When an Old Problem Returns: Lessons from the Screwworm
There are moments when being an old man with old books becomes surprisingly useful.
There are moments when being an old man with old books becomes surprisingly useful.
Over the last few weeks, I have found myself pulling some of my older worn volumes from my shelves from the 1950’s and 1970’s. Books I inherited from a former County Extension Agent when he retired, and he in turn inherited from a retiring agent.
As concern grows about the return of the New World screwworm to North America, old references from the 1950s and 1970s suddenly feel less like history and more like field manuals.
As an entomologist, I spent most of my career teaching people about insects they could see. Fire ants. Mosquitoes. Termites. Caterpillars. The screwworm was different. By the time I entered the profession, the screwworm was already becoming history.
I even remember arguing with my insect taxonomy teacher about having to identify museum specimens of insects none of us would see in real life - boll weevils, bed bugs, and screwworms. I’ve apologized to him because he was right after all. I’ll never forget when a bed bug crossed my desk for the first time, and nobody would believe me, but that is another story.
Most Americans under the age of sixty have never seen a screwworm infestation. Most veterinarians have never treated one. Most physicians have never encountered a human case. That is precisely why the current situation deserves attention.
We are confronting a problem that previous generations knew intimately but that modern agriculture has largely forgotten.
More Than a Livestock Pest
The first lesson from the older literature is that screwworm is not simply an agricultural problem. It is a biological problem with agricultural, veterinary, wildlife, and medical consequences.
The New World screwworm, Cochliomyia hominivorax, is an obligate parasite. Unlike ordinary blow flies that feed on dead animals and decaying material, screwworm larvae develop in living tissue. The name itself means “man-eater.” That is not hyperbole.
Female flies seek wounds on warm-blooded animals and deposit eggs along the margins. The larvae hatch within hours and begin feeding on living flesh. A small scratch, tick bite, branding wound, dehorning wound, surgical incision, or the navel of a newborn calf may be sufficient to start an infestation.
Humans are susceptible as well. The older medical literature describes infestations involving wounds, the nose, ears, eyes, and throat. One, perhaps overly descriptive, account describes a patient whose infestation progressed into the tissues of the head despite repeated attempts at treatment.
The same biology that makes screwworm devastating to livestock makes it a genuine public health concern.
Why It Was So Economically Important
The economic impact of screwworm was staggering. In one Texas outbreak during the 1930s, more than a million livestock cases were reported. The direct losses were obvious:
Animal deaths
Reduced weight gain
Lower milk production
Veterinary expenses
Labor costs
But the indirect costs may have been even greater.
Routine livestock practices suddenly became dangerous: Branding, Dehorning, Castration, Ear tagging, Shearing. Even minor scratches from fences and brush. Everything that created a wound became a potential entry point for screwworm.
Reading the old recommendations feels surprisingly modern. Entomologists repeatedly stressed good animal husbandry, wound management, sanitation, and regular inspection. In today’s language, we would call this Integrated Pest Management, or IPM.
The concept existed long before the acronym.
The Biology Behind the Breakthrough
The most fascinating lesson from these old texts is how deeply the scientists understood the insect before they attempted eradication.
They knew several critical facts.
The female mates only once.
The insect has a relatively predictable life cycle.
Populations can be mass reared.
The pest depends on living hosts for reproduction.
Those observations led to one of the most remarkable breakthroughs in the history of entomology. Scientists discovered that radiation could sterilize male flies without preventing them from mating.
A female that mated with a sterile male would produce no offspring. Because she usually mates only once, her reproductive contribution to the next generation becomes zero. This insight became the foundation of the Sterile Insect Technique.
Millions of laboratory-reared sterile flies were released. Generation after generation, the wild population collapsed. For perhaps the first time in history, humans successfully used insect biology itself as the primary weapon against an insect pest.
One of Agriculture’s Great Eradication Success Stories
Agriculture has many examples of pest suppression. True eradication is rare.
The two major agricultural eradication efforts that immediately come to mind are the screwworm eradication program and the boll weevil eradication program. In both cases, success required decades of commitment and funding. Eradication is not an event, it is a process.
Both succeeded for similar reasons. Neither relied on a miracle pesticide. Neither depended on a single technological breakthrough.
Instead, both combined:
Deep biological knowledge
Long-term funding
Regional cooperation
Constant surveillance
Sustained political commitment
Those programs worked because they treated pests as population problems rather than simply killing insects one field or one ranch at a time.
Nature Never Signed a Surrender Document
The old entomologists understood something modern society often forgets. Eradication is not extinction.
The New World screwworm was declared eradicated from the United States in 1966 after one of the most successful insect control campaigns ever undertaken. Yet the fly continued to exist throughout portions of Latin America. Success depended on maintaining a biological barrier and an international surveillance network capable of detecting and responding to new incursions.
That system worked remarkably well for decades. Then the fly began moving north again.
In June 2026, USDA confirmed New World screwworm in a three-week-old calf in Zavala County, Texas—the first confirmed domestic detection in roughly sixty years. The larvae were recovered from an umbilical wound, exactly the type of injury older entomologists repeatedly identified as highly vulnerable to attack.
The biology had not changed. The insect had not evolved a new strategy. The lessons written in agricultural texts seventy years ago were still valid.
Why Is It Returning?
The popular question is why screwworm has suddenly become a problem again. The better question may be why it stayed away so long.
The answer lies in decades of surveillance, international cooperation, sterile insect releases, livestock inspections, movement restrictions, and rapid response programs. Those efforts pushed the fly south and maintained a barrier against reinvasion.
Recent reports suggest that illegal livestock trafficking through Central America, movement of infested animals, and the gradual breakdown of the geographic buffer that once contained the fly have all contributed to the current situation. Since 2023, the outbreak has advanced northward through Central America and Mexico before finally reaching Texas.
As of early June 2026, more than 171,700 animals and approximately 2,000 people had reportedly been affected across Central America and Mexico.
The lesson is straightforward. Global movement of animals and people is faster than at any point in human history. A pest does not need to migrate naturally when it can ride in a truck, trailer, aircraft, or shipping container.
The old books warned about livestock transport spreading infestations. The warning remains relevant.
The Forgotten Medical Story
Most news coverage focuses on cattle losses.That is understandable. But the older literature repeatedly emphasized that screwworm is also a human health problem.
Medical entomology texts describe infestations of wounds, eyes, ears, nasal passages, and other body openings. Historical outbreaks produced hundreds of human cases throughout the Americas. The current outbreak has affected not only livestock and wildlife but people as well, reminding us that screwworm has always been both a veterinary and medical concern.
Most physicians have never encountered a case. In a sense, medicine has suffered from the same success as agriculture. We solved the problem so effectively that entire generations lost firsthand experience with it.
What the Old Books Teach Us
The most important lesson from the old literature is not how to identify a screwworm. It is how to think about one.
The entomologists of the 1940s and 1950s understood that successful pest management begins with understanding biology. They knew the life cycle, the behavior, the mating system, the ecology, and the vulnerabilities of the insect.
That knowledge ultimately led to one of the greatest achievements in agricultural history: eradication through the Sterile Insect Technique. Today we have better genetics, better diagnostics, better communications, and more powerful computers than those scientists could have imagined, but the principles remain unchanged.
Know the biology.
Inspect animals.
Treat wounds.
Move livestock responsibly.
Maintain surveillance.
Act before an isolated infestation becomes a regional outbreak.
When an old problem returns after sixty years, it is good to have modern science. It is also good to have old books.
Because sometimes the future is already written in the past, and waiting patiently on a dusty shelf of a retired County Extension Agent.