Subscribe to The Y’all — a weekly dispatch about the people, places and policies defining Texas, produced by Texas Tribune journalists living in communities across the state.
Recommended Videos
LUBBOCK — The New World screwworm has entered the country, and if history is any indication, the parasite’s devastating effects on the U.S. could last for decades.
The screwworm re-emerged following years of warnings from Central America and Mexico officials of the impending outbreak. Then last year, the Department of Government Efficiency (DOGE) cut funds for screwworm monitoring in Central America.
Government officials continue to work to pinpoint how screwworm entered the U.S. while cases continue to creep up.
While the financial impact of this modern-day outbreak is still unknown, the USDA estimates the industry saved more than $900 million a year as a result of eradication in the past.
Unlike the first outbreak, however, there is now a playbook on how to eradicate the pests.
Edward F. Knipling, one of the lead entomologists who worked on eradication, said the only way to deal with major insect outbreaks is with large-scale responses. Historical documents from the National Agricultural Library show the nearly 60-year battle with the screwworm and the solution that could’ve helped sooner.
“I thought what we really need is some way to control the screwworms before they attack the animals, rather than just wait until after the animals had the screwworm, then try to control it,” said Knipling in a 2000 interview. “I realized you would never truly control the screwworm that way. What we needed was a preventative measure.”
Knipling, who was born and raised in Texas, theorized that the real solution was to reduce or eliminate the screwworm altogether. Long before Knipling’s theory, however, the public was only beginning to learn about the screwworm. After initially confusing the screwworm with a species of blowflies, researchers were able to start studying the parasite in 1933. By then, the screwworm had already spread in the U.S., from the Southwest to Southeast through a shipment of infested animals.
The pest left deadly consequences in its wake — in 1935, 180,000 livestock deaths from the screwworm were reported in under half of Texas’ counties.
In a 1946 letter, Knipling suggested that they bring geneticists in to help eradicate the screwworm — decades before the worst of the outbreaks. Knipling’s idea was about the possibility of creating a mutation to produce sterile, but otherwise healthy, screwworm flies. His idea was not pursued.
While working at a research facility in Menard, Knipling helped develop Smear 62, a thin paste with an active poison that could be applied to wet and dry wounds. One dose of the treatment would kill all screwworms in a wound up to the size of half a dollar. It would also protect against reinfestation until the wound healed.
In a report by the USDA Agricultural Research Administration, scientists wrote: “Fortunately, it is not a repellent to adult flies. Flies therefore continue to lay their eggs on treated wounds, and the larvae die as soon as hatched.”
Then, while Knipling was researching how to control insects that threatened servicemen and women during World War II, he thought of using sterile flies to stop the screwworm. He wrote to Emory Cushing, his supervisor at the time, about the idea. Thirty years later, Knipling discovered Cushing never sent the letter, and all copies were destroyed except for one.
Even though his idea was ignored, Raymond Bushland, another scientist, was also working on a way to sterilize flies. Bushland raised the flies with a special diet and sterilized them with X-ray radiation. Together, Bushland and Knipling developed the Sterile Insect Technique.
As director of the Kerrville lab, Bushland ordered the release of sterile flies on Sanibel Island in 1951. Radiation proved to be an effective way to sterilize the flies, and by 1953, scientists were cautiously optimistic about the technique and ended the experiments on the island.
Then came a bigger test field. B.A. Bitter, a veterinarian on the island of Curacao, 40 miles from Venezuela, wrote to the USDA that same year. He was desperately seeking help for the outbreak they were experiencing and said the infestation was affecting all kinds of animals on the island — not just livestock — and resulting in their death.
“I should like to consider a way of fighting this pest,” Bitter wrote.
Bitter said infestation was inevitable. The warm climate attracted screwworm flies, and livestock frequently broke their skin through barbed wire fences and thorns, giving the screwworm a point of entry. He included a tube with larvae that was found on a dog’s tail.
“For these reasons, only the biological way of destroying the flies seems to be possible,” Bitter said.
Knipling informed Bitter that field tests of the sterile fly theory were underway — some successful, some not. He said the method was complex, but he thought it was worth exploring. Bitter agreed to operate the fly traps on the island, and the island became the ideal testing area for the Sterile Insect Technique.
By 1958, the Florida legislature appropriated funds for a full-scale screwworm eradication program. With the federal government providing matching funds, a large insect production plant was built and, under full production, produced 50 million sterile flies per week, a method that is being explored today.
By early 1959, the screwworms disappeared from Florida and much of the Southeast.
Then, it was Texas’ turn. Southwestern livestock producers, along with the federal government and state lawmakers, brought in funds to fight the screwworm. Ranchers formed a nonprofit called Southwest Animal Health Research Foundation, which raised over $3 million to support eradication. The Southwest eradication program covered a much larger area and was constantly at risk of re-infestation from Mexico.
By 1966, the lab in Mission was producing up to 150 million sterile flies a week. The mass release of sterile flies, along with help from livestock producers in slowing the spread, proved successful.
The USDA declared the U.S. free of indigenous screwworms in 1966. But the threat wasn’t over, and researchers realized keeping the U.S. free from screwworms forever was impossible.
“Texas again had the somewhat dubious honor of recording more cases in 1967 than any other state cooperating in the program, with 835 in 67 counties,” read one report. In the same year, Arizona only recorded 23, and New Mexico had none.
Most of Texas’ cases happened after a hurricane, with most being reported from September through October. The outbreak was back under control by early November. Dolph Briscoe Jr., a Uvalde rancher and chairman of the Southwest Animal Health Research Foundation, credited the Sterile Insect Technique.
“Thus, it has been proven again that the sterile screwworm fly technique can stop outbreaks of screwworm,” Briscoe wrote in a report.
By 1972, the U.S. experienced an even worse outbreak than before, due to lax quarantine measures and warm, moist weather in Mexico and the U.S. Texas alone confirmed 90,000 cases after only seeing 444 the year before and had a confirmed case in almost every county.
In 1976, producers spent $132.1 million in response to the screwworm, which included loss from deaths, animal weight loss, medication, and extra labor. The total economic loss for Texas that year was nearly $330 million. When adjusted for inflation, the loss would be closer to $1.8 billion if it happened in 2024. The outbreak spurred U.S. Secretary of Agriculture Earl Butz and Mexican Secretary of Agriculture Manuel Bernardo Aguirre to sign an international agreement establishing a joint Mexico-United States Screwworm Eradication Commission. It also inspired another public awareness campaign — this one to “stamp out screwworms forever.”
“The weather is on our side. The fly strain is on our side. Mexico is on our side. Are you on our side?” reads one brochure from 1977.
By 1980, northern Mexico states were free of the screwworms. Two years later, the last case of the screwworm was reported in the U.S., and only a handful of imported cases were reported until this year.
“We cannot deal with these pest problems by just trying to control them year after year, on a farm-by-farm basis,” Knipling said in 2000. “Just like we never would’ve controlled the screwworm that way.”