There is an ongoing program in Panama that keeps flesh-eating flies out of the USA and Mexico that not many people know about. How are these flies kept at bay and are they anything to worry about?

Cochlyomia hominivorax, or the New World screwworm fly, is a species of fly native to the Americas. Their scientific name means “man-consuming fly” and, as the name suggests, they are known for eating the flesh of mammals. After mating, the females seek out mammals with open wounds and once they find one, they lay their eggs on the wounds. Once the eggs hatch, the larvae begin to feed on the exposed flesh of the host. There is no vaccine or antibiotic that is able to remedy the screwworms, making them a particularly gory problem for the people of the Americas a century ago.

Screwworm flies have tormented humans for as long as humans have lived in Central and South America, but the screwworm situation got particularly bad after cattle ranches started to pop up in the Southern USA in the 19th century. Wounded cows would be swarmed with screwworms and suffer greatly. During the 1940s, a scientist with the Department of Agriculture named Edward Knipling came up with an idea that seemed impossible, but the USA was just desperate enough to try it.

Female screwworm flies can lay hundreds of eggs, but only mate with one male before laying their eggs and dying. Knipling hypothesized that if they could release enough infertile male flies into an area then the females would mate with them, but when they laid their eggs, the eggs would never hatch.

Knipling had an idea of how he would get large quantities of sterile male flies as well. At this time in history, radiation was being heavily studied. One study found that a certain amount of radiation could sterilize beetles without physically harming them. Seeing this, Knipling’s team looked for the substances that would provide the perfect amount of radiation to sterilize male screwworm flies. They found two: Cobalt-60 and cesium-137. Both provided the perfect amount of gamma radiation to sterilize the flies.

Knipling’s plan was to raise millions of male screwworm flies in specialized facilities, expose them to the proper amount of radiation, and then release them in areas experiencing screwworm problems to mate with the female flies. Using this method, scientists were able to drive the flies from the Southern USA all the way down to Panama. This is known as sterilized insect technique, or SIT. 

While total eradication of the flies in South America seems unlikely at this point because of their presence in the Amazon and the political conflict between South American countries, other efforts have prevented catastrophes, such as the incident in 1988 when invasive New World screwworm flies were discovered in Libya. The flies threatened to spread across the Mediterranean and even down the Nile, but the flies were eradicated in Africa after a successful SIT program.

SIT facilities can use up to 70 tons of meat along with thousands of gallons of blood to raise these flies. While this may seem like a lot, facilities usually produce 1,000 flies per dollar spent, which is a pretty good deal for the estimated $900 million it saves each year for the livestock industry.

Currently, the governments of the United States and Mexico fund facilities in Panama to keep the flies at bay, though there have been some incidents, such as the outbreak in the Florida Keys in 2016. Lauren Walker pondered, “…will Trump’s ‘America First’ initiative potentially change the funding of these sites?”. While a rapid decrease in SIT funding is yet to happen, it is all possible under the frequent cuts that have happened during Trump’s presidency.

While the eradication of screwworm flies in North America is generally seen as a positive thing, Hannah Givney points out the possible ecological damage, asking, “How does that affect the habitat? You will never know until you do it.”

So are New World screwworm flies anything to worry about? The answer is no, not as long as the SIT facilities continue to be funded. Additionally, because of Hingham’s temperate climate, screwworm flies can’t survive here. So flesh-eating flies remain a distant nightmare rather than an approaching threat.