(CNN) -- Much hope is being placed on large, red-eyed flies with bright green backs, sterile, being bred by scientists from the United States, Mexico and Panama.
The US government has just released the billionth fly in Texas, coming from a facility opened in June in Chiapas, Mexico, near the border with Guatemala, and a sister facility in Panama, to combat a pest that could cause up to $2.5 billion in damage in the southwestern United States alone.
The New World screwworm lays its eggs on warm-blooded animals, including humans, and then burrows its larvae into living tissue, which can lead to infections and death. Although it is not considered a food safety problem, it may threaten the food supply.
This pest has long been thought to have been eradicated from the United States, Mexico and most Central American countries decades ago. But in 2023, the carnivorous fly breached containment lines in the Darien Gap, a dense rainforest between Colombia and Panama, and spread north.
The region's governments launched a massive response that included traps, trained dogs, drones, and most importantly, sterile flies. Despite these efforts, the screwworm infected thousands of animals in Panama and Mexico, and in June the US Department of Agriculture confirmed the first cases of infection in the United States in years.
Decades ago, the production and release of sterile flies contributed to interrupting the pest's reproductive cycle and eliminating it. But scientists fear the screwworm could threaten wildlife, pets and livestock - the country's most valuable agricultural commodity - for years due to an inability to produce sterile flies quickly enough.
US Agriculture Secretary Brooke Rollins expressed no concern about the pace of the administration's efforts. It stated in a statement last week that “the Trump administration moved extremely quickly to expand the production of sterile flies in Texas, Mexico and Panama, to protect the large American cattle herd.”
“Through this joint effort between federal, state, and local agencies and private ranchers, we have been able to keep the number of cases in America low, with only 48 cases recorded, including 30 in June, 14 in July, and only 4 cases in August,” she added. “This is a far cry from the popular narrative in early June, that cattle herds had been decimated, wildlife wiped out, and a decade would be needed to recover. We currently have only three active cases, and no No infection among livestock has been recorded for more than a month.”
Potential for accelerated growth
Fewer than 50 screwworm infections have been recorded in the United States this year, according to the USDA. That may not seem like a large number, but entomologist Dr. Dave Taylor, who has spent years developing methods to eradicate the New World screwworm at the department's Agricultural Research Service, cautioned that the numbers likely will not remain low.
Although flies only live about 21 days, an adult female may lay about 3,000 eggs during her life, and the eggs can turn into adult flies in less than a week, according to the US Centers for Disease Control and Control.
In 1971, only 473 confirmed cases were recorded, but the following year the number rose to about 100,000 cases in the United States.
"That's the reproductive capacity of this fly," Taylor said. "I'm not saying it's going to happen this time, but there could be a lot more cases this time next year."
That could happen if ranchers don't report all cases to the USDA, or if infections aren't detected among Texas wildlife, a $31 billion industry, he added.
Taylor, who also served as editor-in-chief of the Journal of Medical Entomology, died on August 6 at the age of 71. When he spoke with CNN in July, he said he was frustrated after 40 years of working to keep screwworm out of the country, "to see all that progress undone in three or four years."
Striving to produce 500 million flies
The United States succeeded in eliminating screwworm by raising millions of flies, sterilizing them with radiation, and then releasing them into infected areas. Female New World screwworms only mate once, so when they mate with sterile males and do not reproduce, their local numbers decline until they become extinct.
Rollins says that controlling the pest requires the production of huge quantities of sterile flies. When reaching maximum capacity, the Panama and Mexico facilities can produce about 100 million flies per week each, but reaching this capacity in the Mexican facility will take about 6 months.
The United States is also working to increase its production capacity, with the construction of a new facility in Texas, and investment in a sterile fly propagation facility in Arizona.
But even with this increase, production may still fall short of the 500 million flies per week, a rate that once helped wipe out screwworms. Also, half of the flies produced are not useful in controlling the pest because they are female.
There is no place for females
The United States is developing an initiative to produce male-only genetically modified flies, known as Novofly, which is currently undergoing field tests in Panama.
Producing only males would represent a "huge improvement," Taylor said, because it would double the number of males available for control. He added that sterile female flies may distract males from fertile wild females, so eliminating them may make males more focused on mating with wild females, possibly reducing the number of males needed to combat the pest.
“Theoretically, that would be a very big improvement,” he added.
The sterile fly program comes in parallel with efforts to monitor wildlife, examine animals, and cooperate with livestock breeders in areas where the fly has been spotted, to encourage them to conduct regular checks to detect infections. The ministry says landowners have welcomed the efforts and even want more sterile flies.
But not everyone is confident that the ministry's efforts will solve the problem anytime soon.
“I don’t think the USDA has the tools to do the job,” said Maximilian Sonik, executive director of Screwworm-Free Future, a nonprofit that advances science and policy to eradicate the parasite. “There are a bunch of new technologies that can be accelerated, but they need support to get there.”
In January, the USDA launched the Screwworm Grand Challenge, a $100 million funding program to support “innovative projects that advance the production of sterile New World screwworm flies.”
Sonic said Novofly would significantly accelerate these efforts.
