of living flesh. They eat and damage healthy tissue. Left untreated, a single infestation can expand quickly and become life‑threatening, especially for young or injured animals. For much of the twentieth century in the southern United States, Mexico, and Central America, this wasn’t a rare curiosity. It was a routine, devastating threat to livestock, wildlife, and, occasionally, people.
Now, there are some things you never really forget once you've seen them. Years ago, during field work in south Texas, I spent part of a summer trapping wild rodents. Almost every day we handled animals carrying parasites. Ticks clustered...sometimes there'd be so many you could hardly find bare skin. Lots of bot flies—whose large larvae were developing beneath the skin, each occupying its own swollen pocket. Even after all these years, those images still come back to me. Seriously unsettling. But they weren't screwworm. Bot flies usually develop alone. One larva. One small opening. Their survival depends on keeping the host alive long enough to complete their development. Screwworm flies play by entirely different rules.
As the larvae feed on living flesh, they cause a condition known as traumatic, or wound, myiasis—the infestation of living tissue by fly larvae. Female screwworm flies are drawn to existing wounds, whether they're caused by barbed wire, branding, castration, tick bites, the navel of a newborn calf, or even the moist tissues around the eyes, nose, or mouth. At the edge of a wound, a single female can lay more than 300 eggs.
Within about 12 to 24 hours, those eggs hatch, and the larvae immediately begin feeding. As they enlarge the wound, they release odors that attract even more female flies, each laying hundreds of additional eggs. What begins as a small injury can quickly become a self-perpetuating infestation, with successive waves of larvae feeding in the same wound.
The larvae get their name from the rings of backward-pointing spines that wrap around each body segment, giving them the appearance of a tiny screw. They don't actually twist their way into the wound like a drill, but those spines anchor them firmly within the surrounding tissue, making them difficult to remove while allowing them to burrow progressively deeper as they feed. After passing through three stages of development over several days, the mature larvae drop from the wound to the ground, burrow into the soil, and pupate. In warm tropical conditions, the entire life cycle—from egg to adult fly—can be completed in less than three weeks.
Left untreated, the consequences can be devastating. Animals often die not simply from tissue destruction, but from the combined effects of overwhelming infestation, secondary bacterial infection, and the inability to feed or recover. Newborn calves are especially vulnerable. So are lambs. Deer. Pets. Wildlife. Even people, although human cases are much less common.
Before eradication in 1966, for ranchers across the American South, the Southwest, Mexico, and Central America, this wasn't a rare catastrophe. It was simply part of life. Every calving season meant inspecting animals for wounds. Every branding season carried risk. Every injury, no matter how small, had the potential to become something far worse. Generations grew up believing this was simply the price of raising livestock in a warm climate.
By the middle of the twentieth century, screwworm had become so familiar that most people focused on treating the consequences rather than imagining a cure. Veterinarians cleaned wounds. Ranchers inspected their herds. Scientists searched for better insecticides. The assumption was simple: if there are flies, kill the flies. But one entomologist at the USDA wondered whether everyone was asking the wrong question. His name was Edward Knipling. Knipling wasn't thinking about poisons or traps. He was thinking about populations.
Imagine, for a moment, that you're trying to eliminate every screwworm fly across thousands of square miles. Even if you could kill 99 percent of them, the remaining one percent would reproduce and the population would rebound. So instead of asking how to kill every fly... ...Knipling asked how do screwworm flies reproduce? The answer contained an extraordinary opportunity. Female screwworm flies typically mate only once during their lives. One mating. One male. After that, she stores enough sperm to fertilize every batch of eggs she'll ever lay. If she mates with a healthy male, hundreds of offspring may survive. But what if......the only male she ever mates with......is sterile? She still lays her eggs. She doesn't know anything is different. But every one of those eggs fails to develop. Her entire reproductive future ends with that single mating. And because she never mates again, there's no second chance. Every sterile male becomes a biological dead end. Suddenly, the problem isn't about killing flies. It's about changing the odds. Release enough sterile males, and eventually more and more females will encounter them instead of fertile ones. Those females produce no offspring. The next generation is smaller. With fewer fertile flies, the odds improve again. Each generation becomes smaller than the last, until the population begins to collapse under the weight of its own mathematics. No pesticides. No blanket spraying across the countryside. No contaminating rivers or pastures. Just a remarkably elegant understanding of how one species reproduces. It was a solution so simple that many people thought it couldn't possibly work.
It was a beautiful idea.
On paper, the math worked. But nature rarely behaves as neatly as equations do, so many scientists remained unconvinced. Could enough sterile flies actually be produced? Would they survive long enough after release? Would wild females even mate with them? And if they did...would releasing millions of flies somehow make the problem worse instead of better? There was only one way to find out.
Knipling's elegant idea had to survive its encounter with the real world. That responsibility fell to a team of scientists and field researchers, including entomologist Alfred Baumhover, who would spend years transforming a mathematical insight into a working eradication program. Together, they needed a place where they could test the theory without new screwworms constantly flying in from neighboring regions.
They found it on the Caribbean island of Curaçao.
For screwworm, it was the perfect laboratory. The island was isolated, the fly population well established, and its geography meant that any changes could actually be measured. If the strategy worked there, scientists would finally have proof that the impossible might actually be possible.
So they did something no one had ever attempted before. They bred millions of screwworm flies. The flies were sterilized using carefully controlled doses of radiation—high enough to make them infertile, but low enough that they could still fly, compete for mates, and behave like any other screwworm. Week after week, aircraft released the sterile flies across the island while researchers monitored what happened next.
Looking back decades later, Baumhover admitted he believed they had only about a fifty-fifty chance of success. Every evening, he and his team examined egg masses collected from sentinel goats across Curaçao. Every day, they wired the results to Edward Knipling, waiting to see whether the mathematics would hold in the real world.
Somewhere, a wild female found a mate. She laid her eggs and none of them hatched. Another female did the same. And another. Generation by generation, it was working. The population didn't simply shrink. It unraveled. With every sterile mating, fewer fertile flies entered the next generation. Making successful reproduction less and less likely.
By 1954, screwworm had been eliminated from Curaçao. For the first time in history, an insect pest had been eradicated from an entire region—not with pesticides, not by destroying habitat, but by turning the species' own biology against itself. It was one of those moments that changes more than the problem it set out to solve. The obvious question became: Could they do it across a continent?
Scaling up?
They proved the idea could work, but eliminating screwworm from the United States, where flies could move across open landscapes and reinvade from untreated regions, was another problem entirely. The campaign could not simply wipe out screwworm and declare victory. It had to create a system that would keep the parasite from coming back. Spoiler alert. They did it.
That system became one of the most ambitious biological control programs ever attempted. Screwworm flies were raised by the millions in production facilities, sterilized, packed, transported, and released from airplanes over targeted regions.
On the ground, that strategy didn’t look like equations. It looked like calving seasons with fewer animals eaten alive. A rancher in south Texas who once expected screwworm in every branding pen suddenly saw wounds that healed instead of turning into spiraling infestations. Year by year, sterile flies quietly converted what had been a routine, deadly problem into something younger ranchers only heard about in their parents’ stories.
As the campaign succeeded, the barrier moved. Screwworm was pushed out of Florida and the southeastern United States, then out of the Southwest, then farther south through Mexico. Eventually, the effort expanded through Central America until the permanent barrier came to rest in Panama, where sterile flies could be released continuously across a narrower geographic front. It was not a wall in the usual sense. There was no concrete, no fence, no visible line separating safety from risk. It was an ecological barrier, maintained from the air, made of millions of sterile insects released again and again to prevent fertile flies from moving north.
That image is one of the most remarkable parts of this story: a constant aerial release of sterile insects acting almost like an invisible immune system. Not a cure given once, but a living form of maintenance. The sterile fly barrier did not mean screwworm had vanished from the planet. It meant humans had built a system capable of holding it back.
And this is a big deal because truly successful eradication campaigns are remarkably rare. Many control programs reduce pest populations for a time, only to see them rebound. Mosquitoes can quickly recolonize treated areas. Mediterranean fruit flies have been eliminated from specific outbreaks only to be reintroduced later. Other campaigns succeed locally but struggle to hold their gains across large geographic regions. Screwworm was different because several rare conditions aligned. Female screwworm flies mate only once. The insects could be mass-produced. Sterile males could still compete successfully in the wild. Governments coordinated across borders. Funding continued long enough to move the barrier south. And surveillance remained in place to detect breaches before they became disasters.
So this was not luck. It was infrastructure. The victory over screwworm was not a single discovery or a single campaign or a single heroic moment. It was a system: production, transportation, release, monitoring, cooperation, response. The science made the breakthrough possible, but maintenance made it real.
That may be the most important lesson in the whole story. Victories in infectious disease and parasite control are not usually permanent achievements. They are ongoing agreements. They last only as long as the systems that sustain them. Screwworm was defeated once before, but it stayed defeated because people kept doing the unglamorous work after the headlines were gone. Victory, in this case, was maintenance.
The paradox of prevention
The remarkable thing about successful disease control is that, eventually, it disappears into the background of everyday life. Children vaccinated against diseases that no longer circulate grow up never seeing them. Communities protected by clean drinking water rarely think about the epidemics that once followed contaminated wells. When a public health victory lasts long enough, it stops feeling like a victory at all. It simply becomes normal.
That was true of screwworm in the United States. Entire generations of veterinarians never diagnosed a case. Most ranchers had never watched a healthy calf become infested with flesh-eating larvae because the eradication campaign had prevented it from happening. The absence of screwworm wasn't an accident. It was the visible result of an invisible system that had been quietly doing its job for decades.
We rarely notice the disasters that never happen because surveillance systems, laboratories, production facilities, and international partnerships quietly keep them from happening in the first place.
That quiet success can become politically vulnerable. Not because ranchers or veterinarians stopped caring—they didn't. In fact, as screwworm continued moving north through Mexico, livestock producers and veterinary organizations repeatedly warned that the United States needed to strengthen surveillance and expand sterile fly production capacity.
Instead, in March 2025, a stop-work order halted a USAID program that had been supporting New World screwworm surveillance and response efforts in Central America. At the same time, broader federal staffing and budget reductions affected parts of the public health and agricultural infrastructure responsible for monitoring and responding to biological threats (the USDA lost ~20,000 employees from January 2025 to January 2026, while the USDA's Animal Plant and Health Inspection Service (APHIS) lost 2,009 employees or 23% of the service).
Parasites, of course, don't care why a surveillance system weakens. They don't distinguish between budget cuts, administrative restructuring, political priorities, or simple neglect. They respond only to ecological opportunity. A weakened monitoring network, reduced production capacity, or disruptions to international coordination create openings that a parasite will exploit regardless of how those gaps came to exist.
For decades, the screwworm program succeeded because the United States, Mexico, Panama, and other partners continued investing in a system whose greatest achievement was that most Americans never had to think about it. That is both the triumph—and the paradox—of prevention.
FAFO
Now, after gutting the systems that protected the US from screwworm, by politicians who don't like science, screwworm is back. In June 2026, New World screwworm stopped being a historical story and became a present‑day problem...again. USDA confirmed the first case in a Texas calf on June 3, 2026, and by late June, at the time of this recording, the agency’s screwworm dashboard showed 27 confirmed animal cases in domestic livestock across several Texas counties. (29 on July 1 when the episode dropped). In response, officials have activated infested and surveillance zones, tightened livestock movement, and ramped up sterile fly production and aerial releases to keep the outbreak contained.
Responding to that threat has required rebuilding parts of the very system that once seemed so routine. Sterile fly production has had to expand. Additional personnel have been deployed. Surveillance has intensified. New facilities are being brought online, while existing programs have been stretched to meet a challenge that many people assumed belonged to history.
Let's be clear. The parasite didn't suddenly became stronger. Sterile insect releases still work. Surveillance can still detect new cases before they become widespread. The science is as solid as ever. What changed was the environment surrounding that science and upkeep of eradication. Maintaining a continental-scale biological barrier requires funding, international cooperation, manufacturing capacity, trained personnel, veterinary networks, and constant monitoring. It's a system built on thousands of routine decisions made year after year. Like any complex system, it is strongest when every part is functioning and it is vulnerable to destruction by policy makers who don't understand the science behind a public health infrastructure that's been functioning successfully for decades.
Defeating screwworm now depends on taking that infrastructure and scaling it back up to meet a renewed threat. But one of the greatest challenges facing the current response isn't simply rebuilding the infrastructure that existed before. It's adapting it to today's realities. Livestock surveillance remains the backbone of detection, but wildlife represents a significant surveillance gap. Domestic animals can be inspected regularly. Wild animals cannot. A deer carrying screwworm through dense brush doesn't report itself, and unless it's seen by a rancher, hunter, trail camera, or wildlife biologist, that infestation may go entirely undetected.And if screwworm is circulating in wildlife, the official case count may tell us where the parasite has been detected—not necessarily everywhere it exists.
That is one of the weakest points in the current response. Sterile fly releases can still work, but they work best when officials know where the active infestations are and can release enough sterile males over the right areas at the right density. Missed wildlife cases make that map harder to draw. They create uncertainty about where the leading edge of the outbreak really is, how far the parasite has spread, and whether new cases in livestock are isolated introductions or signs of a larger hidden problem.
And while the Sterile Insect Technique sounds simple, in practice it means producing millions of sterile males every week and delivering them across vast landscapes with enough density that most wild females are likely to mate with sterile males instead of fertile ones. Facilities in places like Panama and the United States don’t just breed flies; they maintain specialized diets, controlled environments, and irradiation systems calibrated to damage reproductive cells without destroying the flies’ ability to compete in the wild. Aircraft and ground crews then carry out regular releases over infested and buffer zones, turning the sky into a delivery system for biological control. For the current outbreak, ramping up this work has required expanding production lines, reassigning and hiring staff, securing additional funding, and coordinating closely with state and international partners so that sterile flies arrive where they’re needed, when they’re needed, at the scale the mathematics demand.
The screwworm story is repeating itself because our greatest public health victories are not self-sustaining. They require us to keep believing that the quiet work of prevention is worth doing, even when its greatest success is that nothing happens.
Conclusion
Turning points in infectious disease sometimes begin with someone asking a question that everyone else overlooked. Edward Knipling didn't discover a new insecticide or invent a more powerful weapon against screwworm. He looked at a fly that generations had accepted as inevitable and wondered if understanding its biology might be more powerful than trying to destroy it. The Sterile Insect Technique went on to become one of the most important innovations in biological control. Rather than blanketing landscapes with poisons, it demonstrated that it was possible to reshape an ecosystem by targeting a single species with extraordinary precision. It remains one of the clearest examples of how understanding biology can produce solutions that are both effective and remarkably selective.
But perhaps the most enduring lesson is historical. Every generation inherits public health systems built by the people who came before it—vaccination programs, surveillance networks, laboratories, international partnerships, production facilities, and countless other pieces of infrastructure that quietly protect us every day. Those systems continue working only if each generation decides they are worth maintaining.
A year ago, ranchers were basking in the aftermath of an amazing public health success that was being maintained. Today they are wondering who among them will survive this fight. It will get worse before it gets better.
"On the fly" means improvising when the systems we depended on aren't there anymore. For decades, we didn't have to respond on the fly to screwworm because we had built a system that quietly prevented the crisis from happening in the first place. That's the difference between emergency response and public health. One reacts. The other makes reaction unnecessary. And victories in public health aren't moments...they're committments.
Thanks for joining me. If you enjoyed this episode, you'll find annotated references, additional historical resources—including some fascinating letters and firsthand accounts from the original eradication campaign—after my signature.
And don't forget to subscribe to my free weekly newsletter, Field Notes, where I continue the conversation beyond each episode, share the stories I'm following, provide outbreak updates, and take you behind the scenes of Infectious Dose.
Next week, we're staying with this month's theme of turning points in infectious disease history, but shifting from Texas ranches to a Texas military base. We'll explore the military's long struggle against one of its oldest and most persistent enemies: infectious disease. Long before vaccines changed childhood, armies were learning that microbes could be deadlier than bullets. It's another example of how victories against infectious diseases are committments events.
Annotated References and Resources
Accessing the literature: Whenever possible, I've linked directly to free full-text articles and books that are legally available online. For subscription-only journal articles, check whether a free version is available through an institutional repository, or many researchers are also happy to share a personal copy of their work if you contact the corresponding author. For books, I've included links to free online lending copies or library catalogs when available. If a title isn't freely available online, try searching WorldCat to locate it at a nearby library or ask your local library about interlibrary loan.
U.S. Department of Agriculture, Animal and Plant Health Inspection Service. Confirmed Detections of New World Screwworm.
🌐 Free online resource
→ This USDA APHIS dashboard provides the current official count of confirmed New World screwworm animal cases and wild fly detections in the United States. It is the best place to follow where detections have been confirmed, which animals are affected, and how active and inactive cases are being tracked over time.
Hailu KT, Kasagga A, Haddad RR. 2025. The New World Screwworm in the United States: A Narrative Review Anchored to the 2025 Travel-Associated Human Case. Cureus.
🆓 Open access
→ This recent review uses a 2025 human screwworm case as a starting point to explain the parasite's biology, life cycle, diagnosis, treatment, and public health significance. It's an excellent, accessible overview of why New World screwworm remains a concern today and how the current response connects to one of the most successful eradication campaigns in history.
USDA Nat Ag Library (1953–1955). Selections from the Screwworm Eradication Program Records. U.S. Department of Agriculture, National Agricultural Library.
🌐 Free online resource
→ This remarkable digital archive brings the early screwworm eradication campaign to life through letters, memoranda, photographs, and other original documents from the scientists and officials who made it happen. If you enjoyed the story of the Curaçao field trial, this collection offers a fascinating glimpse into the conversations, challenges, and excitement as one of the most successful parasite eradication campaigns in history unfolded in real time.
Hall MJR, Wall RL, Stevens JR. 2016. Traumatic myiasis: a neglected disease in a changing world. Annual Review of Entomology.
🔒 Subscription required but here's the free prepublication version from the University of Exeter Institutional Repository
→ One of the best scientific overviews of myiasis—the infestation of living animals by fly larvae. This review explains how screwworm differs from most other flies, why it is such a devastating parasite, and how changes in climate, travel, and agriculture continue to shape its distribution.
Knipling EF. 1955. Possibilities of Insect Control or Eradication Through the Use of Sexually Sterile Males. Journal of Economic Entomology.
🔒 Subscription required – Publisher paywall; see access note above.
→ This landmark paper introduced what became the Sterile Insect Technique. Edward Knipling's idea was revolutionary: instead of trying to kill every pest, he proposed disrupting reproduction itself. Few scientific papers have had such a lasting impact on agriculture and public health.
Bushland RC, Hopkins DE. 1951. Experiments with Screwworm Flies Sterilized by X-rays. Journal of Economic Entomology.
🔒 Subscription required – Publisher paywall; see access note above.
→ The first successful experiments showing that radiation could sterilize screwworm flies without preventing them from competing for mates. These early studies transformed Knipling's theoretical idea into something that could be tested in the real world.
Baumhover AH, et al. 1955. Screw-Worm Control through Release of Sterile Flies. Journal of Economic Entomology.
🔒 Subscription required – Publisher paywall; see access note above.
→ This paper describes the famous Curaçao field trial—the first large-scale demonstration that releasing sterile flies could eliminate an entire screwworm population. It remains one of the defining moments in the history of biological control.
Baumhover AH. 1997. A Personal Account of Programs to Eradicate the Screwworm. Florida Entomological Society Website.
🌐 Free online resource
→ Written by one of the scientists who helped carry out the original screwworm eradication campaigns, this memoir combines history with firsthand stories from the laboratory and the field. It offers a fascinating behind-the-scenes look at the challenges, improvisation, and determination that turned one of the boldest ideas in public health into reality.
Wyss JH. 2006. Screwworm Eradication in the Americas. Annals of the New York Academy of Sciences.
🔒 Subscription required but you can request a copy from the author through ResearchGate:
https://www.researchgate.net/publication/227832046_Screwworm_Eradication_in_the_Americas or this link if you have access:
→ A fascinating account of how the eradication campaign expanded from the United States through Mexico and Central America. It highlights the decades of international cooperation, logistics, and sustained investment required to build one of the most successful eradication programs ever attempted.
U.S. Department of Agriculture, Animal and Plant Health Inspection Service (USDA APHIS). New World Screwworm Eradication Program.
🌐 Free online resource
→ This resource explains how sterile flies are produced and released, why the biological barrier was established in Panama, and how surveillance and international cooperation continue to protect North America.
National Cattlemen's Beef Association. New World Screwworm Resources and Policy Statements.
🌐 Free online resource
→ These statements provide the perspective of U.S. cattle producers, documenting industry concerns about the northward spread of screwworm and calls for expanded surveillance and increased sterile fly production capacity. They offer an important reminder that eradication is not only a scientific achievement but also an economic one.
Texas Animal Health Commission. New World Screwworm Information for Livestock Owners and Veterinarians.
🌐 Free online resource
→ This practical resource explains what producers and veterinarians should look for, how to report suspected cases, and why rapid detection remains one of the most important tools for preventing screwworm from becoming reestablished.
The National Sustainable Agriculture Coalition (NSAC) Blog
🌐 Free online resource
→ This analysis uses federal personnel data to document major USDA staffing losses between January 2025 and January 2026, including reductions across USDA agencies and state-level offices. It is useful background for understanding how agricultural response capacity depends not only on emergency funding, but also on experienced people in place before a crisis begins.
Ward O. 2025. Bird flu, screwworm monitoring among foreign aid programs killed by Trump. Agri-Pulse.
🌐 Free online resource
→ This Agri-Pulse report documents the termination of U.S.-funded FAO animal disease monitoring programs, including projects focused on avian flu and New World screwworm in Central America. It helps explain how international surveillance and containment programs can affect the United States long before a parasite or pathogen reaches the border.
Informative and enjoyable read!