2016: The Sex Life of the Screwworm Fly

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

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

AWARDEES: Edward F. Knipling and Raymond C. Bushland

SCIENCE: The Sex Life of the Screwworm Fly

FEDERAL FUNDING AGENCY: U.S. Department of Agriculture Agricultural Research Service


Screwworm—the name alone might make you chuckle. But these agricultural pests are no laughing matter. Through the early to mid-20th century, screwworm flies killed livestock, wildlife, and even some people across the American South. Based on polls of ranchers in the 1950s, losses and round-the-clock efforts to monitor and treat cattle for screwworm infections were costing American ranchers a staggering $200 million per year, nearly $1.8 billion in today’s dollars.

But thanks to the pioneering fundamental research led by U.S. Department of Agriculture (USDA) scientists Drs. Edward F. Knipling and Raymond C. Bushland, most people living in North and Central America today have never even heard of these costly pests. With their ingenious “sterile insect technique,” the pair laid the groundwork for an international campaign to eradicate the screwworm fly. Beyond that, their technique informs fights to this day against a variety of pests, including disease-spreading insects like the Aedes aegypti mosquito.

The Screwworm Challenge

Edward F. Knipling (left) and colleagues doing field research.   Photo courtesy of World Food Prize Foundation.

Edward F. Knipling (left) and colleagues doing field research. Photo courtesy of World Food Prize Foundation.

By infesting even the smallest of wounds, screwworm flies can bring down a full-grown cow in less than two weeks. Female flies lay their eggs in open wounds, often targeting the navels of newborn calves, and their larvae hatch as maggots that feast within the animal’s wound and literally eat it alive. Every ranch used to employ ranch hands to “ride herd” and “doctor wormies” day and night—at great expense.

But the deadly screwworm flies turned out to be no match for a pair of researchers who friends knew as “Knip” and “Bush.”

Growing up in Texas, Knipling knew firsthand the ravages of pests on agriculture and livestock, from the boll weevil to the dreaded screwworm fly. His earliest assignments as a USDA scientist to develop treatments and controls for livestock insects during the Great Depression of the 1930s gave him time to learn even more, as he spent long hot days trapping and counting flies. It was then that Knipling made a crucial early observation: screwworm flies were not that prevalent; other species of so-called “blowflies” significantly outnumbered them.

A Friendship Forms

In 1937, Knipling took this new knowledge with him to the USDA research station in Menard, TX, where he met his lifelong collaborator, Dr. Raymond C. Bushland. “Bush” had also been working with screwworm flies, overseeing the aptly named “stinkhouse” where they bred large numbers of the nasty flies in dying rabbits for use in experiments. Driven to end the expensive and disgusting current practice (and improve his own working conditions), Bushland developed a less expensive, non-living medium for breeding flies that was easy to produce in large quantities.

Edward F. Knipling.   Photo courtesy of World Food Prize Foundation.

Edward F. Knipling. Photo courtesy of World Food Prize Foundation.

In Menard, Knipling and Bushland struck up a fast friendship, spending many an afternoon just sitting together in the fly breeding facility, watching the flies in their cages. It was here that Knipling made the next breakthrough observation: the female screwworm fly seemed to be monogamous—confining itself to only one mating experience during its life. Males, however, were promiscuous in the extreme.

Combining the females’ monogamy and the researchers’ ability to breed large numbers of flies on the cheap, Knipling realized they might be able to make the flies work against themselves. If they could somehow sterilize large numbers of male flies and release them to overwhelm the native fly population, they could trick wild females into “wasting” their one mating experience on an infertile male. They could make screwworm flies exterminate themselves.

This idea was so far outside the mainstream, Knipling was wary of sharing it even with other entomologists, let alone policymakers at the USDA or the public. But at Bushland’s insistence, they shared the “sterile insect technique” with some fellow scientists. The responses ranged from intense skepticism to outright laughter. “You just can’t castrate enough flies,” their colleagues would say. And this was where their idea rested for over a decade.

The Breakthrough

When World War II broke out, both Knipling and Bushland went to work on pest control for the USDA and U.S. Army, earning many commendations for their contributions to the fights against Typhus and other diseases transmitted by biting insects and afflicting troops overseas.

After the war, Knipling went to the Washington headquarters of the USDA Agricultural Research Service (ARS) to serve as head of the entomology division, while Bushland headed to a lab in Kerrville, TX to continue working on screwworms. In Kerrville, Bushland set back to work on their “sterile insect technique,” with Knipling’s support from headquarters. While Bushland was able to confirm that female flies were monogamous, work on chemical sterilants didn’t pan out, so the same problem remained: how could they sterilize enough flies?

Raymond C. Bushland.   Photo courtesy of World Food Prize Foundation.

Raymond C. Bushland. Photo courtesy of World Food Prize Foundation.

The year 1950 brought the answer, when Knipling and colleagues came across an article by Nobel Laureate geneticist Dr. Hermann J. Muller describing the dangers of radiation and the perils of nuclear war. In it, Muller referenced his earlier work showing that exposure to excessive amounts of radiation rendered fruit flies sterile. Upon reading this, Knipling wrote Muller to describe their pest control idea, and the geneticist replied, “I know nothing of screwworms, but your theory is sound,” and encouraged them to proceed.

In Kerrville, Bushland scraped together the resources to prove out this new methodology. At first, he convinced a friend at a nearby Army hospital to allow them to use the hospital’s x-ray machine, and with it he and colleagues were able to prove they could indeed sterilize the screwworms, without otherwise affecting their…romantic endeavors. The x-ray machine was good enough for these early experiments, but impractical for sterilizing the vast numbers of flies they would need, so Bushland traveled to Oak Ridge National Laboratory and procured a more potent source: radioactive Cobalt, a byproduct of nuclear reactors.

Putting the Technique to the Test

This sterilization experiment was all well and good in the laboratory, but would it work in the wild? They needed a place to test their theory—a place with a secluded population for a controlled experiment.

Their answer came in 1953 in the form of a letter from the small island of Curaçao. The letter asked Knipling, in his capacity as head of the ARS’s Entomology Division, if they could help with a problem—screwworms were decimating the local goat population, a source of both milk and meat. When Knipling pulled out a map to locate the island, he found the perfect site for the controlled experiment he envisioned. Within a year, having secured a small amount of federal funding, Knipling, Bushland, and their colleagues led the first demonstration of the sterile insect technique, successfully eradicating the population of screwworms on Curaçao.  

On May 25, 1955, headlines around the United States reported on the successful test, proclaiming the victory of “atom and romance” in the fight against the screwworm pest. After three months of dropping millions of USDA-raised and irradiated male screwworms in entomological air raids, the team was no longer able to find a single viable egg among those laid by local females. The sterile insect technique, maligned by some of their colleagues, had worked even better than predicted.

The victory on Curaçao formed the model for the screwworm eradication program that would run in the southern United States and elsewhere over the coming decades. By 1959, with the help of local cattlemen’s associations, state and federal governments, and some unusually cold winters, sterile insect technique was used to eliminate the screwworm east of the Mississippi. And by 1966, the screwworm fly was completely gone from the United States. Over the next forty years, the program moved down through Central America, achieving eradication all the way down to Panama by 2006. Today a barrier zone of sterile flies is maintained there to prevent reinfestation of wild screwworm flies from South America. An investment of approximately $250,000 for fundamental research on screwworms spread over several decades from the 1930s to 1950s resulted new technologies and an operational program to eliminate the deadly pest with annual cost savings of over $200 million (in the 1950s) for meat and dairy suppliers in the U.S. alone.

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

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

Ridicule and Renown

Knipling and Bushland, once literally laughed out of rooms for what some colleagues called “Knip’s pipe dream,” were honored with the 1992 World Food Prize for their groundbreaking work—just one of many accolades the duo has received since the eradication of the screwworm fly in America.

The federally funded research on the sexual behavior of the screwworm fly came before Senator William Proxmire’s “Golden Fleece Award,” which often ridiculed odd-sounding science. But it did not escape such ridicule. 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, coming under fire especially during a particularly nasty influx of screwworms from Mexico in the 1970s, and again in the 1990s, when the ongoing eradication effort appeared in the Citizens Against Government Waste’s inaugural “Pig Book” of what the group deemed wasteful federal spending.

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. For example, damaging tropical fruit fly species have been eradicated or held in check in parts of Japan, the United States, and Guatemala. 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. 


Download one-page summary