U.S. Deploys Billions of Sterile Flies to Fight Flesh-Eating Pest

Washington, D.C. — The U.S. government is preparing to breed and release billions of sterilized flies over Mexico and southern Texas in a bid to combat a parasitic larva that consumes living flesh. Though it may sound like the plot of a horror film, the strategy is part of a federal effort to protect the country from an insect that could devastate livestock, decimate wildlife, and even kill pets.

A Proven Biological Control Method

The U.S. Department of Agriculture (USDA) is reviving a decades-old biological control tactic to halt the spread of the New World screwworm (Cochliomyia hominivorax), a larva that feeds on living tissue and poses risks to livestock, wild animals, and humans. The plan involves mass-rearing male flies, sterilizing them with radiation, and releasing them to mate with wild females. The resulting eggs fail to hatch, gradually reducing the population.

“It’s an exceptionally effective technology,” said Edwin Burgess, an adjunct professor at the University of Florida specializing in livestock parasites. “It’s one of the best scientific applications for solving a large-scale problem.”

The method, first deployed successfully in the mid-20th century, eradicated the screwworm north of Panama. Since then, a facility in Panama has produced sterile flies to contain the pest. However, late last year, infestations were detected again in southern Mexico, prompting renewed action.

Expansion of Sterile Fly Production

The USDA aims to open a new fly production facility in southern Mexico by July 2026. Additionally, a distribution center in southern Texas is planned before year’s end to import and distribute flies from Panama if needed. The agency has allocated $8.5 million for the Texas site and $21 million to repurpose a fruit fly facility in southern Mexico for screwworm production.

The Panama plant currently produces up to 117 million sterile flies weekly, but the USDA seeks to increase capacity to 400 million to meet demand.

A Deadly Threat to Livestock and Wildlife

Unlike most fly larvae, which feed on dead tissue, the screwworm consumes living flesh. Females lay eggs in open wounds or exposed mucous membranes, and the larvae burrow into the host.

“A 450-kilogram cow can die within two weeks,” warned Michael Bailey, president-elect of the American Veterinary Medical Association. Though treatments exist, infestations cause severe pain and debilitation.

Don Hineman, a retired cattle rancher from western Kansas, recalled the stench of infected livestock in his youth. “It smelled awful,” he said. “Like rotting meat.”

Challenges in Fly Rearing and Release

Raising billions of flies requires precise conditions. Cassandra Olds, an entomology professor at Kansas State University, explained that females must be induced to lay eggs, and larvae need a nutrient-rich diet. Past USDA research used horse meat and honey, while Panama’s facility feeds larvae a mix of powdered egg, beef blood cells, and plasma.

Releasing the flies also carries risks. Last month, a plane crash near the Mexico-Guatemala border killed three people during a sterile fly dispersal mission. Historically, flies were dropped from aircraft using specialized chutes or “Whiz Packer” machines.

A Cautionary Tale

Burgess praised the USDA’s mid-20th-century development of the technology as a landmark achievement. However, he and other experts warn against complacency.

“Something we think is completely under control—and declare victory over—can resurface at any time,” Burgess cautioned.

The screwworm, a tropical species, cannot survive cold winters but remains a seasonal threat in warmer regions. Between 1962 and 1975, the U.S. and Mexico released over 94 billion sterile flies to eradicate it. The strategy hinges on releasing enough sterile males to outcompete wild ones, aided by the fact that females mate only once in their short adult lives.

Border Measures and Ongoing Vigilance

In May, U.S. authorities temporarily closed the southern border to live cattle, horse, and bison imports due to screwworm concerns. Full reopening is not expected before mid-September.

The USDA emphasizes that sterile fly releases remain the most effective and environmentally friendly solution, far preferable to pesticides. Yet experts stress that long-term vigilance—and sustained production capacity—are essential to prevent future outbreaks.