A fly that was once wiped out in the U.S. is back. Screwworm flies return in Mexico and pose a risk to U.S. farms. If untreated, animals die in weeks from flesh-eating maggots.
DiGiFX Media from Pixabay
SNS - The hum of the cargo plane’s engines was steady but distant, drowned beneath the weight of anticipation. Dr. Lena Mireles leaned against the cool fuselage, eyes fixed on the pale glow of morning rising over the Gulf of Mexico. Below them, a swath of farmland, scrub brush, and winding rivers awaited the release. Behind her, row after row of aluminum canisters held billions of sterile male flies — tiny, winged soldiers bred in a lab, irradiated, and readied for war against a flesh-eating parasite that once again threatened to crawl northward. She tapped her tablet, reviewing the flight path as the countdown ticked closer. In just minutes, the belly doors of the aircraft would open, scattering the living payload across the borderlands of southern Texas and northern Mexico. The plan was simple, almost elegant: drown the wild screwworm population in a tide of infertile mates. But Lena knew it wouldn’t feel elegant if they failed. The New World screwworm was already burrowing into livestock flesh in Chiapas and Campeche. If it crossed into U.S. herds, the economic and ecological damage would take decades to undo. The cabin lights dimmed as the pilot radioed clearance. Lena stepped closer to the viewing port, watching the earth spin slowly beneath them. It was strange, she thought, to fight something so ancient with something so engineered. The flies would be gone in days, their work done in silence. No guns, no poison — only radiation, instinct, and time. Yet the stakes couldn’t be higher. This wasn’t just pest control. This was containment. Survival. A race between biology and biotechnology, she was flying at 12,000 feet over the front line yet again. This sounds like a scene from a made-for-Netflix science-fiction movie, right? Actually, billions of irradiated male flies will soon rain from airplanes over southern Texas and northern Mexico as the U.S. government accelerates efforts to contain the alarming resurgence of the New World screwworm — a parasitic fly species that threatens livestock, wildlife, and food security across North America. This scenerio might actually happen in the years ahead. The plan, announced this week by the U.S. Department of Agriculture (USDA), represents the latest escalation in a long-running battle against a pest that was once eradicated from the United States but has now breached containment lines and advanced to within 500 miles of the U.S.-Mexico border. The U.S. government is preparing to drop billions of flies from airplanes over southern Texas and northern Mexico to stop the screwworm. This flesh-eating fly lays eggs in animals' wounds, and its maggots eat living flesh. If not treated, the infestation can be fatal in just two weeks.
Photo: Babs Müller/Pixabay
The U.S. Department of Agriculture (USDA) is using this strategy to stop the insect from spreading into the United States. The pest was once eliminated from North America, but in recent years, it has returned, moving north through Central America and into Mexico. The economic and health risks are growing, especially in Texas, where cattle populations are the highest in the country. Officials are increasing efforts to contain the spread before it reaches U.S. herds.
What is the New World Screwworm?
The New World screwworm (Cochliomyia hominivorax) is a type of fly that attacks warm-blooded animals. Female flies lay eggs in open wounds or body openings. When the eggs hatch, the maggots dig into living tissue. As more larvae grow and feed, the wounds get larger and deeper. Untreated, this infestation can lead to death.
This parasite affects livestock, pets, wildlife, and people. Animals that have given birth, had surgery, or have open wounds are most at risk.
The adult fly is slightly larger than a housefly, with orange eyes, a metallic blue-green body, and three dark stripes on its back. Maggots can often be seen in wounds, and animals may act restless, stop eating, or isolate themselves.
Eradicated before, but now it’s back
The U.S. removed screwworm from the country in 1966 using a process called the Sterile Insect Technique (SIT). This method involves releasing large numbers of male flies that have been sterilized using radiation. These sterile males mate with wild females, but no larvae hatch. Over time, this lowers the pest’s population.
SIT worked well for decades. A biological barrier was created in Panama to stop the screwworm from moving north again. But in 2023, that barrier was broken. Since then, screwworm has spread through Central America and into Mexico.
New scientific models show that screwworm is most likely to enter the U.S. through southern Mexico. Areas with warm climates and large livestock populations, such as Texas and Florida, are at the highest risk. The fly can travel up to 12 miles to find a host.
Cold weather limits its survival, but summer weather and the movement of animals or wildlife can carry the pest into new regions, including northern states.
Serious threat to farmers and the economy
Texas has about 12.5 million head of cattle — the largest number in the country. A USDA study estimated that a screwworm outbreak could cost Texas nearly $2 billion each year. This number includes lost livestock, lower meat and dairy production, higher veterinary costs, and labor shortages during an outbreak.
If the pest spreads to other states, the economic damage could rise even more. Past outbreaks, such as the one in 1976, required a large number of workers to manage. Today, there are fewer workers in agriculture, making it harder to handle a crisis.
The screwworm is also a threat to food supply chains and public health. The pest does not only harm farm animals — deer, wild hogs, pets, and even humans are at risk.
What the U.S. is doing now to fight back
To prevent an outbreak, the USDA is building a new sterile fly factory in southern Mexico, expected to open in July 2026. Until then, a fly distribution center in southern Texas will help deliver sterile flies from an existing factory in Panama.
Sterile flies will be dropped from airplanes over high-risk areas in Texas and Mexico. The goal is to stop wild screwworms from reproducing by filling the environment with sterile males. This method is safer for the environment than chemical spraying and only targets screwworms.
At the same time, Texas has begun forming state response teams to monitor and respond to new cases. These efforts are focused on protecting livestock and keeping the pest from crossing into U.S. herds.
Photo: Kylee Alons/Unsplash
Early signs and what to watch for
Farmers and veterinarians are key to spotting screwworm early. Watching animals closely is the best way to catch an infestation before it spreads.
Common signs include:
- Foul-smelling wounds with visible maggots
- Animals licking or biting at their wounds
- Lesions at dehorning or branding sites
- Unusual behavior such as restlessness or not eating
A Race against time
The reappearance of the New World screwworm shows how quickly old threats can return. While the USDA and its partners work to stop the pest, experts warn that control will take time and constant effort. Warmer months increase the risk of spread, and infected animals can quickly spread the larvae across large areas.
While the method of dropping sterile flies is proven and safe, it works slowly. It requires months - sometimes years - of regular releases to lower populations. But doing nothing is not an option. Without action, the pest could take hold again in the U.S., harming animals and causing long-term economic loss.
Stopping the screwworm now may save American farmers and ranchers billions in the future.
