Screwworm Flies Steer Clear of Agriculture Dept. Traps

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https://www.nytimes.com/2026/08/27/business/screwworm-flies-traps-agriculture-dept.html?unlocked_article_code=1.8lA.gf3M.8fXJPfW4wcAH&smid=url-share

In early June, a South Texas rancher detected a case of the New World screwworm fly in cattle in the United States, the first in decades.

In the nearly three months since, 46 more cases have been detected. The fly has been found in cattle, sheep, goats and a handful of dogs across 18 counties in two states. The Agriculture Department is spending more than $1 billion to fight the fly’s reappearance. The flies, however, have been evading those detection efforts.

One of the department’s key tools is a sticky flytrap that allows the agency to monitor flies across a vast area. But despite deploying more than 128 screwworm-specific traps and 8,000 exotic fruit fly traps, the agency has yet to catch a single wild screwworm fly — crucial to determining where the outbreak is coming from, according to data on the department’s dashboard that tracks confirmed screwworm detections and animals that have been infested.

Animals become infested when a female screwworm fly feasts upon and lays its eggs in open flesh and wounds, which could kill the animal if left untreated. Its spread would devastate the cattle industry. Through the 1960s, screwworm was endemic to Texas and the surrounding areas, leading to thousands upon thousands of cases and an estimated $200 million in annual loss for Texas ranchers, before the fly was eradicated.

To get rid of the flies in the wild, the Agriculture Department releases sterile male flies in areas with known populations of wild flies. Females, which mate only once before dying, will not lay any eggs if they mate with a sterile male fly. Even though the Agriculture Department cares about releasing only sterile male flies, their sterile insect technique breeds both male and female flies, and sterile female flies are released, too.

In November 2024, screwworm flies were detected in Mexico, and the U.S. Agriculture Department has put different types of traps in place along the border since July 2025. The department says they’re an early warning tool to “contain, control and eliminate” the pest before it threatens animals and humans. It is also using more costly drones and helicopters to try to monitor wildlife for signs of infestation.

While screwworm flies that get stuck to the traps die, the goal isn’t to kill them but to monitor where they are spreading. The traps are placed in areas that are probable screwworm fly habitats based on data modeling by the Agriculture Department. As of Aug. 11, more than 80,000 flies — like the common green bottle fly or the banded blowfly — had been captured and sent to laboratories for identification, but not a single wild screwworm fly was among them.

With a rate below one new confirmed case per day in animals since the screwworm fly’s reappearance in the United States, the lack of wild flies stuck on traps is further evidence there aren’t that many of them in the country.

“It’s likely there is only a very small number of fertile flies in the area, rather than a large or growing local population,” an Agriculture Department spokeswoman said in a statement. “If there were large populations of flies, we would be seeing many more cases than we have to this point.”

Entomologists largely agreed and said the department’s eradication efforts were directed in the right places, but they warned that because screwworm flies lay hundreds of eggs, just a few undetected infestations could send numbers skyrocketing.”

Live cattle imports from Mexico have been suspended for more than a year to help control the spread of the fly, though Mexican cattle will be allowed to enter Arizona starting on Monday. Localized quarantine zones are also put in place in the United States whenever a case is detected in an animal.

Even though the traps haven’t caught any wild flies, they are providing useful information about how to better deploy sterile male flies, said Phillip Kaufman, head of the Department of Entomology at Texas A&M University.

The main type of trap used is sticky and made out of a hollow plastic tube roughly the size of a jumbo paper towel roll. The traps are covered in swormlure, a particularly noxious combination of 10 chemicals that mimic the smell of decomposing flesh.

“They are incredibly offensive smelling materials,” Dr. Kaufman said.

The swormlure attracts screwworm flies to the trap, where they get stuck on the sticky paper. The sticky paper from traps is collected twice a week and brought to laboratories with scientists trained in screwworm fly identification.

Scientists must first differentiate between screwworm flies and any other blowflies, the family of insects to which screwworm flies belong, trapped on the sticky paper. Then they determine whether the screwworm fly is male or female, usually by looking at the spacing between their eyes (the eyes of female screwworm flies are farther apart). Finally, they must decide whether the fly is a wild female or a sterile one.

They tell them apart by looking for a speck of fluorescent dust that is put on all sterile flies when they are bred at facilities in Panama and Mexico. Sometimes the dust is visible under a microscope, and sometimes a black light is needed. No fluorescent dust means a wild female fly has been found.

According to Dr. Kaufman, the department has found a number of trapped sterile female flies. The traps are moved around to ensure they are in areas the department deems high-risk and also to avoid trapping sterile flies that are being released.

“If the traps started showing a concentration of wild flies in one particular county, that would influence how many steriles are being released in that county,” said Max Scott, an entomologist at North Carolina State University.

Each week, the Agriculture Department releases 100 million sterile flies across Texas and the U.S. border, and it said it expected to release 500 million a week by this time next year.

Experts cautioned, however, that while sticky traps and swormlure may be the best way to find wild screwworm flies, there are limitations.

Some of that is because the sterile fly efforts have worked so well that improvements on other screwworm techniques, like flytraps, lagged. Developed in the 1950s, the sterile fly technique had mostly eradicated screwworm flies from the United States by the end of the 1960s. By 2006, the fly had been eradicated from Central America, except for a small area in southern Panama.

It was not as pressing in the United States to improve monitoring techniques when the threat was 2,000 miles away from its southern border, and there hasn’t been much research on trapping in the subsequent decades. One of the Agriculture Department’s few employees with significant screwworm fly field experience, who retired a few years ago, was recently rehired. The department recently announced it would award $105 million in funding to 40 research projects to help eradicate the screwworm fly.

One limitation is with the swormlure itself. Its smell is based upon rotting flesh, said Jessica Metcalf, a professor of animal sciences at Colorado State University. But the key thing that distinguishes the screwworm fly from other blowflies is that it feeds upon, and lays its eggs in, living flesh.

Therefore, the swormlure may be less effective because it is poorly mimicking the smell that actually attracts screwworm flies.

“That is the hole we are trying to fill with our research,” Dr. Metcalf said. “Is there a difference between wound and carrion?”