'Flying blind': A livestock-killing pest is set to make a comeback — thanks to Trump

To a throng of goats foraging in a remote expanse of Sanibel Island, Florida, the low whir of a plane flying overhead was perhaps the only warning of what was to come. As it passed, the specially modified plane dropped scores of parasitic New World screwworm flies through an elongated chute onto the herd.

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Then the plane’s whir gave way to the swarm’s buzz. It was 1952, and the U.S. Department of Agriculture was conducting a series of field tests with male screwworm flies that had been sterilized with gamma radiation. The experiment’s aim was to get them to mate with their female counterparts, reduce the species’ ability to reproduce, and gradually shrink the population — and its screw-shaped larvae’s propensity to burrow into living mammals before swiftly killing their host — into oblivion.

It didn’t fully work, but the population did diminish. So the team of scientists tried again; this time in an even more remote location — Curaçao, an island in the Dutch Caribbean. That quickly proved to be successful, a welcome development after a decades-long battle by scientists, farmers, and government officials against the fly, which was costing the U.S. economy millions annually and endangering colossal numbers of livestock, wildlife, and even the occasional human. Within months, the screwworm population on Curaçao fell, and the tactic would be replicated at scale.

The USDA took its extermination campaign first throughout much of the south, and then all the way west to California. From then on, planes loaded with billions of sterilized insects were also routinely flown over Mexico and Central America. By the 1970s, most traces of the screwworm had vanished from the U.S., and by the early 1990s, it had all but disappeared from across the southern border and throughout the southernmost region of North America.

Since 1994, the USDA has partnered with the Panamanian government to control and wipe out established populations all the way down to the country’s southeastern Darién province, where the Comisión Panamá–Estados Unidos para la Erradicación y Prevención del Gusano Barrenador del Ganado, or COPEG, now maintains what’s colloquially called the “great American worm wall.” Each week, millions of sterilized screwworms bred in a nearby production facility are dropped by plane over the rainforest along the Panama-Colombia border — an invisible screwworm biological barrier zone, complete with round-the-clock human-operated checkpoints and inspections. But questions are now surfacing about its efficacy.

The pest is attracted to open wounds as small as tick bites and mucous membranes, such as nasal passages, where the female fly lays her eggs. A single female can lay up to 300 eggs at a time, and has the capacity to produce thousands during her short lifespan. Those eggs then hatch into larvae that burrow into the host animals with sharp mouth hooks and feed on living flesh.

To save the host, the larvae must be removed from the infested tissue. Otherwise the infestation can cause serious harm, and can even be fatal within a matter of days.

Female flies generally mate only once in their lifespan, but can continuously lay more than one batch of eggs every few days, which is why the sterile-insect technique has long been considered a fail-safe tactic for wiping out populations, when accompanied by surveillance and host treatment and quarantine. The best way to prevent infestation, according to the Centers for Disease Control and Prevention, is to avoid exposure.

About 20 years after the “worm wall” was created, the screwworm was spotted in the Florida Keys, the first sighting in the Sunshine State since the 1960s. An endangered deer population in Big Pine Key was discovered with the telltale symptoms of gaping wounds and erratic, pained behavior. The USDA responded rapidly, deploying hordes of sterilized flies, setting up fly traps in affected areas, and euthanizing deer with advanced infections. In totality, the parasite killed more than 130 Key deer, a population estimated at less than 1,000 before the outbreak. Though the threat was contained by the following year, the incident stoked concerns throughout the country.

No one really knows why the worm wall has started to fail. Some believe that human-related activities, such as increasing cattle movements and agricultural expansion, have allowed the flies to breach the barrier that, until recently, had been highly effective at curbing the insect’s range expansion. Max Scott, professor of entomology and genetics at North Carolina State University, researches strains of livestock pests for genetic control programs, with a focus on the screwworm.

“Why did it break down after being successful for so long? That’s the million-dollar question,” said Scott.

Bridget Baker, a veterinarian and research assistant professor at the University of Florida Institute of Food and Agricultural Sciences, thinks climate change may have had something to do with the screwworm’s sudden reappearance in the Florida Keys. “There was a major storm just prior to the outbreak. So the question is, were flies blown up from Cuba, for example, into the Florida Keys from that storm?” said Baker. Though invasive in the U.S., the screwworm is endemic in Cuba, South America, Haiti, and the Dominican Republic.

“And if there’s more major storms, could that potentially lead to more of these upward trajectories of the fly?” she added. “With climate change, all sorts of species are expected to have range shifts, and so it would be reasonable to assume that the flies could also experience those range shifts. And those range shifts are expected to come higher in latitude.”

In the past few years, we may have seen just that happen. In 2023, an explosive screwworm outbreak occurred in Panama — the recorded cases in the country shot up from an average of 25 cases annually to more than 6,500. Later that year, an infected cow was found in southern Mexico not far from the border of Guatemala. In response, last November, the USDA halted Mexico’s livestock imports from entering Texas and increased deployments of sterile screwworm males south of the border. Early this year, the suspension was lifted, after both nations agreed to enhanced inspection protocols.

Then, on May 11, the USDA suspended live cattle, horse, and bison imports from Mexico yet again. The fly had been spotted in remote farms in the Mexican states of Oaxaca and Veracruz, only 700 miles from the southern U.S. border. Experts worry it may just be on the verge of resurging in the U.S.

If the screwworm does regain its stronghold in the U.S., estimates suggest it will result in billions in livestock, trade, and ecological losses, and the costs of eradication will be steep. It could also take years to wipe out again, and decades for sectors like the cattle industry to recover. But with President Donald Trump’s USDA overtly refusing to acknowledge climate change or fund climate solutions, and federal cuts resulting in a skeleton agency to tackle the issue, any attempts to halt the range expansion of the fly may ultimately be doomed.

In a press release about the temporary ban, the USDA noted that it would be renewed “on a month-by-month basis, until a significant window of containment is achieved.”

“This is not about politics or punishment of Mexico, rather it is about food and animal safety,” stated Agriculture Secretary Brooke Rollins, who previously criticized Mexico for imposing restrictions on a USDA contractor conducting “high-volume precision aerial releases” of sterilized flies in its southern region.

New Mexico Senator Ben Ray Luján, a Democrat and member of the Senate Committee on Agriculture, Nutrition, and Forestry, co-sponsored the STOP Screwworms Act, a bill introduced to the Senate on May 14 that would authorize $300 million for the USDA to begin construction on a new sterile fly production facility.

“It is vital that Congress act to pass this legislation to protect our farmers and ranchers and prevent an outbreak in the U.S.,” Luján told Grist. When asked about the absence of climate change in the USDA’s messaging about the screwworm, Luján said he’d “long fought to ensure our agricultural communities have the tools they need to confront climate change and its growing impact on farmers and ranchers. Unfortunately, this administration does not share those priorities.”

The bill has bipartisan support, but another major concern is the USDA’s shrinking capacity to contain the screwworm threat. As part of an effort by the administration to gut spending across most federal agencies, the USDA has cut more than 15,000 staffers since January, leaving behind a skeleton workforce. Several hundred were employees at the Animal and Plant Health Inspection Service who were working to prevent invasive pest and disease outbreaks. The budget reconciliation bill currently making its way through Congress includes proposals to further cut USDA spending and gut the agency’s research arms.

A spokesperson for the USDA declined to comment for this article, and did not respond to Grist’s questions about the role of climate change in escalating the screwworm expansion risk.

Andrew Paul Gutierrez, professor emeritus at University of California, Berkeley, has been investigating the relationship between invasive pests and weather since the 1970s. In 2014, he found that the screwworm moves northward to new regions on anticyclonic winds, or a high-pressure weather system, which scientists believe warming may be affecting — leading to prolonged and more intense heatwaves and shifting wind patterns.

Before it was widely eradicated, the screwworm had been considered somewhat of a seasonal problem in more northern climates where it wasn’t endemic, as it was routinely killed off by freezing temperatures. Though the metallic green-blue fly thrives in tropical temperatures, it doesn’t tend to survive in conditions lower than 45 degrees Fahrenheit, though the movement of livestock and wildlife has shown that colder spells aren’t a silver bullet. As the planet heats up, rising temperatures are creating more favorable conditions for a legion of agricultural pests, like the parasitic fly, to spread and thrive.

Thirty-year average coldest temperatures are rising almost everywhere in the U.S., a new Climate Central analysis found. Future climatic modeling predicts those average temperatures will only continue to climb — further influencing which plants and insects thrive and where across the country.

“With climate change … if it becomes warm enough, and you can get permanent establishment in those areas, then we got a problem,” said Gutierrez.

As the USDA attempts to skirt the role of climate change and weather dynamics in escalating the threat, Gutierrez questions whether the agency’s response and longer-term plan to combat the threat from screwworm flies is destined to fall short. The agency’s response is missing what Gutierrez designates “really critical” insight into how screwworms interact with temperature conditions, and what climate-induced shifts in those means for its survival and reproduction.

The USDA, said Gutierrez, “spends an awful lot of money” on dealing with the screwworm issue, but he argues that is being hindered by a lack of understanding of the weather-pest-biology relationship, or how weather drives the dynamics of such a species. “And if you don’t know that, then you can’t, say, model the interaction of the invasive species and its natural enemies, or the effects of weather on the invasive species itself,” he said.

“Without that kind of platform, you’re kind of flying blind.”

This article originally appeared in Grist at https://grist.org/food-and-agriculture/trump-climate-denial-screwworm-fly-make-comeback/.

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