The Decline of the Screwworm Fly – Edward Knipling vs Cochliomyia hominivorax

Hundreds of screwworm larvae can infest
a single wound, doing unspeakable damage
and often causing death. Image courtesy
of the US Agricultural Research Service.

In July 2013 a British woman came to the attention of both the medical community and the media when she returned from Peru with maggots in her ear. The fly larvae had created a cavity connected to the ear canal and were feeding on healthy tissue in Rochelle Harris's head.

According to a Medical Daily story by John Ericson, Harris endured “unbearable pains emanating from one side of her face, pains that were eventually accompanied by menacing scratching sounds from inside her head” ("British Woman Discovers Flesh-Eating Maggots Inside Her Ear," July 16, 2013).

This is a horrific story, and many people reading it will have heard of the dreaded New World screwworm fly – Cochliomyia hominivorax - for the first time. In North America, too, we're unfamiliar with the fly - we've forgotten that it once infested the southern United States, spreading northward each summer as far as Canada. If not for entomologist Edward Knipling, it might still plague us, as it does parts of South America.

Edward Knipling Meets the Screwworm Fly

As a boy in the early 20th century, Edward Knipling worked long hours on his parents' Texas farm. There he encountered all manner of insect pests, including the maggots of the screwworm fly, which infested healing umbilical cords of newborn animals and any other open wound that the female fly could find.

Left alone, screwworm maggots feed on healthy flesh, quickly turning a small wound into a large, spreading one, and the odor generated by their activities attracts more female flies to lay their eggs. Infested animals often die. Humans aren't immune, with infested wounds and nasal sinuses most often reported.

According to biographers Adkisson and Tumlinson, Knipling “decided at an early age that he wanted to make a bigger contribution to agriculture than treating screwworm infested calves or pulling a sack down a cotton row” ("Edward F. Knipling, 1909 – 2000; Biographical Memoirs"). But Knipling likely had no inkling that his contribution would be to discover a way to eradicate C. hominivorax from the United States, Central America, and even Libya, in Africa, where it was accidentally introduced in the 1980s.

Knipling and the Idea of Screwworm Eradication

Edward Knipling combined scientific
knowledge and innovative thinking
to pioneer the sterile insect
technique. As a result, the screwworm
fly has been eradicated from much of
its range. Image courtesy of the US
Agricultural Research Service.

Edward Knipling sought a science career in entomology, ultimately earning a PhD in entomology from Iowa State University. During his studies, the United States recorded thousands of cases of screwworm infestation in livestock annually; some years saw hundreds of thousands of cases. The occasional human infestation occurred as well. Screwworm caused serious agricultural losses and horrible human illness.

By 1937 Knipling was studying the life cycle of C. hominivorax with colleague R. C. Bushland, and  the two made an important observation: female screwworm flies mate only once. Knipling knew that  he might be able to turn this against the pest, but the puzzle of just how one might use it to advantage had to wait until the end of WWII, when he had time to return to it.

Adkisson and Tumlinson write that Knipling thought “that if male flies could be produced in large numbers, sterilized, and released into the environment they might out-compete... the wild fertile males in breeding with females... If a sufficient number of sterile males could be released into the wild population," Knipling thought, the sterile males might “breed the screwworm population into extinction.”

How to Sterilize a Screwworm Fly

Knipling's idea raised two questions: how do you sterilize a male screwworm fly without damaging it in other ways, and how do you rear large numbers of screwworm flies in the laboratory? He knew that radiation could render insects sterile because of work done on fruit flies.

Now working in Washington DC, Knipling couldn't do the experiments himself, but Bushland, still in Texas, took it on. With scarce resources himself, Bushland had to innovate: he used X-ray equipment at the Brooke Army Hospital in San Antonio, Texas, to investigate sterilization, by trial and error. Meanwhile, Knipling worked on determining just how many sterile male flies were needed.

Bushland had already devised a laboratory diet for raising screwworms; now he, and others, worked to modify it to make it cheaper and better suited to mass production. The final test would be trials to see if  the sterile male approach actually worked.

The screwworm fly, Cochliomyia hominivorax is
aptly named: hominivorax means man eater.Image
courtesy of The Mexican-American Commission
for the Eradication of the Screwworm.

Screwworm Eradication in the United States

The first screwworm eradication trial took place on Sanibel Island, off the coast of Florida. Researchers released 39 sterile males per square kilometer each week for eight weeks, and saw the screwworm population drop off to virtually zero; however, screwworm flies continued to migrate from the mainland, so the island wasn't rendered screwworm free.

A second trial on the island of Curaçao in 1954 used four times as many sterile flies, and eradicated C. hominivorax in just three and a half months. The method worked.

The years that followed saw the construction of large facilities devoted to producing millions of sterile screwworm flies, and eradication programs, beginning in Florida. Cochliomyia hominivorax was eradicated in Florida in 1959; the process took longer in southwestern states because of continual reintroduction from Mexico, but the US was free of screwworm by 1982.

Screwworm Eradication in Central America

By 1984, there were no screwworm flies north of the Isthmus of Tehuantepec in southern Mexico, and today Central America is screwworm free all the way to the Panama - Columbia border.

Affected countries in South America are now using integrated pest management, including active surveillance, insecticides, chemical attractants and traps, and the sterile male technique to control and, one day, eliminate C. hominivorax from the last of its range.

If Rochelle Harris returns to Peru when she's older, she might not have to worry about another encounter with the screwworm fly, but if she does meet it again, she'll be ready. John Ericson quotes her, after her ordeal: "I'm no longer as squeamish as I was about bugs," she [says] "How can you be when they've been inside your head?"

Additional Reading

Adkisson, P., Tumlinson, J. 2003. "Edward F. Knipling, 1909 – 2000." Biographical Memoirs. 83.

Ericson, John. Jul 16, 2013. "British Woman Discovers Flesh-Eating Maggots Inside Her Ear." Medical Daily.

Mastrangelo, T., Welch, J. B. 2012. "An Overview of the Components of AW-IPM Campaigns Against the New World Screwworm." Insects. 3.

Novy, J. E. 1991. "Screwworm Control and Eradication in the Southern United States of America." In: New World Screwworm Response to an Emergency. World Animal Review. Special issue.