Gene-edited sterile flies offer new path to pest control without chemical sprays

A three-week-old calf in Zavala County, Texas, arrived at a veterinary clinic in early June with fly larvae burrowing into an open wound. The animal survived, but the discovery marked something Americans had stopped worrying about decades ago: the New World screwworm, a parasitic fly whose larvae consume living flesh from cattle, wildlife, and occasionally people. The United States had eradicated the pest in 1966 through an ambitious international campaign with Mexico and Central American nations. Yet the fly persisted in South America, crossed back into Panama in 2023, and by summer 2026 had reached Texas. Within weeks, state officials tallied more than forty cases across cattle, dogs, and other animals.

To fight back, the government deployed an old weapon: releasing millions of sterilized flies from aircraft and ground stations. The strategy sounds counterintuitive but works with brutal efficiency. When a wild female mates with a sterile male, she produces no offspring. Flood a region with enough sterile males for long enough and the population collapses. This sterile insect technique, developed by USDA entomologist Edward Knipling in 1937, succeeded so thoroughly that Americans largely forgot the screwworm existed. Now genetic engineering promises to make the approach far more powerful.

In June 2026, the Environmental Protection Agency granted emergency authorization and proposed commercial approval for NovoFly, a genetically engineered, male-only strain of screwworm developed by the U.S. Department of Agriculture and North Carolina State University. The fly contains a genetic switch that kills female embryos unless workers add tetracycline to the breeding colony's diet. In the batches destined for release, without the tetracycline, nearly all females die as embryos, leaving almost entirely males that are still sterilized through radiation. This approach could roughly double the number of useful insects a factory produces and may allow lower radiation doses, making released males more competitive with wild flies. The USDA plans to incorporate NovoFly into its sterile insect program as it builds out a new Texas facility.

A second engineered fly, Knockout SWD, targets Spotted-wing drosophila, one of the most damaging pests of berries and cherries. It uses a technique called precision-guided sterile insect technique, or pgSIT, which relies on CRISPR gene editing. Two engineered breeding lines are crossed: one carries Cas9, a protein that acts like molecular scissors, and the other carries guide RNAs that direct those scissors to specific genes. When offspring inherit both, Cas9 cuts genes required for normal female development and male fertility. The females fail to develop; the surviving males are sterile. Researchers are now trying to extend this approach to screwworm itself. The Foundation for Food & Agriculture Research committed $150,000 this summer toward a project with Agragene and North Carolina State to use CRISPR to produce sterile males without irradiation.

For farmers and ranchers, these engineered insects offer something chemical sprays cannot: a living alternative that searches for its own targets. Spotted-wing drosophila, unlike most fruit flies, cuts into healthy ripening berries and cherries to lay eggs inside, where larvae are protected from insecticides once they hatch. Growers spray preventively during ripening and harvest, often weekly, rotating among different insecticides to slow the evolution of resistance. Some fly populations have already evolved resistance to pyrethroids and spinosyns, including spinosad, a mainstay of both conventional and organic production. Genetic biocontrol could allow growers to spray less often by suppressing and potentially eradicating fly populations before they damage crops, saving money while sparing bees, predatory insects, and other organisms harmed by conventional insecticides.

The potential extends far beyond agriculture. Government agencies across the United States rely heavily on insecticides to suppress mosquito species that transmit dengue, Zika, West Nile virus, and other diseases. Repeated spraying is expensive, often misses breeding sites, exposes non-target insects, and selects for resistance. Researchers have already adapted pgSIT to Aedes aegypti, the mosquito species that primarily spreads dengue and other viruses in urban areas. Other self-limiting engineered mosquitoes have reduced local Aedes populations in field trials. Gene drives, a more powerful and less reversible approach, could eventually control disease vectors and agricultural pests that make people sick. Researchers are developing genetic tools to control ticks, though the work remains perhaps a decade or more behind mosquitoes, according to Dr. Gulia-Nuss at the University of Nevada, Reno. Ticks' longer life cycle and need to feed on a live host make sterile release less practical, but once genome engineering techniques are better established, gene drives might suppress populations of cattle fever ticks and eventually species that carry Lyme disease and alpha-gal syndrome.

The original screwworm campaign succeeded because the Department of Agriculture supported it from research through scale-up, funding initial work and then conducting the actual rearing, dispersal, and monitoring. The next generation of genetic biocontrol needs similar institutional commitment. NovoFly grew out of decades of USDA research and collaboration with North Carolina State. PgSIT emerged from university research supported by the National Institutes of Health, the Defense Advanced Research Projects Agency, and other public funders. Private companies can build upon this foundational work, but they need government as a partner in developing insect factories and studying ecological impacts. EPA's review of NovoFly and Knockout SWD could establish a clear pathway for other companies, helping unlock financing and interest in the field. The agency must clarify what evidence it needs, what studies are required, and what protocols must be followed for deployment. The return of the screwworm has brought an old lesson: pest control does not have to mean spreading poison and hoping it reaches the target. Sometimes the best insecticide is one that hatches and flies away, leaving little trace and no descendants.