Hundreds of millions of flies dropping from planes in the sky might sound like a horrible nightmare, but experts say such a swarm could be the livestock industry’s best defense against a flesh-eating threat poised to invade the southwestern border of the United States.
An outbreak of New World screwworms — the larval form of a type of fly that’s known to nest in the wounds of warm-blooded animals and slowly eat them alive — has been spreading across Central America since early 2023, with infestations recorded in Panama, Costa Rica, Nicaragua, Honduras, Guatemala, Belize and El Salvador. Most Central American countries hadn’t seen an outbreak in 20 years.
The fly reached southern Mexico in November, sparking concern among US agricultural industry officials and triggering the closure of several border-area cattle, horse and bison trading ports.
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It wouldn’t be the first time the US has had to battle these invasive bugs. The nation mostly eradicated the New World screwworm populations in the 1960s and 1970s by breeding sterilized males of the species and dispersing them from planes to mate with wild, female flies.
The strategy — essentially fighting flies with flies — slowly degraded the insects’ populations by preventing them from laying more eggs. Now, as the insects continue to spread north officials are hoping the approach could work again.
However, today only one facility in Panama breeds sterilized New World screwworms for dispersal, and hundreds of millions more sterile flies are needed to slow the outbreak, according to a June 17 letter from 80 US lawmakers.
The next day, the US Department of Agriculture announced plans to open a “fly factory” in a yet to be determined town near the Texas-Mexico border. But the process of defeating the screwworm may not be quick — or inexpensive.
Screwworm dangers
New World screwworms are the parasitic larva of a metallic blue blow fly species called Cochliomyia hominivorax. Unlike all other blow flies native to the Western Hemisphere, the New World screwworm feeds on the flesh of living animals, rather than dead ones, said Dr. Phillip Kaufman, a professor and head of the department of entomology at Texas A&M University.
The flesh-eating maggots go for most warm-blooded animals, including horses and cows.
They have also been known to infect domestic pets and even humans in rare cases, Kaufman said.
“After mating, the female fly finds a living host, lands on its wound, and will lay up to 200 to 300 eggs,” Kaufman explained. “After 12 to 24 hours, those eggs all hatch, and they immediately start burrowing and feeding on the tissue of that animal, causing very, very large wounds to form.”
After the larvae feed on the tissue with their sharp mouth hooks for several days, they drop from the animal and burrow into the ground to emerge later as fully grown adult flies, according to Thomas Lansford, the deputy executive director and assistant state veterinarian for the Texas Animal Health Commission.
Since the outbreak began in 2023, there have been more than 35,000 New World screwworm infestations reported, according to statistics listed on the Panama–United States Commission for the Eradication and Prevention of Screwworm Infestation in Livestock (COPEG) website. Of those cases, cows make up about 83% of the affected animal species.
Treatment for infested cattle often involves cleaning, antiseptic treatment and coverings for the wounds, Lansford said.
If left untreated, the flies can kill an animal in a matter of one to two weeks and spread to others, posing a threat to the livelihood of ranchers.
“It’s a daily chore to provide those inspections to our livestock, just to make sure they’re not infested,” said Stephen Diebel, a rancher and the first vice president of the Texas & Southwestern Cattle Raisers Association. “We know the incredible economic impact an infestation would cause.”
There are no known vaccinations or effective repellant methods to prevent infestation, Diebel said. Instead, during warmer months, ranchers should avoid branding, tagging and other procedures that create potential entry points for the screwworms in livestock, he recommended. The tropical fly is less active in the winter.
While regional cattle trading is thought to be a major way the fly populations travel, Diebel said infestations can also affect wildlife such as deer, birds and rodents, making surveillance of the parasite’s spread even more challenging.
Fighting flies with flies: How it works
Just like a caterpillar goes into a cocoon before becoming a butterfly, the New World screwworm becomes a black, pill-sized pupa before emerging as an adult fly, Kaufman explained.
In a sterile fly production facility, the pupae are subjected to high-energy gamma rays that break down the DNA of the males, damaging their sex chromosomes, according to the USDA. The result: impotent adult flies that cause female mates to lay unfertilized eggs.
The amount of radiation the male flies are exposed to does not pose a danger to animals or humans, according to the USDA. But since the female flies only mate one time in their short, 20-day lifespan, once populations are exposed to sterile males, the populations die out over the course of months or years, depending on the size of the outbreak.
While it is unclear how dispersal would work in the US in the event of an outbreak, Kaufman said the adult flies are typically loaded into temperature-controlled containers and dropped from planes. However, there’s no need to panic about the fly drops coming to a suburb near you, he said — they usually target sparsely populated rural areas, since the flies have no interest in urban environments.
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A pricey battle
At the COPEG facility, about 100 million sterilized flies are produced and dispersed aerially in affected regions each week.
Currently, the dispersal efforts have been focused in the southern regions of Mexico and throughout Central America, where cases of infestation have been reported, according to COPEG’s website.
The new US dispersal facility is expected to be located at the Moore Air Base in Hidalgo County, Texas, and to cost $8.5 million, per the release. The location and price tag of the production facility, or the “fly factory” itself, has not been revealed, but lawmakers estimate it could cost around $300 million.
In addition to the new sterile fly facilities, the USDA also announced $21 million plans to renovate an old fly factory in Mexico by late 2025.
While the plans are expensive, it’s a price worth paying to save the multibillion-dollar livestock industry, Diebel said.
‘When you offset the $300 million to the $10 billion of economic impact these flies would have, it’s an easy trade-off to understand,” Diebel said. “Having (a domestic production facility) here is super important … to control the distribution of those sterile flies more efficiently.”
Shortly after the June 18 announcement, the USDA shared plans to begin reopening livestock trading ports in Arizona, Texas and New Mexico that closed last year, citing “good progress” in surveillance and sterile fly dispersal efforts throughout Mexico.
COPEG did not immediately respond to request for comment on further details about the current progress of the US dispersal initiatives.
Source: https://www.cnn.com/2025/07/09/science/new-world-screwworms-outbreak-fly-factory
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