At the southern border, a new threat is creeping into the United States—and no wall can stop it. Enter, the New World screwworm: a small fly that lays eggs into open wounds of a bird or mammal, where the larvae feast on live flesh. If untreated, the wound will become infected and likely fatal.
The flies target almost all warm-blooded species—humans occasionally included—but the most common victims are deer and cattle. Wildlife experts are warning that an imminent invasion could have devastating effects on the populations of both animals. The risk to deer, especially, should be of concern to hunters.
“The raw pedicle left after antler shedding is very attractive to New World screwworms,” Terry Hensley, with the Texas A&M Veterinary Medical Diagnostic Lab, told Farm Progress last month. “A lot of vital organs are very close to that pedicle, so it can be devastating.”
That devastation was witnessed once already in the 1950s, when a screwworm outbreak culled deer populations by up to 60% in some southern states, like Florida. In Texas, 80+% of the whitetail fawn crop was lost every year to screwworms, according to Jason Sawyer, Chief Science Officer at the East Foundation. (Listen to a full interview with Sawyer and Communications Director James Powell in Episode 383 of Cal’s Week in Review podcast.)
Newborn fawns are particularly susceptible because the open wound where the umbilical cord is severed after birth is a prime target for the flies. Bucks can also fall victim during the rut, when they sustain injuries from fighting.
Now, if you’re scratching your head wondering why you haven’t heard of screwworms before, it’s because they haven’t been present in the United States for nearly 60 years. The 1950s outbreak was successfully contained by what was, at the time, a novel technique: male flies were bred in captivity, sterilized by radiation, and then released into the wild. In Ryan Callaghan’s words, “We released millions of these little infertile dudes into the wild, but they were shooting blanks. Their attempts at procreation failed, and over the course of many years, the wild populations dwindled down to nothing.” By 1966, screwworms were considered fully eradicated from the United States.
However, south of the border in more tropical, Central American areas, fly populations remained viable. In 2024, reports of screwworm outbreaks began to surface in Panama, and then in Guatemala and Honduras. In the time since, the insects have continued to creep northwards. As of early June of this year, the flies were reported 700 miles from the US border, but there’s reason to believe they may already be in Texas by now.
Stopping the outbreak, though, is proving to be a tall task. Sterile fly production facilities in Texas, Florida, and Mexico were shuttered following the eradication success in the 60s. Only one breeding facility remains in Panama. In mid-June, Secretary of Agriculture Brooke Rollins announced a $21 million federal investment to renovate fly factories in Mexico, and $8.5 million for a facility in southern Mexico. Soon, the USDA is expecting to supply 300 million sterile bugs per week.
Still, experts say that’s not nearly enough to stymie an outbreak. In that vein, the Texas Department of Agriculture has developed a screwworm flytrap and poison called “TDA Swormlure” that the department claims can kill 90% of screwworms upon contact. The system is currently being trialed in Panama, but according to the Texas Agriculture Commissioner, “the traps are expected to outperform anything else available.” The agency is also looking into the possibility of feeding ivermectin-laced food to deer as a preventative measure along the border.
In all, it’s going to take as many resources as possible to stop the flies from wreaking havoc on US deer populations. “If you look at the historical effort here, it was a combination of private landowners and organizations, conservation groups, livestock organizations, and the federal government that came together to create the solution,” Jason Sawyer said. “That’s where we see ourselves today, and this is what government is for—to safeguard the resources and productivity of the nation. It’s that sort of an issue.”
Hunters—especially those in the South—will likely be hearing more about the screwworm in the coming months.
Feature image via The Texas Wildlife Association Facebook.
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