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How The Screwworm's Sex Life Saved Your Steaks

A screwworm larva grasps the flesh in a wound with its mouth's tusklike protrusions and screws itself in.
Science Source
A screwworm larva grasps the flesh in a wound with its mouth's tusklike protrusions and screws itself in.

The screwworm fly is a cringeworthy creature.
Philippe Psaila/Science Source
The screwworm fly is a cringeworthy creature.

Edward F. Knipling (right) and his colleagues inspect cans of ground meat where they grew screwworm flies.
USDA National Agricultural Library
Edward F. Knipling (right) and his colleagues inspect cans of ground meat where they grew screwworm flies.

A team member gets a canister of screwworm pupae ready for the irradiation chamber.
USDA National Agricultural Library
A team member gets a canister of screwworm pupae ready for the irradiation chamber.

Over the past 70 years, the U.S. has been waging a war against a miniature menace: the New World screwworm.

The story of how we eradicated the critter has it all: triumph of the little guys, a medical treatment that uses bacon (i.e., "bacon therapy") and a new technology to wipe out horrible diseases — one that scientists are using today to try to stop the Zika virus.

The screwworm is arguably the most cringeworthy creature on Earth. Seriously, if you're squeamish at all, you might want to skip the next few paragraphs.

The screwworm isn't a worm at all. It's a fly. And its Latin name, hominivorax, means "man eater." There's a good reason for that. The fly survives by eating warm-blooded flesh.

Here's how it works: A fly lands on a wound in the skin and lays hundreds of eggs. The eggs hatch into swarms of wormlike larvae, which then burrow into the wound. The larvae have little ridges on their surface, which makes them look like screws inserted into skin. The larvae gorge on the flesh for a few days until they're full and then fall out of the wound.


Take, for instance, the case of an unfortunate 12-year-old girl who was invaded by screwworms while traveling in Colombia in 2008. Doctors in Connecticut had to manually extract 142 larvae from the girl's scalp.

In other instances, doctors have laid bacon fat over the wound to coax the larvae out of a patient's skin. A team at Massachusetts General Hospital dubbed this form of treatment "bacon therapy."

Most of the time, thankfully, screwworms don't infect people, but rather feast on livestock, such as sheep and cattle.

In the early 20th century, the critters were wreaking havoc on the beef industry. They were costing farmers millions of dollars each year, not just in the U.S. but also in Central and South America.

One infection could "kill a fully grown steer in 10 days," The New York Times wrote in 1977. So in the 1930s, the U.S. Department of Agriculture hired a bunch of scientists to study the fly. One of them was Edward F. Knipling, a young entomologist who had grown up on a Texas cattle farm. He had a crazy idea: Develop birth control for the male flies.

Specifically, Knipling wanted to sterilize a bunch of male flies in the lab and then unleash them into the wild. With enough impotent flies out there courting the ladies, the fly population would eventually plummet, Knipling theorized.

Other scientists balked at the idea. They didn't think it would work. "Scientists would say, 'You just can't castrate enough flies,' " says Knipling's son, Edward B. Knipling.

"Telling people you're going to study the sex life of the screwworm gets some chuckles even today," the son says. "But in the 1930s, it was such a brand new idea. The scientific community thought my dad was pulling their leg."

But Knipling was dead serious. And for more than two decades, he worked on the fly sterilization project with his colleague Raymond Bushland.

They devised a way to grow millions of flies in the lab, using big vats of ground beef, warmed up to body temperature. They figured out how to sterilize the flies using gamma rays — a new technology that came out of research on the atomic bomb.

By 1958, Knipling and Bushland had convinced the U.S. government to start airdropping the sterilized male flies across Florida. Each week they unleashed 50 million flies.

And what do you know? It worked.

Screwworm flies started to disappear. Cattle no longer died from larval intrusion. By early 1959, the screwworm had disappeared from the entire Southeast U.S.

"From there, the snowball got rolling," Edward B. Knipling says. The government started airdropping the flies across Texas, the Southwest and eventually into Mexico and Central America.

By 1997, the project had wiped out screwworms all the way from Texas to Panama. Even today the USDA continues to release flies in Panama to prevent fertile males from sneaking out of South America and reinfecting the U.S. "It creates a buffer zone," Edward B. Knipling says.

Eradication of the screwworm has saved farmers in North and Central Americas billions of dollars, the USDA says. It has reduced the price of beef. The U.N. called it one of the "greatest achievements in animal health" in the 20th century.

This September, Knipling and Bushland will posthumously be given the Golden Goose Award, which honors "seemingly obscure, federally funded research" that has led to big breakthroughs.

But the impact of Knipling and Bushland's work extends beyond the screwworm. Their idea to squash insect populations by sterilizing males has been used to keep the Mediterranean fruit fly out of North America and to eradicate the melon fly from Okinawa.

And now scientists have their eyes set on using the technique for mosquitoes. Just last March, a biotech company released 300,000 sterilized male Aedes aegypti mosquitoes in Brazil — that's the species that carries Zika — in hopes of slowing the spread of the virus.

The jury's still out on whether this strategy will be useful, but there's one thing we know: You can indeed "castrate enough flies."

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