Return of the Flesh-Eaters

What It Covers

Radiolab traces the New World screwworm — a flesh-eating fly parasite eradicated from North America using USDA entomologist Edward Nippling's sterile insect technique starting in 1957 — and examines its current northward resurgence from Panama toward the U.S. border, alongside the ethical debate over using gene drives to permanently extinct the species.

Key Questions Answered

• •Sterile Insect Technique (SIT): Nippling's method works by mass-producing radiation-sterilized male screwworm flies — sterilized precisely between days 5.5 and 5.7 of testes development — then releasing them by aircraft to overwhelm wild populations. Since females mate only once, mating with sterile males permanently removes them from reproduction, collapsing population numbers without chemical pesticides.
• •Economic scale of eradication: The Panama sterile insect barrier currently costs $15 million annually to maintain while preventing an estimated $1 billion per year in livestock losses — a 66:1 return. The current outbreak response involves building new fly factories in Texas and Mexico targeting 500 million sterile flies released per week, costing hundreds of millions of dollars.
• •Current outbreak severity: Screwworm cases in Panama surged from roughly 25 per year to thousands beginning in 2023, with infestations confirmed across Costa Rica, Nicaragua, Honduras, El Salvador, Guatemala, and Mexico — reaching within 70 miles of the U.S. border. Over 1,000 human cases have been recorded, with vulnerable populations including unhoused individuals and immunocompromised people at highest risk.
• •Gene drive mechanics and permanence: Unlike conventional genetic modification where a lethal gene passes to only 50% of offspring and dilutes across generations, gene drives force nearly 100% inheritance rates. Introducing a female-lethal gene via gene drive into screwworm populations could theoretically self-propagate to extinction without ongoing releases — but the outcome is irreversible at a species level.
• •Extinction ethics framework: A multidisciplinary panel convened at Arizona State in May 2024 evaluated screwworm extinction candidacy across ecological, aesthetic, educational, and intrinsic value criteria. Screwworm ranked as a minor pollinator with no critical predator dependencies. The panel reached consensus that screwworm qualifies as a strong extinction candidate, noting that de-extinction via frozen specimens remains theoretically possible as a safeguard.