It was the 1950s. In the southern U.S. screwworms were literally screwing cattle, and figuratively doing the same to ranchers who were annually losing the equivalent of $1.8 billion in today’s dollars. The somewhat comical-sounding name has nothing to do with the worm’s mating habits. And the screwworm isn’t even a worm! It is the larva of Cochliomyia hominivorax, commonly known as the “screwworm fly.” The creature does look somewhat like a worm, and the little spikes that cover its body gives the appearance of the thread of a screw. It is these spikes that allow the “worm” to burrow into the flesh of animals as a screw might do. Then the screwworm starts chomping away, eating the animal alive. Ranchers were at their wits end. Fortunes were being lost in dead cattle. As if that weren’t enough, “hominivorax” literally means “man-eater.” Indeed, there have been cases, mercifully very few, in which wounds in humans became host to the screwworm and its voracious dining habits.
Scientists were finally able to find a way to proverbially bring the screwfly to its knees, but it took a critical observation about the female’s sexual appetite, a laboratory full of rabbits, a Nobel laureate’s concern about nuclear war, an X-ray machine, and a letter from the Caribbean island of Curaçao!
The life cycle of the screwworm fly is only about three weeks, but that is long enough to cause catastrophic damage. A female can lay up to 400 eggs in one shot, preferring an open wound where the eggs can hatch into larvae that resembling a screw being driven into an object, immediately dive deeper into tissues. Such wounds may come from branding, dehorning, castrating or just being scratched by barbed wire. An animal can die from damage to critical organs or from secondary bacterial infections of open wounds. Once the larvae have gorged themselves, they drop to the ground where they burrow into topsoil, pupate, and emerge as flies to start the dreadful cycle again.
In the 1930s, entomologists Edward Knipling and Raymond Bushland were struggling to find a solution to the screwfly problem at the U.S. Department of Agriculture’s research station in Menard, Texas. Rabbits inflicted with lesions proved to be an ideal breeding ground, and as the flies multiplied, Knipling made a crucial observation. I can’t imagine how, but he noted that while male flies were promiscuous, females mated only once! An idea was quickly hatched! If somehow male flies could be sterilized and released into the wild, females, unaware of the males’ lack of potency, would be lured into mating but would produce no offspring. Even to colleagues the “sterile insect technique” idea seemed far-fetched. One amusingly commented that the duo could not possibly castrate enough of the male flies.
Realistically, of course, Knipling and Bushland did not have any sort of insect vasectomy or surgical removal of the flies’ gonads in mind. They were floating the idea of some sort of chemical agent that would sterilize males. There already was a model for such a possibility with diethylstilbestrol, a synthetic oestrogen developed in the 1930s to prevent miscarriage that was also found to inhibit sperm production. However, none of the chemicals they tried was effective in sterilizing flies. The researchers’ frustration continued until 1950 when Knipling happened to come across an article by Dr. Hermann Muller who had received the 1946 Nobel Prize in Medicine and Physiology for “the discovery that mutations can be induced by X-rays.” The article had nothing to do with screwworms, but warned about the dangers of radioactive fallout from nuclear testing with Muller alluding to his observation that fruit flies exposed to X-rays become sterile. This prompted Knipling to contact Muller and solicit his thoughts about sterilizing the screwworm fly with X-rays. Muller’s response that the idea was sound got the ball rolling.
By this time, much to the relief of rabbits, Knipling and Bushland had found that large numbers of screwworm flies could be raised on ground meat. Luckily, they also had a friend at a hospital with access to x-ray equipment. Off they went with a brood of flies to give the theory a shot. The X-rays did indeed render the flies infertile, and they did that without affecting their love life. But a lab experiment was one thing. Would releasing the flies into the wild have an effect? And where could this be tried? Once again, chance opened the door.
Curaçao was experiencing a screwworm fly infestation and goat herds were being decimated. An official reached out to Knipling, who had made already made a name for himself in the scientific literature with his publications on diseases transmitted by biting insects. The entomologist jumped at the opportunity to carry out a real-world experiment! X-ray machines were not a practical way to sterilize large numbers of flies, but it turned out that radiation from Cobalt-60, a byproduct of nuclear reactors, fit the bill. In 1953 millions of sterile flies were dropped from the air over Curacao and the screwworm fly was eradicated! The success of that experiment led to a widespread program of sterile fly release in the U.S. and by 1966 cattle were no longer tormented, ranchers relaxed, and consumers enjoyed lower beef prices.
Sporadic infestations by screwworm flies migrating from South America have occurred since, but today a large facility in Panama continuously raises sterile flies ready to be released at the first sign of an infestation. Curiously, given that government funded research resulted in the development of an effective method to control this devastating insect, the granting of government funds to “study the mating habits of the screwworm fly” is still sometimes brought up as an example of wasteful squandering of public money. That’s a nonsensical view. Without such funding, researchers would not have been able to successfully screw the screwfly.
