#764 1954 · USDA (Edward Knipling and Raymond Bushland) · Agricultural pest control / veterinary science
Screwworm flies were eating livestock alive across the Americas and pesticides couldn't stop them — so two USDA scientists stopped trying to kill the flies and sabotaged their sex lives instead
the problem
Pesticides couldn't stop a flesh-eating livestock parasite
background
New World screwworm was a livestock plague across the southern United States, Mexico, and Central and South America through the first half of the twentieth century: female flies lay their eggs in any open wound, and their larvae eat living flesh, not dead tissue, killing an infected animal within about ten days if untreated. The industry's standard response was chemical -- pesticides sprayed on animals, soil and pasture at ever-larger scale -- and it never worked for long: flies developed resistance, reinfestation was constant, and the losses (roughly $20 million a year in the mid-20th-century US alone) kept mounting regardless of how much poison was applied.
The obvious next move, when a pesticide campaign stalls, is a stronger pesticide or a wider spray radius -- more force against the same target. Working at a USDA lab in Menard, Texas from the late 1930s, entomologist Edward Knipling looked instead at a detail of the fly's own reproductive biology that had nothing to do with killing it at all: a female screwworm fly mates exactly once in her life, then lays fertilized eggs from that single mating for the rest of it. Colleague Raymond Bushland had separately solved the harder practical problem of mass-rearing screwworms on ground meat in a lab.
what everyone would do
The standard 20th-century answer to any devastating agricultural pest was more and stronger pesticide, sprayed at ever-larger scale as resistance and reinfestation kept undoing whatever ground was gained.
what they saw
Knipling saw that killing screwworms directly was the wrong target entirely -- the leverage point was the species' own reproductive bottleneck. A female screwworm fly mates exactly once and then reproduces from that single mating for life, which meant a single sterile mating didn't just kill that pairing's offspring, it permanently removed the female from the breeding population as effectively as killing her outright, without needing to touch her at all.
the move
Knipling and Bushland exposed lab-reared male screwworm flies to radiation, sterilizing them without otherwise impairing their ability to compete for mates, then released them by the millions into infested areas. A wild female that mated with a sterile male produced no viable offspring at all -- and because she never mated again, that one bad pairing removed her permanently from the breeding population. The first field trial, in 1954, saturated the 176-square-mile island of Curaçao with sterile males; screwworm was eradicated there within four months.
why it works
Because females mate only once, every sterile male released has a chance to convert one wild female's entire remaining reproductive output to zero -- unlike a pesticide, whose effect ends the moment the chemical degrades, a sterile mating is permanent for that individual. Flood an area with enough sterile males relative to wild fertile ones and the population's own reproduction collapses faster than pesticide resistance can evolve around it, because there is no chemical signature to adapt to -- only a numbers game the technique wins by mass-producing and irradiating flies faster than the wild population can out-reproduce sterile competition.
the payoff
The technique eliminated screwworm from the continental United States by 1966, then from Mexico and Central America through the 1970s-90s via a joint US-Mexico eradication program, and stopped a 1991 outbreak in Libya -- the first time the parasite had reached the Eastern hemisphere. A permanent sterile-fly production facility on the Panama-Colombia border has released tens of millions of sterile flies a week ever since to maintain the barrier; a joint Costa Rica-US eradication program alone returned an estimated $168-448 million in benefits against a $41 million cost. Knipling and Bushland won the 1992 World Food Prize; then-Secretary of Agriculture Orville Freeman called it 'the greatest entomological achievement of the 20th century.'
where it breaks
It depends on maintaining a high enough ratio of sterile to wild males continuously -- suppression, not extermination in the wild, requires an unbroken supply chain of mass-reared, irradiated insects, and any interruption (the COVID-19 pandemic disrupted Panama's production facility) lets the population rebound, as the 2023-2026 Central American resurgence showed. It also only works on species with the right biology: females that mate multiple times, or species that can't be reared and sterilized without losing mating competitiveness, don't yield the same leverage, and the technique requires an isolable barrier (an island, a narrow land bridge) to hold a border once eradication is achieved.
what came after
Sterile insect technique became the standard tool for area-wide eradication of screwworm, Mediterranean fruit fly, tsetse fly and other pests worldwide, and remains in continuous use today. It is also its own cautionary tale about maintenance: interruptions in sterile-fly production during the COVID-19 pandemic, combined with other pressures on the Darién Gap barrier, let screwworm breach north into Central America and Mexico again starting in 2023, with the first new US human case since eradication reported in Maryland in 2025 -- proof the technique suppresses the population only for as long as sterile flies keep being released, not permanently.
references
- [1]New World Screwworm: Rise, Fall and ResurgenceAmerican Society for Microbiology, 2025asm.org
- [2]Why the government breeds and releases billions of flies a yearNational Geographic, 2019nationalgeographic.com