Recently however, the screwworm’s steadily made its way north, back toward the United States. The U.S. needed Mexico’s help to control the screwworm, and it wasn’t getting it.
For years, the U.S. Department of Agriculture has strategically dropped sterilized screwworms from airplanes to prevent their spread. Rollins said that authorities in Mexico held this up with bureaucratic delays and taxes on equipment.
“The sterile flies on the planes that we’re trying to land, that is what will push that New World Screwworm back into the south of Mexico and into southern Latin America,” Rollins said. “We were having all sorts of trouble landing those planes. They wouldn’t let us land them.”
So Rollins wrote a letter to Julio Berdegué Sacristán, Mexico’s agriculture and rural development secretary, saying that if these delays continued, the U.S. would stop accepting imports of Mexican cattle.
The impasse between the two countries broke shortly afterward. But a bigger problem remains: pushing screwworms back south again before they reach the United States.
Controlling the screwworms
Generations of American ranchers fought screwworms.
“In the past when we did have screwworms, and we’re going back to the 1960s, ’70s, I remember reading about cases in the Dakotas,” said Max Scott, a professor of entomology at North Carolina State University.
Their screw-shaped larvae burrow into the sensitive spots of mammals, causing disease, infection and death.
“This is a critter that we don’t want back in the U.S.,” said David Anderson, a livestock professor and extension economist with Texas A&M University. “If you have livestock, you’re going to be out there all the time checking your animals for any wound. I think would be a pretty devastating thing if we were to get it back.”
A Texas entomologist named Edward Knipling was one of the researchers who made a major breakthrough in screwworm control, called the sterile insect technique.
Since female screwworms mate only once, Knipling realized he could collapse their population by releasing sterilized male screwworms. Labs in Kerrville and Mission sterilized billions of screwworms over the second half of the 20th century with nuclear radiation.
Over the decades, wave after wave of sterilized screwworms pushed the insects all the way to Panama’s border with Colombia. The barrier is maintained by a joint venture between Panama and the United States called COPEG, which conducts air-drops of sterile insects and on-the-ground inspections. It’s saved an untold number of animals’ lives, as well as billions of dollars.
“The screwworm sterile release program had been very effective for you know, 20 years longer, maybe,” Scott said.
A renewed threat
In 2022, however, the flies broke through COPEG’s barrier and started to spread north. No one knows exactly why this happened, but the illegal movement of people and cattle, as well as regulators being stretched thin by the pandemic, likely contributed.
“The fly started to spread northward through Panama, up into Costa Rica and Central America and then, the first detection November of 2024 in Mexico,” said Jenny Lester Moffitt, who was in charge of the screwworm program at the time as USDA undersecretary for marketing and regulatory programs. Moffitt was a Biden administration appointee and left the agency in January.
When USDA got word that screwworms had reached Mexico in November, they temporarily shut down cattle imports, like Rollins threatened to do last week. During that time, the agency installed inspection equipment in southern Mexico and started dropping sterile insects in the country. To keep them from reaching the U.S., Moffitt also realized that they would need more flies.
“Starting in November when we had the first detection in Mexico, realizing that we were going to need to have more flies to release, and we needed to really not just rely on the one production facility that we have in Panama,” she said.
The facility in Panama can produce 100 million sterile flies per week.
“The 100 million flies out of that Panamanian facility are not enough flies to push it back down to Colombia probably in my lifetime. It’s going to take an increased amount of flies,” said Wayne Cockrell, a rancher from College Station who chairs the cattle health committee for the Texas and Southwestern Cattle Raisers Association.
In early April, Cockrell went to Panama with a group of other Texans to see where the flies are made. He was impressed by its size, its staff, and its continuous operation.
“It was absolutely much larger than I anticipated,” Cockrell said. “They have their own water treatment facility, their own sewer treatment facility, backup power. They’re operating 24 hours a day they can’t have a breakdown and say ‘Oh hey, they’ve ordered the part; it’s going to be here in a week.’”
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