Fortunately, they're not a very pleasant species. Screw worms enjoy laying eggs in the mucus membranes of mammals, and telling their friends to do the same. A little applied radiation really helped. Perhaps.

Above, a screw worm fly larvae under an electron microscope.

The Unpleasant Habits of Infant Screw Worms

It was in the 1950s that people got seriously sick of the screw worm. These animals were flies that, when they decided to lay eggs, would head for the nearest wound of the nearest mammal. If they couldn't find an outright wound, they'd go for mucus covered tissue, like that around the nose or eyes.


The best thing you could say about them was they had a cooperative spirit. Once a female found a good place for her eggs, she'd leave a chemical that would allow other females to find the same spot. This was a kindness to the flies, but not to the host. When the larvae hatched, they'd start eating the flesh nearby. Unlike maggots, they didn't care for dead flesh, and feasted only on healthy tissue. They'd also screw themselves into the flesh to get more food, earning them their name and making the wound much worse and more likely to get infected.

Radiation to the Rescue?

Although they cause plenty of pain to humans, it was their threat to livestock, especially cattle, that really drew ire. And in the 1950s, there seemed to finally be a way of eradicating them. Radiation, after being revered through the 1930s as a possible health-giving phenomenon, had lost its cuddliness in after the debut an testing of atomic bombs. It was finally understood as a destructive force, if a mysterious one.


Thomas Knipling, who worked at the USDA, read a book about the use of radiation on fruit flies. Exposure of male fruit fly larvae to radiation caused sterility. Unlike many other methods of causing sterility – like hormones or removal of sexual organs – this did not change their behavior during mating season. They competed as actively, and successfully, for females as any other of their kind. There simply weren't any offspring.

Knipling finally hit on an idea. It was easy to raise tens of thousands of screw worm larvae, and their size made it relatively easy to irradiate them all. If they could sterilize tens of thousands of worms and send them out into the wild as healthy, breeding flies, the population would necessarily go down. Keep it up on a grand scale for a few years, and who knew what might happen?

The operation started on a twenty-mile island off the coast of Florida. It worked. Having proved the concept on a small scale, Knipling looked for larger ground. When a Dutch controlled island, about 170 miles long, became the center of a large outbreak, he headed over. Tens of thousands of sterile flies were dropped from airplanes. Screw worms reproduced in a three week cycle. The initial cycle after the first drop showed that over seventy percent of egg cases were sterile. The second cycle had an eighty percent rate of sterility. After the third cycle, few people could find any eggs that weren't sterile.

In 1961, after extensive programs implementing the irradiation and release of male flies, the United States declared the screw worm eradicated. Soon, though, people realized that other, similar worms could move in to the void they'd left. Over the next decades, they took the program to other species of screw worms in Mexico, and then slowly downward toward Central and South America. While some don't consider the screw worm completely eradicated – one scientist called the programs reported success "the grand delusion" – and other people argue against the eradication, the program seems a success.

Until we get invaded by giant, mutated screw worms.

[Via Nature, Screw Worm Timeline, NCBI, Wicked Bugs.]