The most grotesque legacy of Joe Biden’s border policy may be a fly. The migration surge his administration helped unleash overwhelmed governments across Central America, enriched the cartels engaging in human and livestock trafficking, and weakened the systems meant to prevent the illegal movement of animals. As caravans of illegal immigrants bound for the United States’ southern border pressed northwards, so did the New World screwworm, a flesh-eating parasite once confined south of the Darién Gap. On June 3, 2026, after more than four decades of effective exclusion, screwworm was detected in Texas livestock.
The New World screwworm, Cochliomyia hominivorax (hominivorax meaning “devourer of men”), is one of the most destructive parasites in the hemisphere. As an adult, it is an unremarkable-looking fly, not much different from the common housefly. But when the time comes for a screwworm fly to reproduce, it must find a host. Female screwworm flies lay their eggs by the hundreds in open wounds on virtually any warm-blooded creature. The wound in question can be minuscule; a single tick bite is enough to attract the attention of screwworm flies.
Less than 24 hours after screwworm eggs have been deposited on their unfortunate victim, the larvae hatch and begin their work of destruction. Screwworms are born with sharp, hooked teeth—huge in proportion to the small size of their bearers—and use these mandibles to gorge on the flesh of their hosts. When disturbed, the larvae “screw” in, burrowing deeper into the wound to avoid being dislodged or removed, a grisly maneuver that earned the species its common name. Severe infestations of screwworm are nearly always fatal, but even smaller infestations in inopportune places—the belly, for example, or the head—can quickly kill the host.
For most of the United States’ history, screwworm was a significant hazard for ranchers and farmers in the American South. The parasite, which thrives in warm weather, was endemic to the Southwestern United States—Texas, Arizona, and California, specifically—and to Florida, but seasonal infestations of screwworm could spread widely through the rest of the South during the warmer months. The fly could travel still more widely, and devastating outbreaks sometimes occurred in unexpected areas when infested animals were moved to new locations. At times, screwworm infestations reached as far north as South Dakota.
Cattle are particularly susceptible to screwworm infestation. They are largely defenseless against the fly, and normal ranching practices like branding and dehorning leave many wounds the parasite can exploit. During severe outbreaks, livestock died by the tens of thousands. In 1935, for example, the state of Texas recorded 3 million cases of screwworm that killed 180,000 head of cattle. (The following year, the U.S. Department of Agriculture estimated that screwworms cost the cattle industry $10 million annually—nearly $240 million in today’s dollars.)
But while screwworm was a major issue for Americans in 1936, few Americans in 2026 had heard of it before the most recent outbreak. This is because the parasite, technically speaking, has been a solved problem for over half a century; we have known how to eliminate screwworm populations since 1957, and the fly was completely eradicated from the United States in 1982. The U.S. government, in cooperation with Mexico and other Central American countries, pushed screwworm out of North America almost entirely over the course of the 20th century, driving it back to a small quarantine zone in southern Panama. With a few trivial exceptions, the U.S. has been free from screwworm for more than 40 years.
The process for screwworm eradication was first proposed in the 1930s by entomologist Edward F. Knipling, who theorized that the flies could be prevented from reproducing by introducing large numbers of sterilized male flies into the breeding population. Female screwworm flies would mate with sterile male flies and the resulting unfertilized eggs would never hatch. Repeat this process over several generations, and the flies would eventually die out.
It took some time for this to become practicable, but advances in science during and after the Second World War paved the way for screwworm eradication. Tests of the process in the early ‘50s showed that the process worked as Knipling had predicted, and the American cattle industry began clamoring for its widespread implementation. The state of Florida collaborated with USDA to begin an eradication program there in 1958, and when the program was successful it was expanded to begin eradication in the rest of the country.
The buildout required for the process was substantial. The U.S. government constructed facilities across the South to breed sterile flies by the millions. By 1967 the screwworm eradication program was blanketing an area over 1,000 miles long and 400 miles wide, while churning out over 6 billion sterile flies a year. The program worked well enough that in 1966 USDA declared the U.S. free of endemic screwworm, but outbreaks continued to occur, including a particularly severe one in 1972. Screwworm flies can lay eggs nearly 200 miles away from the site at which they emerge, and entomologists concluded that in order to eradicate screwworm in the U.S., it would have to be eradicated from Mexico as well.
The Mexican government proved happy to cooperate, and the U.S. and Mexico agreed to a joint extermination project in 1972. Sterile fly production was ramped up, including at a new facility built in southern Mexico, and the Joint Mexico–United States Screwworm Eradication Commission steadily eliminated the pest in northern Mexico throughout the 1970s. By 1984, screwworm had been eradicated from Mexico north of the Isthmus of Tehuantepec, the agreed-upon barrier zone, but it was decided that it would be most effective to continue the program and eliminate the parasite in the rest of Central America. The new barrier zone was set at the Darién Gap in Panama, a sparsely populated stretch of jungle just 60 miles wide that separates Central America from South America.
With cooperation from the various countries in the region, the U.S. continued its program of screwworm eradication, dropping as many as 600 million flies per week as it advanced the eradication sector southwards, and the fly was pushed back to the barrier zone in Panama by 2006. A new screwworm production facility was built there to create the flies needed to maintain the barrier, and the old plant operating in Mexico was shuttered. Screwworm, which by then had already been absent from the U.S. for decades, was considered an essentially solved problem, and the eradication program was turned over to the Panama–United States Commission for the Eradication and Prevention of Screwworm Infestation in Livestock (COPEG), which is responsible for monitoring livestock in the area and maintaining the barrier through the continued release of sterile flies.
All seemed well for nearly two decades; any minor outbreaks were quickly smothered by a massive release of sterile flies in the area. But warning signs started to go off in August 2022, when a few cases of screwworm were detected beyond the barrier zone of the Darién Gap. COPEG deployed additional sterile flies, but to no avail. By July 2023, screwworm had spread far past southern Panama into neighboring Costa Rica, and by 2024 infestations were reported throughout Central America and into Mexico, massively exceeding the capacity of COPEG to produce and disperse enough sterile flies to control the outbreak. The advance of the parasite was at that point impossible to stem, and by June 2026 screwworm was spreading in the U.S. as well.
Nobody knows exactly how the screwworm barrier failed. An array of contributing factors, including the growth of the livestock industry near Darién and power outages at the sterile fly production plant have been suggested. But one important explanation has received far too little attention: The unprecedented migration surge created by the Biden administration’s border policies.
The wave of illegal immigration produced by the Biden administration was unprecedented in the history of the United States, and it produced a similarly unprecedented surge of traffic through the Darién Gap: Between 2010 and 2019, the Panamanian National Migration Service recorded an average of 911 illegal transits of the Darién Gap per month. By 2021, average monthly crossings had risen to 11,136, and in August of 2022 a total of 31,055 illegal crossings were recorded—over 30 times more than the average month in the decade preceding 2019. The wave continued to accelerate into 2023, when over half a million people (520,085, to be precise) were recorded crossing the Darién Gap, an average of nearly 45,000 every month, most headed for the United States.
The pressure exerted by this flood of humanity rushing through the Isthmus of Panama is difficult to describe. The border and immigration authorities, in Panama and in other Central American countries along the route to the U.S., were completely swamped; normal inspection and questioning was rendered impossible by the sheer volume of people to be processed. (For Panama, a country with a population of 4.4 million people, dealing with 520,000 migrants would be proportionally equivalent to 40 million migrants crossing the United States’ border with Mexico.)
It is possible that this migration surge was directly responsible for the outbreak; screwworm flies will infest humans if given the opportunity, and the hundreds of thousands of people making the dangerous passage through Darién would have provided plenty of opportunities for the flies to encounter humans with nicks, bugbites, and other wounds. Many migrants also brought with them pets, especially dogs, which are common targets for the parasite.
But while immigrants and their pets may have contributed to some outbreaks of screwworm beyond the barrier, the most likely culprit was neither the migrants themselves nor the pets they brought with them. Rather, it was the trade in illegal cattle that the migrant surge facilitated, which is conducted principally by cartels and other organized criminal groups.
Cattle are the most attractive host for the screwworm fly, and are particularly vulnerable to large infestations. Historically, severe outbreaks of screwworm have frequently been caused by movement of infested cattle, which can quickly introduce large numbers of flies into a region. Cattle smuggling consistently moves animals northwards towards the U.S., the largest market for cattle and beef, providing an obvious vector for the spread of the parasite—cattle smugglers are not overly solicitous of the state of their livestock, and smuggled cattle are not subject to the normal inspections for disease and parasites, including screwworm.
Alexis Sandí, the former head of Costa Rica’s National Animal Health Service, explicitly identified cattle smuggling as the most likely source of the screwworm outbreak in Costa Rica in a July 2023 interview. “There is no way for the fly to arrive here by its natural flight,” Sandí noted. “It must have arrived with some animal. Our theory is that it could have arrived through animals that entered the country illegally, because it did not come by legal import.”
Cattle smuggling is not generally the kind of crime that comes to mind when discussing cartels, but the trade can be a very lucrative one. “Cattle trafficking from Central America to Mexico is a multimillion-dollar business,” InsightCrime’s report on the business noted. Cartels are more than happy to diversify away from their core business of drug trafficking where it makes sense, and both cattle and human trafficking were major opportunities for them during the huge migration surge of 2021–24. Human Rights Watch (HRW) reported that the Clan del Golfo, a major Colombian cartel, “regulates the routes that migrants and asylum seekers can use, decides who can assist them on the way, extorts people who benefit from migrant flows, and establishes rules of conduct for locals and migrants alike.” This is big business: HRW estimates that the cartel earned $57 million on human trafficking through Darién between January and October 2023 alone.
The migrant surge created by the Biden administration’s policies benefitted both human and cattle traffickers, making the monitoring of border crossings and the interception of trucks carrying contraband cattle far more difficult. The infrastructure used by human trafficking is essentially identical to that used for the illegal movement of cattle, and criminal organizations that expanded their operations and territorial control to take advantage of migrant flows could use the same guides, routes, contacts, and corrupt officials they used to illegally transport immigrants to smuggle livestock.
Most of the illicit cattle trade happens farther north in Central America, but livestock trafficking during the migration surge became a noted problem in Panama and Costa Rica as well. In April 2022, Panamanian legislator Miguel Fanovich, the second vice president of the country’s Chamber of Deputies, excoriated the director of Panama’s National Customs Authority for the proliferation of cattle smuggling across the Panama–Costa Rica border. Fanovich raised the issue again the next year, complaining in April 2023 that “cows, calves, and even bulls” were being illegally trafficked into Costa Rica. The situation was severe enough that in June 2023 TVN-Panama reported that Panamanian butchers in the province of Chiriquí, which borders Costa Rica, were complaining that illegal cattle smuggling from Panama to Costa Rica was causing a shortage and driving up consumer prices for beef.
Additional evidence that the migrant surge drove increases in illegal cattle trafficking shows up in Panama’s crime statistics. According to the Panamanian Ministry of Public Security, livestock theft jumped by 40 percent between 2020 and 2023, with over 1000 cases of cattle theft reported in 2023. Cattle theft is typically carried out by organized criminal groups, and most of the stolen livestock was almost certainly smuggled out of the country to be sold in a more friendly jurisdiction. Some may have made it as far as Mexico, where hundreds of thousands of head of cattle are smuggled in every year to take advantage of the massive American demand for imported Mexican beef—potentially carrying screwworm along with them.
With the election of President Donald Trump in 2024, the migrant surge came to a sudden halt. Monthly traffic through the Darién Gap, which had reached the tens of thousands in 2023, has cratered to double digits; Panama recorded just 43 illegal crossings in both March and April 2026. But while illegal immigration and many of its attendant troubles have largely ceased, the screwworm outbreak they contributed to has not. New, active cases of screwworm continue to occur in the U.S., although currently the outbreak remains small and manageable (on July 21, at time of writing, there were 12 active cases in the U.S., all located in Texas).
Phillip Kaufman, head of the Department of Entomology at Texas A&M University, told The American Conservative that “the cases that we’re seeing in Texas now do indicate that we have populations of the adult fly in the state that are laying the eggs that are being found in these infested animals, so we do indeed have active screwworms in Texas.”
The stakes of a larger, uncontained outbreak are significant: USDA has stated that screwworm “threatens over $100 billion in United States economic activity tied to the cattle and livestock industry alone.”
To prevent such an outbreak, the U.S. government has taken dramatic steps to halt the spread of screwworm in the U.S. The Biden administration prohibited any imports of live cattle from Mexico in November 2024, a restriction that was briefly relaxed by the new Trump administration but was enacted again in July 2025. The measure is politically costly—normally the U.S. imports over a million head of cattle every year from Mexico, and the sudden constraint on supply has helped push up beef prices, enraging consumers. But the restrictions have so far been effective in preventing any large outbreaks of screwworm.
Starting in December 2023, USDA’s Animal and Plant Health Inspection Service (APHIS) began activating hundreds of millions of dollars of emergency funding to stem the outbreak, much of which has gone into ramping up the production of sterile flies. Currently, the U.S. does not have enough sterile fly production to completely halt the spread of the screwworm fly. The production facility in Panama is producing 100 million sterile flies per week, all of which are being used by APHIS to control the situation on the border. That number is far from sufficient for eradication, however. Earlier this year, the U.S. began construction of a major sterile fly production facility at Moore Air Force Base in Texas and has also converted a facility in Metapa, Mexico. By mid-2027, when these plants are working at full capacity, the U.S. should be able to disperse 500 million flies per week, as many as the eradication program was dispersing at its height in the 1980s.
USDA is even trialing a new strain of fly that could potentially double the productivity of its current facilities. The breed, called NovoFly, is engineered to enable production facilities to raise only the male flies that contribute to screwworm eradication, allowing them to use the same resources to raise twice as many flies.
Working with state agricultural agencies, APHIS has established monitoring, reporting, and animal-movement controls designed to detect infestations quickly and prevent them from spreading. “Historically, this fly moved across Texas because humans moved animals that had infestations,” Kaufman said. “We didn’t really understand that back in the 1960s, and so we were our own worst enemy, moving livestock around the state without recognizing that we were inadvertently moving the flies to parts of Texas that they may never have reached.” To avoid repeating that mistake, officials saturate areas surrounding confirmed cases with sterile flies, while ranchers in at-risk regions must have their livestock inspected by state experts before moving them.
If the spread of screwworm in the U.S. remains under control, APHIS will begin pushing screwworm southward back to the barrier region of the Darién Gap once the new sterile fly production facilities come online. The process will probably take years, but should be done much more quickly than the original eradication program. “We continue to leverage all tools at our disposal to tackle this issue,” a spokesperson for USDA told TAC. “USDA has previously eradicated screwworm from the United States using a proven combination of surveillance, animal movement controls, sterile insect releases, and public outreach.”
Kaufman believes that the United States’ response has been a success. “We are in much better shape this time than we were 44 years ago when we last had the fly. We have better technology. We have better pesticides. We have a better understanding of the fly’s biology, we have better modeling. We have genetic technologies like the NovoFly,” he said. “We’re only at 37 cases, and this fly has been in Texas for about almost six weeks. In the past, we would have seen a lot more cases.”
That’s not to say there aren’t still risks that could result in a severe outbreak in the U.S. if things go awry.
Bill Bullard, CEO of R-CALF USA, a cattleman’s trade association, believes that the most significant threat to the U.S. is political. “The greatest concern is that we continue to reintroduce the screwworm from Mexico,” he told TAC. “If the border were to be opened, there would be a huge incentive for cattle to move northward in anticipation of entering the U.S. market. Mexico needs to completely eradicate the pests from within its entire country before the United States begins to relax any of the mitigation measures that are currently in place, including the closed border.”
And there are powerful people pressuring the Trump administration to do just that. On July 16, POLITICO published an article titled “Trump’s rancher allies push Rollins to reopen border to cattle,” noting that sections of the cattle industry are lobbying Agriculture Secretary Brooke Rollins to end prohibitions on cattle imports. Many feedlot operators and meatpackers rely heavily on Mexican cattle, and they are eager for the border to open up, screwworm or no screwworm.
A major weather event, such as a hurricane, could also disrupt APHIS’s monitoring and inspection process and make screwworm dispersal impossible for a time, creating conditions that could lead to the parasite’s spread. But, barring any disasters natural or political in the next 12 months, it appears that the U.S. will be able to prevent any major infestation and push the screwworm back down to Panama.
The current battle with screwworm is one the United States should never have needed to fight. The parasite had already been eradicated from North and Central America, and was confined to the Darién Gap for nearly two decades. With greater foresight, it could likely have remained there. Instead, chronic underinvestment and reckless policy decisions combined to teach the United States two costly lessons.
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First, immigration policy matters far beyond the U.S. border. The Biden administration’s decision to throw open the border produced a massive windfall for cartels and other organized criminals, which exploited the resulting migration surge to accumulate money, infrastructure, and control over the routes used to move people, contraband, and cattle. The chaos created by that surge helped turn a containable incursion of screwworm into a regional outbreak.
Second, prevention is much cheaper and more effective than response. Maintaining the biological barrier in Panama cost about $15 million a year—a pittance beside the expense of eradicating screwworm again. But the program’s success bred complacency. Sterile-fly production was consolidated in a single facility designed to maintain the barrier and reserve capacity disappeared. When the outbreak began spreading, the program could not produce enough sterile flies to stop its advance northward.
The United States knows how to defeat screwworm, and it will eradicate the parasite again. But we should ensure we don’t need to do so a third time.
