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The Sex Life of the Screwworm Fly: How an Odd-Sounding Study Saved Ranchers Billions

Raymond C. Bushland (standing) and Edward F. Knipling (seated at microscope). Photo courtesy of World Food Prize Foundation.

Providing scientists with funding to spend hours watching flies in breeding cages might seem like a waste of money. And in fact, Raymond C. Bushland and Edward F. Knipling, were often ridiculed, and were the butt of many jokes. But we’re lucky they persisted, because their work has eradicated the screwworm fly from North and Central America, saving ranchers about $1.8 billion a year. Their research is also giving us a way to fight the mosquitoes that carry the Zika virus.

As described in the video below, the screwworm fly is a nasty pest. The female lays its eggs in even the smallest wounds, or the navels of newborn calves. When the eggs hatch as maggots, they begin feeding within the animal’s wound, literally eating it alive. A full-grown cow can be killed in less than two weeks.

What Bushland and Knipling discovered from their hours of watching the breeding screwworm flies was that the female was monogamous, mating with only one male. The males on the other hand were “promiscuous in the extreme.” The two thought that if they could sterilize the males and release them into the wild, they could breed with wild females who would waste their one breeding experience on an infertile male. Over time, they believed they could make screwworm flies exterminate themselves.

The problem was figuring out how to sterilize enough flies. The answer came from noble laureate Hermann J. Muller. Muller was studying the genetic effects of radiation and had published an article showing that exposure to radiation rendered fruit flies sterile. To see if it would work on screwworm flies, Bushland convinced a friend at an Army hospital to let them use the hospital’s x-ray machine. Sure enough, the x-rayed flies were sterilized. Their first test in the wild, the small island of Curaçao, started in 1954. The dropped millions of irradiated male screwworms from airplanes, and by 1955, the fly was eradicated from the island.

With the support of local cattlemen’s associations and state and federal governments, the sterile insect technique was used to eliminate the screwworm fly east of the Mississippi, and by 1966 it was completely eradicated from the United States. By 2006 it was eradicated down to Panama. As you’ll see in the video, there is barrier zone of sterile flies maintained there to prevent wild screwworm flies from South America from reinfesting Central and North America.

In 2016, Bushland and Knipling were posthumously honored with the Golden Goose Award. The award honors scientists whose work may have been considered silly, odd or obscure but that has resulted in great societal benefits. As the steering committee notes in saluting the two scientists:

Raymond C. Bushland (left) and Edward F. Knipling receiving Hoblitzelle National Award for Agriculture. Photo courtesy of World Food Prize Foundation.

The “sex life of the screwworm” has been a favorite target for some Members of Congress eager to talk to their constituents about “Washington waste.” And the story has had an amazingly long shelf-life. It has been variously attacked or used in the defense of silly-sounding research regularly since the 1950s.

Yet by helping to eradicate the screwworm, Knipling, Bushland and their colleagues have saved the U.S. livestock industry billions of dollars over the last 50-plus years. For U.S. consumers, this has translated into an estimated 5 percent reduction in the price of beef at the supermarket. And for others in less developed parts of the world, the pest control technique they pioneered is a crucial component of food security and public health.

Today, the technique and variations of it hold promise for controlling disease-carrying mosquitoes. The international screwworm eradication program is a classic example of odd-sounding basic science producing enormous returns.”

Not bad for an investment in research of $250,000 spread over three decades!

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Kathy Voth
Kathy Voth
I am the founder, editor and publisher of On Pasture, now retired. My career spanned 40 years of finding creative solutions to problems, and sharing ideas with people that encouraged them to work together and try new things. From figuring out how to teach livestock to eat weeds, to teaching range management to high schoolers, outdoor ed graduation camping trips with fifty 6th graders at a time, building firebreaks with a 130-goat herd, developing the signs and interpretation for the Storm King Fourteen Memorial trail, receiving the Conservation Service Award for my work building the 150-mile mountain bike trail from Grand Junction, Colorado to Moab, Utah...well, the list is long so I'll stop with, I've had a great time and I'm very grateful.


  1. Thank you very much for the great article. I saw one, just one, last year and none this year. Science at it’s finest! walk in beauty

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