A billion flies are now in the screwworm fight. Will there be enough?
By Jen Christensen, CNN
(CNN) — A lot is riding on the shiny green backs of large red-eyed sterile flies that scientists from the United States, Mexico and Panama have been raising.
The US government just released its billionth fly into Texas from a facility that opened in June in Chiapas, Mexico, near the Guatemalan border, and its sister operation in Panama in order to fight a pest that could potentially cause $2.5 billion dollars in damage in the Southwest US alone.
The New World screwworm lays eggs on warm-blooded animals – which can include humans – and its larvae screw themselves into the living flesh, leading to infections and sometimes death. While not considered a food safety issue, it can be a food supply problem.
Screwworm had been considered eradicated from the US, Mexico and most of Central America decades ago. In 2023, to the frustration of cattlemen and officials alike, the flesh-eating fly broke through containment lines in the Darien Gap, a stretch of dense rainforest between Colombia and Panama, and spread north.
Governments in the region launched a massive response, including traps, sniffer dogs, drone surveillance and most importantly, sterile flies. Despite those efforts, New World screwworm infested thousands of animals in Panama and Mexico, and in June, the US Department of Agriculture confirmed the first US cases in years.
Decades ago, the creation and release of sterile flies to interrupt the pest’s breeding cycle was key to driving it out. But scientists said they’re worried the screwworm could threaten wildlife, pets and cattle — the country’s highest-value agricultural commodity — for years, largely because sterile flies can’t be produced fast enough.
USDA Secretary Brooke Rollins said she is not concerned about the pace of the administration’s efforts.
“The Trump Administration has moved at warp speed to ramp up sterile fly production across Texas, Mexico, and Panama to protect our Great American Cattle Herd,” Rollins said in an emailed statement last week. “Through this all of federal, state, local, and private rancher effort, we have been able to keep cases in America low with only 48 total – 30 in June, 14 in July, and just 4 cases in August. This is a far cry from the common narrative in early June that the herds would be decimated, wildlife would be destroyed and it would take a decade to recover. Right now, we only have three active cases, and not a single case in cattle in over a month.”
Potential for exponential growth
There have been fewer than 50 screwworm cases recorded in the US this year, according to the USDA. That may not sound like much, but entomologist Dr. Dave Taylor, who spent years developing methods to defeat New World screwworm with the USDA’s Agricultural Research Service, warned that the numbers probably won’t stay low.
Although the flies live for only about 21 days, an adult female may lay up to 3,000 eggs during her lifespan, and they will grow into adult flies in less than a week, according to the US Centers for Disease Control and Prevention.
There were only 473 confirmed cases in 1971. The next year, the US recorded nearly 100,000 cases.
“This is the reproductive potential of this fly,” Taylor said. “I’m not saying that’s going to happen this time, but there could be many, many, many more cases at this time next year.”
That is particularly true, he added, if ranchers don’t report all the cases they see in their animals to the USDA, or if cases go undetected in Texas’ wildlife population — a $31 billion industry.
Taylor, who was also former editor-in-chief of the Journal of Medical Entomology, died August 6 at the age of 71. When he spoke with CNN in July, he said he was frustrated, after 40 years of successful work keeping screwworm out of the country, “to see all that progress wiped out in three to four years.”
The quest for 500 million flies
The only way the US has succeeded in eradicating screwworm is by breeding millions of flies, sterilizing them using radiation and dropping them in areas with infestations. Wild female New World screwworms mate only once, so when sterile males mate with them and they don’t reproduce, the local population dies off.
Even before cases were identified in the United States, Rollins described the need for more flies.
“I went to the president, and I said, ‘This is going to be a massive problem because we need 400 million to 500 million flies per week to beat this back,’” Rollins said in June. “We only have one facility in our entire hemisphere, and it’s only producing 100 million flies per week.”
When the US initially beat back screwworm, it did so by releasing 500 million sterile flies per week in the 1960s and ’70s. That production capacity was lost when the immediate threat passed. The plants closed long ago.
“It’s totally my opinion, but I think it was somewhat overconfidence with the success of the program,” Taylor said.
Taylor had worked with the USDA’s screwworm program until the 1990s, when he was reassigned to work with other biting flies until his retirement about five years ago. His first assignment with the screwworm was with a plant that had been producing sterile flies in Mexican state of Chiapas since 1976.
The plant closed in 2012, eventually leaving only the Panama plant to produce sterile screwworm. “There was no contingency for an outbreak such as this,” he said.
Dr. Joe Annelli, a former senior adviser for the USDA’s Animal and Plant Health Inspection Service Veterinary Services, said the government has often been reactionary about threats like screwworm.
“Unless there’s a fire in front of us, we don’t want to put the effort in to maintaining preparedness,” said Annelli, who also managed USDA’s involvement in the White House Avian and Pandemic Preparedness implementation plan.
“We don’t value the preparedness aspect of a disease. How do you measure something that doesn’t happen? It becomes difficult to justify continuing to spend money on a situation that hasn’t been a problem.”
The Trump administration is working to rebuild capacity, but it will take time to reach that level again.
Rollins says the nation is moving at “Trump-speed” to fight the pest. At capacity, the facility in Panama can produce 100 million flies a week. The new plant in Mexico should also be able to generate 100 million per week but a senior USDA official told CNN that it takes about six months from the time to facility opened to reach that production level.
The USDA also broke ground on a plant in Texas in April. It’s set to open late next year and should be able to generate 100 million flies per week by the end of 2027. By late 2028, according to the USDA, the Texas plant could produce 300 million more flies per week.
In July, the USDA said it would also invest $25 million in a sterile fly dispersal facility in Arizona. The agency did not give a timeline for when that facility would open.
Even when the Texas facility reaches full production, the country would likely still be below the 500 million benchmark that worked before. The facilities in Panama and Mexico do not produce flies exclusively for the US; they are shared with those countries.
And half the flies they produce don’t help because they’re female.
No (fly) girls allowed
The US is developing an initiative to create genetically altered male-only flies called Novofly, which the USDA says are currently in field trials in Panama.
Male-only flies would be a “tremendous improvement,” Taylor said, since they would double production. And, he said, sterile female flies can be a distraction to the released sterile males. “So if you eliminated those sterile females, the sterile males would more than likely be concentrated upon just the fertile wild females,” and they may not even need as many males to do the job. “In theory, it should be a major, major improvement.”
A senior USDA official told CNN that the ongoing results from field tests “are great” and that they were optimistic about the outcome.
The sterile fly program complements efforts to monitor wildlife, inspect animals and work with ranchers in areas where the fly has been spotted to encourage regular checks for infestations. The USDA says that the reception from landowners has been positive and that landowners want the flies.
But not everyone is confident the USDA’s effort will solve the problem any time soon.
“I don’t think that USDA is equipped with the tools that it needs to do the job,” said Maximillian Seunik, executive director of the Screwworm Free Future, a nonprofit dedicated to advancing science and policy to eradicate the parasite. “There’s a whole round of new technologies that could be fast-tracked, and they need support to do that.”
In January, the USDA launched the Screwworm Grand Challenge, a $100 million funding opportunity to support “innovative projects that enhance sterile NWS fly production.”
Seunik said Novofly would accelerate the effort significantly.
“We just can’t produce enough flies with what we have right now,” he said. “If it was deployed and scaled, it would really ramp up the capacity of the US just to produce flies.”
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