The US plans to begin breeding billions of flies to fight a pest. Here
is how it will work
[July 02, 2025]
By JOHN HANNA
TOPEKA, Kan. (AP) — The U.S. government is preparing to breed billions
of flies and dump them out of airplanes over Mexico and southern Texas
to fight a flesh-eating maggot.
That sounds like the plot of a horror movie, but it is part of the
government's plans for protecting the U.S. from a bug that could
devastate its beef industry, decimate wildlife and even kill household
pets. This weird science has worked well before.
“It’s an exceptionally good technology,” said Edwin Burgess, an
assistant professor at the University of Florida who studies parasites
in animals, particularly livestock. “It’s an all-time great in terms of
translating science to solve some kind of large problem.”
The targeted pest is the flesh-eating larva of the New World Screwworm
fly. The U.S. Department of Agriculture plans to ramp up the breeding
and distribution of adult male flies — sterilizing them with radiation
before releasing them — so they can mate ineffectively with females and
over time cause the population to die out.
It is more effective and environmentally friendly than spraying the pest
into oblivion, and it is how the U.S. and other nations north of Panama
eradicated the same pest decades ago. Sterile flies from a factory in
Panama kept the flies contained there for years, but the pest appeared
in southern Mexico late last year.
The USDA expects a new screwworm fly factory to be up and running in
southern Mexico by July 2026. It plans to open a fly distribution center
in southern Texas by the end of the year so that it can import and
distribute flies from Panama if necessary.
Fly feeds on live flesh
Most fly larvae feed on dead flesh, making the New World screwworm fly
and its Old World counterpart in Asia and Africa outliers — and for the
American beef industry, a serious threat. Females lay their eggs in
wounds and, sometimes, exposed mucus.
“A thousand-pound bovine can be dead from this in two weeks,” said
Michael Bailey, president elect of the American Veterinary Medicine
Association.
Veterinarians have effective treatments for infested animals, but an
infestation can still be unpleasant — and cripple an animal with pain.
Don Hineman, a retired western Kansas rancher, recalled infected cattle
as a youngster on his family's farm.
“It smelled nasty,” he said. “Like rotting meat.”
How scientists will use the fly's biology against it
The New World screwworm fly is a tropical species, unable to survive
Midwestern or Great Plains winters, so it was a seasonal scourge. Still,
the U.S. and Mexico bred and released more than 94 billion sterile flies
from 1962 through 1975 to eradicate the pest, according to the USDA.
The numbers need to be large enough that females in the wild can't help
but hook up with sterile males for mating.
One biological trait gives fly fighters a crucial wing up: Females mate
only once in their weekslong adult lives.
Why the US wants to breed more flies
Alarmed about the fly's migration north, the U.S. temporarily closed its
southern border in May to imports of live cattle, horses and bison and
it won't be fully open again at least until mid-September.
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In this Jan. 2024 photo provided by The Panama-United States
Commission for the Eradication and Prevention of Cattle Screwworms (COPEG),
a worker drops New World screwworm fly larvae into a tray at a
facility that breeds sterile flies in Pacora, Panama. (COPEG via AP)
 But female flies can lay their eggs
in wounds on any warm-blooded animal, and that includes humans.
Decades ago, the U.S. had fly factories in Florida and Texas, but
they closed as the pest was eradicated.
The Panama fly factory can breed up to 117 million a week, but the
USDA wants the capacity to breed at least 400 million a week. It
plans to spend $8.5 million on the Texas site and $21 million to
convert a facility in southern Mexico for breeding sterile fruit
flies into one for screwworm flies.
How to raise hundreds of millions of flies
In one sense, raising a large colony of flies is relatively easy,
said Cassandra Olds, an assistant professor of entomology at Kansas
State University.
But, she added, “You’ve got to give the female the cues that she
needs to lay her eggs, and then the larvae have to have enough
nutrients."
Fly factories once fed larvae horse meat and honey and then moved to
a mix of dried eggs and either honey or molasses, according to past
USDA research. Later, the Panama factory used a mix that included
egg powder and red blood cells and plasma from cattle.
In the wild, larvae ready for the equivalent of a butterfly’s cocoon
stage drop off their hosts and onto the ground, burrow just below
the surface and grow to adulthood inside a protective casing making
them resemble a dark brown Tic Tac mint. In the Panama factory,
workers drop them into trays of sawdust.
Security is an issue. Sonja Swiger, an entomologist with Texas A&M
University’s Extension Service, said a breeding facility must
prevent any fertile adults kept for breeding stock from escaping.
How to drop flies from an airplane
Dropping flies from the air can be dangerous. Last month, a plane
freeing sterile flies crashed near Mexico’s border with Guatemala,
killing three people.
In test runs in the 1950s, according to the USDA, scientists put the
flies in paper cups and then dropped the cups out of planes using
special chutes. Later, they loaded them into boxes with a machine
known as a “Whiz Packer.”
The method is still much the same: Light planes with crates of flies
drop those crates.
Burgess called the development of sterile fly breeding and
distribution in the 1950s and 1960s one of the USDA’s “crowning
achievements.”
Some agriculture officials argue now that new factories shouldn’t be
shuttered after another successful fight.
“Something we think we have complete control over — and we have
declared a triumph and victory over — can always rear its ugly head
again,” Burgess said.
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