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 life cycle 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.