US Fights Parasites With 400 Million Sterile Flies Per Week

The screwworm fly just made its first appearance in the US since 1966, but scientists already have a game plan that sounds like science fiction but actually works.

A calf in southern Texas tested positive this week for New World screwworm, a flesh-eating parasite that once killed hundreds of thousands of cattle every year. Female flies lay eggs in open wounds on animals, and when those eggs hatch, the maggots feed on living tissue before becoming adult flies.

Here's where it gets interesting. Back in the 1950s, researchers discovered they could use radiation to make male screwworm flies sterile while keeping them otherwise normal. When these sterile males mate with wild females, the eggs don't hatch and no offspring are produced.

The technique worked so well that scientists completely eliminated screwworms from the entire United States by 1966 and pushed them as far south as Panama by 2006. On the island of Curaçao, it took just seven weeks to wipe out the pest and save vital goat herds.

The secret is that female screwworm flies only mate once in their lifetime. If that one mating is with a sterile male, the cycle stops completely.

"The sterile insect technique is probably the most eloquent example of a completely successful biologic control mechanism," says Sally DeNotta, a veterinary medicine professor at the University of Florida. "The life cycle stops. There's no progeny produced."

The flies started breaking through barriers in Central America in 2022, so US officials were ready. They're already air-dropping 4 million sterile flies per week in southern Texas and have now added targeted truck releases in a 12-mile zone around the infected calf.

The Bright Side

The USDA is ramping up to 400 million sterile flies per week to beat back this outbreak. They're investing $21 million to convert an existing facility in Mexico that should produce up to 100 million flies weekly by summer.

A brand new $750 million facility near the Texas-Mexico border will open in 2027, ensuring long-term protection for American livestock and wildlife. The agency shifted its border strategy back in February, anticipating exactly this scenario.

This approach uses no chemicals, leaves no environmental damage, and has a perfect track record. DeNotta expects more cases to pop up in the short term since one female can lay thousands of eggs, but she has zero doubt the US will eradicate screwworms again using this proven method.

Americans just beat this parasite before, and they're going to do it again with science that turns the pest's own biology against it.