flies

Texas prepares for war as invasion of flesh-eating flies appears imminent

Past success

As the flies’ host and geographic range expand, pressure is intensifying to control the flies—something many countries have managed to do in the past.

Decades ago, screwworms were endemic throughout Central America and the southern US. However, governments across the regions used intensive, coordinated control efforts to push the flies southward. Screwworms were eliminated from the US around 1966, and were pushed downward through Mexico in the 1970s and 1980s. They were eventually declared eliminated from Panama in 2006, with the population held at bay by a biological barrier at the Darién Gap, at the border of Panama and Colombia. However, in 2022, the barrier was breached, and the flies began advancing northward, primarily through unmonitored livestock movements. The latest surveillance suggests the flies are now about 370 miles south of Texas.

The main method to wipe out screwworms is the sterile insect technique (SIT), which exploits a weakness in the fly’s life cycle since they tend to only mate once. In the 1950s, researchers at the US Department of Agriculture figured out they could use gamma radiation to sterilize male flies without affecting their ability to find mates. They then bred massive amounts of male flies, sterilized them, and carpet-bombed infested areas with aerial releases, which tanked the population.

Panama, in partnership with the US, maintained the biological barrier at the Colombian border with continual sterile-fly bombings for years. But as the flies approached this year, the USDA shifted its aerial deliveries to Mexico. In June, the USDA announced plans to set up a new sterile fly facility in Texas for aerial deliveries to northern Mexico. And last month, the USDA halted livestock trade from southern entry points.

Miller said in the announcement today that SIT is no longer enough, and Texas is taking its own steps. Those include the new bait, insecticides, and new feed for livestock and deer laced with the anti-parasitic drug ivermectin. Miller also said that the state aims to develop a vaccine for cattle that could kill larvae, but such a shot is still in development.

Screwworms are coming—and they’re just as horrifying as they sound

flies, health, parasite, screwworm / Beth Washington / May 2, 2025

We’re on the verge of being screwwormed.

The biological barrier was breached, they’re slithering toward our border, and the US Department of Agriculture is now carpet-bombing parts of Mexico with weaponized flies to stave off an invasion.

This is not a drill. Screwworms are possibly the most aptly named parasites imaginable, both literally and figuratively. Screwworms—technically, New World Screwworms—are flies that lay eggs on the mucous membranes, orifices, and wounds of warm-blooded animals. Wounds are the most common sites, and even a prick as small as a tick bite can be an invitation for the savage insects.

Once beckoned, females lay up to 400 eggs at a time. Within about a day, ravenous flesh-eating larvae erupt, which both look and act like literal screws. They viciously and relentlessly bore and twist into their victim, feasting on the living flesh for about seven days. The result is a gaping ulcer writhing with maggots, which attracts yet more adult female screwworms that can lay hundreds more eggs, deepening the putrid, festering lesion. The infection, called myiasis, is intensely painful and life-threatening. Anyone who falls victim to screwworms is figuratively—well, you know.

Previous victories

Screwworms aren’t a new foe for the US. Decades ago, they were endemic to southern areas of the country, as well as the whole of Central America, parts of the Caribbean and northern areas of South America. While they’re a threat to many animals, including humans, they are a bane to livestock, causing huge economic losses in addition to the carnage.

In the 1950s, the US began an intensive effort to eradicate screwworms. The successful endeavor required carefully inspecting animals and monitoring livestock movements. But most importantly, it relied on a powerful method to kill off the flies.

The ploy—called the Sterile Insect Technique—throws a wrench into the unique lifecycle of screwworms. After the larvae feast on flesh, they fall to the ground to develop into adults, a process that takes another seven days or so during warm weather. Once adults emerge, they can live for around two weeks, again depending on the weather. In that time, females generally only mate once, but don’t worry—they make the most of the one-night stand by retaining sperm for multiple batches of eggs. While females lay up to 400 eggs at once, they can lay up to 2,800 in their lives.

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