The US Has a Plan to Combat Screwworm. It Involves a Lot More Flies

A flesh-eating parasitic fly that poses a major threat to livestock has returned to the United States after 60 years. This week, the US Department of Agriculture confirmed the presence of New World screwworm in a calf in southern Texas.

Eliminated in the US in 1966 and as far south as Panama by 2006, its recent reemergence in Mexico made it likely that the screwworm would eventually enter the country again, with modeling showing that it could arrive as soon as summer 2025. It took slightly longer, but the screwworm has arrived. And to head off an outbreak, officials are deploying a tried-and-true technique: releasing lots and lots of adult screwworm flies.

A screwworm infection occurs when a female fly lays its eggs in open wounds or other body parts of warm-blooded animals. When the eggs hatch, maggots emerge and feed on living tissue before turning into flies. As adults, screwworm flies do not bite or feed on flesh. Scientists in the 1930s and 1940s thought if they could prevent female flies from reproducing, they could break the cycle. At the time, New World screwworms killed hundreds of thousands of cattle annually, mostly in the American South and Southwest.

In the 1950s, researchers at the USDA made a breakthrough when they applied radiation to male screwworms and rendered them sterile. When released into an infected area, the sterile males mate with wild female insects and produce unviable eggs. No offspring are produced, and the population crashes. Known as sterile insect technique, it was first used successfully on the island of Curaçao, off the coast of Venezuela. It took just seven weeks to eliminate the pest, and the effort saved goat herds on the island that were a vital food source.

The technique takes advantage of the fact that female New World screwworm flies only mate once in their lifetime. "The sterile insect technique is probably the most eloquent example of a completely successful biologic control mechanism,” says Sally DeNotta, associate professor of veterinary medicine at the University of Florida. “The life cycle stops. There's no progeny produced. It’s been very successful.”

For years, the dense stretch of rainforest between Panama and Colombia known as the Darién Gap served as a biological barrier where sterile flies were released to prevent the northward spread of screwworm. But insects started breaking through the barrier in 2022.

To prevent an outbreak in South Texas, the USDA has blocked off a roughly 12-mile zone around the infected calf and is carrying out a targeted release of sterile screwworm flies from trucks. That’s in addition to the 4 million sterile flies per week already being air-dropped in the area. Anticipating the screwworm’s movement north, in February, the agency shifted its efforts to disperse 100 million sterile flies per week to focus on the area along the US-Mexico border.

“While this development is a serious threat to our livestock and wildlife, it hasn't caught us off guard,” USDA secretary Brooke Rollins said during a House Agriculture Committee meeting on Thursday.

She said around 400 million flies per week are needed to beat back screwworm. Currently, the US can only produce about 100 million flies per week at a facility located in Panama.

A sterile insect facility in Mexico shut down in 2012, but the USDA is investing $21 million to help renovate and convert an existing fruit fly facility in Metapa, Mexico, to produce an additional 60 to 100 million sterile flies per week. That facility is expected to be operational this summer, according to the USDA.

The agency is also fast-tracking the construction of a $750 million sterile fly facility at Moore Air Base in Edinburg, Texas, near the border with Mexico. That facility will not be up and running until November 2027.

Screwworms do not infest meat, fruits, vegetables, or other food sources, but can infect people. Since 2023, there have been at least 2,070 human cases of screwworm in Mexico and Central America, according to the Centers for Disease Control and Prevention.

DeNotta says she has no doubt that the US will eradicate it again, but says in the meantime, more cases could pop up. “A single female fly can lay hundreds to thousands of eggs at a time,” she says. “They’re able to travel to the next site and to the next host, so it's very unlikely that this case is truly the only case.”