Government plans to fight flesh-eating maggot by breeding billions of flies

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Cattle crowd a feedlot in the southern U.S., where officials are working to prevent the spread of flesh-eating screwworm larvae — a parasite that could devastate livestock if not contained.

WASHINGTON - It may sound like science fiction, but the U.S. is once again turning to airborne fly drops to battle a dangerous flesh-eating parasite.

The Department of Agriculture is preparing to breed billions of sterilized flies and release them over parts of Mexico and southern Texas in an effort to stop the spread of the New World screwworm fly, whose larvae feed on live flesh and pose a serious threat to livestock, wildlife, and even humans.

The campaign revives a decades-old biological control method that once helped eradicate the pest from the U.S. using sterile insect technique—a form of pest control that disrupts breeding without the use of pesticides.

Why is the U.S. releasing flies from planes?

The backstory:

The New World screwworm fly lays its eggs in open wounds, where the larvae hatch and begin consuming live tissue. The infestation can prove deadly to cattle and other animals within weeks.

Rather than use chemical sprays, the U.S. will rely on the release of sterile male flies that mate with wild females. Because females only mate once in their lifetimes, the resulting eggs remain unfertilized and the population declines.

"It’s an exceptionally good technology," Edwin Burgess, a University of Florida assistant professor who studies animal parasites, told the Associated Press. "It’s an all-time great in terms of translating science to solve some kind of large problem."

The same technique was used successfully in the 1960s and 70s, when the U.S. and Mexico bred and released more than 94 billion sterile flies to eliminate the screwworm north of Panama.

Where will the fly factories be and how will it work?

What we know:

Officials have laid out a plan to scale up fly breeding and expand drop operations to prevent a major outbreak:

• A new fly factory in southern Mexico is expected to open by July 2026.
• A distribution center in southern Texas could be operational by the end of this year.
• The USDA will spend $29.5 million to expand fly breeding capacity to 400 million flies per week.

What we don't know:

Several aspects of the operation are still unclear or in development:

• Whether the current Panama facility can keep up with demand if U.S. sites are delayed.
• What specific areas of southern Texas will be prioritized for drops.
• If additional funding will be required long term to maintain control over the pest.

What makes the screwworm fly so dangerous?

Why you should care:

Most fly larvae feed on decaying matter, but the screwworm fly is different — it targets live animals.

"A thousand-pound bovine can be dead from this in two weeks," said Michael Bailey, president-elect of the American Veterinary Medicine Association. Infestations can also affect pets, wildlife, and even people, particularly in warm regions.

The fly was largely eradicated from the U.S. decades ago, but a reappearance in southern Mexico last year has raised alarm. In response, the U.S. temporarily closed its southern border to imports of live cattle, horses, and bison. The restrictions are expected to stay in place until at least mid-September.

How do you breed and drop billions of flies?

Big picture view:

Raising flies in massive numbers is a logistical feat. Breeding programs must mimic natural conditions to trigger egg-laying, then feed the larvae enough nutrients to develop.

In the past, USDA facilities used blends of egg powder, molasses, and cattle blood products. Mature larvae are placed in sawdust trays where they form cocoons and grow into adults.

Sterile flies are loaded into crates aboard planes and dropped from the air. The system is efficient — but not without risk. In June, a plane conducting a fly drop near the Mexico–Guatemala border crashed, killing three people.

To prevent accidents and ensure containment, facilities must take steps to keep fertile breeding stock from escaping.