Billions of sterile flies drove a livestock parasite out of North America
A female New World screwworm fly mates just once and stores that sperm for life. Scientists exploited this quirk by breeding flies in factories, sterilising them with radiation and releasing them by the billion. Wild females that paired with sterile males laid eggs that never hatched, and the pest was gradually pushed south to Panama.
The screwworm is a blowfly whose larvae feed on the living tissue of warm-blooded animals, unlike most maggots, which prefer dead flesh. Females lay 250 to 500 eggs at a time in wounds, the navels of newborn animals or even insect bites, and the damage attracts yet more flies. Livestock suffers most, though birds and humans are occasionally affected. A full life cycle takes about 20 days, and a female can travel up to 200 kilometres in her lifetime.
The sterile insect technique was tested on this species first. In 1954 a trial on the Caribbean island of Curaçao eliminated the fly within four months. The southeastern and southwestern United States followed between 1958 and 1960, and from 1962 to 1975 the US and Mexico released more than 94 billion sterile flies. The United States officially declared the pest gone in 1982, and country after country in Central America followed through the 1990s.
Rather than hold a line in southern Mexico, planners decided the narrowest place to stop the flies was the Darien Gap in Panama. A breeding plant opened there in 2003 and released up to 100 million sterile flies every week to keep a barrier between South and Central America. The fly remains widespread in tropical South America and parts of the Caribbean, so imported animals must be inspected or treated.
The barrier has not held perfectly. The fly was detected again in southern Mexico in late 2024, and by 2026 cases had appeared in southern Texas, prompting renewed releases. The USDA estimated in 2025 that a Texas outbreak could cost $1.8 billion, a reminder that eradication must be maintained, not simply achieved once.
Source: Cochliomyia hominivorax
