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

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

The United States has confirmed the first known case of New World screwworm in over six decades, with a calf in southern Texas infected by the parasitic fly. This reemergence of a species that once devastated livestock across the American South and Southwest signals a renewed threat to both agriculture and public health.

The Sterile Insect Technique: A Biological Weapon Against Screwworm

The sterile insect technique (SIT) has long been the go-to strategy for controlling screwworm populations. Developed in the 1950s, the method involves irradiating male flies to render them infertile before releasing them into the wild. These sterile males then mate with wild female insects, producing no viable offspring and causing the population to crash.

The technique was first successfully applied in the 1950s on Curaçao, where it eliminated the pest in just seven weeks. It proved so effective that the USDA used it to eradicate screwworm from the U.S. by 1966. The same method is now being deployed in response to the recent reemergence of the fly.

A Growing Need for Sterile Flies as the Pest Moves North

The recent detection of screwworm in Texas has accelerated the need for a large-scale SIT deployment. The USDA has already begun air-dropping 4 million sterile flies per week in the affected area, but estimates suggest that 400 million sterile flies per week are needed to effectively combat the infestation.

A facility in Panama currently produces about 100 million sterile flies weekly. The USDA is working to reactivate a facility in Metapa, Mexico, which could add 60 to 100 million sterile flies per week. A new $750 million facility at Moore Air Base in Texas is under construction but won’t be operational until November 2027.

The movement of screwworms across the U.S.-Mexico border has also raised concerns about human health. Since 2023, at least 2,070 human cases of screwworm have been reported in Mexico and Central America, according to the Centers for Disease Control and Prevention. The fly can infest humans through open wounds, though it does not transmit disease.

A Race Against Time to Prevent Widespread Infestation

Despite the urgency, the U.S. is still working to expand its capacity for producing sterile flies. Officials have stated that the country is not caught off guard by the reemergence, but the challenge remains significant. The Darién Gap, a rainforest region between Panama and Colombia, had previously served as a natural barrier against the spread of screwworm, but this has been breached in recent years.

Sally DeNotta, a veterinary medicine professor at the University of Florida, warns that a single female screwworm can lay hundreds to thousands of eggs at a time, making it highly likely that the current case is not an isolated incident. "They're able to travel to the next site and to the next host," she said. "It's very unlikely that this case is truly the only case."

With the construction of new facilities and the reactivation of existing ones, the U.S. is aiming to scale up its sterile fly production. But until then, the strategy remains one of containment and aggressive biological control.

The Road Ahead: A Delicate Balance Between Biology and Logistics

The USDA is confident in its ability to once again eradicate screwworm from U.S. territory using the same methods that worked in the past. However, the timeline and logistical challenges of expanding sterile fly production remain uncertain.

As the nation prepares for the next phase of this biological battle, the focus is not only on the science but also on the infrastructure needed to support it. With new facilities coming online in the coming years, the U.S. is hoping to avoid the kind of agricultural devastation that once plagued the country in the mid-20th century. The fight against screwworm is a race against time, but one that, if managed carefully, can be won with the help of millions of sterilized flies.