In the 1940s and 1950s, cowboys carried a stick to treat cattle infested with screwworms. The stick was used to scrape the flesh-eating maggots from infested wounds — eyes, noses, umbilicus — and then used to smear benzol and pine tar oil on the wound to prevent reinfestation. Most maggots and flies feed only on dead or decaying flesh, but screwworms feed on live, healthy tissue, making them a massive threat to the U.S. livestock industry as they edge closer to the southern border. Notably, the New World screwworm is a zoonotic pest, meaning it can and will affect any warm-blooded mammal, including humans.

The cost of screwworms to producers included the direct costs associated with the individual animals and herds that require treatment, but the indirect costs are also significant. Surveillance, detection, quarantine and treatment costs can quickly multiply. The largest U.S. outbreak in 1972, according to Colorado State assistant veterinarian Dr. Maggie Baldwin, affected Arizona, Arkansas, California, New Mexico and Texas. In Texas, where the previous year documented 500 cases, experienced 90,000 cases, and shouldered the majority of the $172 million in losses. McGrath said a 1976 outbreak in Texas cost economic losses of about $330 million, which adjusted for inflation is about $1.8 billion in 2024.

The lifecycle and mating habits of the screwworm have made the sterile fly technique to create a biological barrier possible — that, and the brilliant minds at places like the Panama-United States Commission for the Eradication and Prevention of Screwworm. Female screwworms mate only once during their lifecycle. Biologists release irradiated sterile male flies who then mate with the fertile female flies. The eggs she then lays are unviable and the fertile fly population then dies out during a few lifecycles. It is through this technique officials have held screwworm populations to the southern regions of Mexico and eradicated the pest from the U.S.

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Until recently, this method was employed to maintain a biological barrier at the Panama Canal. Screwworms are endemic south of the canal and it is also home to a facility producing the sterile flies.

U.S. Secretary of Agriculture Brooke Rollins rolled out an $8.5 million sterile screwworm fly dispersal facility in south Texas earlier this month to protect the nation’s cowherd, wildlife and pets. Recent detections, according to the USDA, have been confirmed in Oaxaca and Veracruz, as close as 700 miles to the southern border. These detections led to the closure of the U.S. to Mexican cattle, horses and bison.

An inability to access the U.S. market is a huge financial blow to Mexico’s livestock industry and one Colin Woodall, chief executive of the National Cattlemen’s Beef Association, said was entirely avoidable. Woodall said though the NCBA and other trade organizations have been “sounding the alarm for months,” the “Mexican government created unnecessary bureaucratic hurdles that rendered prevention efforts ineffective and allowed screwworm cases to spread unchecked beyond control points in southern Mexico.”

The border closure also causes economic harm to U.S. producers and will also cause supply chain disruptions. However, the economic damage will be less than a fight to again eradicate screwworm on U.S. soil.

In the 1960s, Moore Air Base was key in eradication efforts and will again be used to house a sterile fly production facility. The base once trained fighter pilots during World War II, and will now deploy sterile flies in close proximity to the border. The current facility in Panama can produce 117 million flies weekly. According to the USDA, 300 million sterile flies per week are necessary to push the pest back to the previous biological barrier. In the 1960s when screwworm was first eradicated from the U.S., 500 to 500 million sterile flies were released weekly.

Researchers at Texas A&M University recently published a white paper about the ethics of eliminating a harmful species from the planet, examining the cases of screwworm; the Anolpheles gambiae mosquito, a vector for malaria; and invasive rodent species like the house mouse and black rat. The findings suggested deliberate full extinction might occasionally be acceptable, but only extremely rarely.

In addition to the Sterile Insect Technique, other genetic methods to eradicate species include female-specific release of insects with a dominant lethal, whereas genetically-modified male insects are released, and their offspring inherit a gene that kills female larvae unless they are exposed to a specific substance and sex-biasing gene drives, biasing the sex ratio of a population, leading to a population crash.

The researchers considered the severity of suffering the species cause, the ecological effect, the effectiveness of existing control methods, the risk of unintended consequences, and the public health threat.

Having studied photos of screwworm infested cattle, I understand the threat and am thankful for, and a bit fascinated with, the science that took the screwworm fight from a tar-covered stick to sterile flies deploying from a military base. Release the flies, men.

Rachel Gabel writes about agriculture and rural issues. She is assistant editor of The Fence Post Magazine, the region’s preeminent agriculture publication. Gabel is a daughter of the state’s oil and gas industry and a member of one of the state’s 12,000 cattle-raising families, and she has authored children’s books used in hundreds of classrooms to teach students about agriculture.