US deploys sterile flies to fight screwworms
For the first time in nearly six decades, the New World Screwworm has turned up on American soil, and the plan to stop it is already running short.
The alarm was raised when a three-week-old calf in La Pryor, Texas, roughly 30 miles from the Mexican border, was found with screwworm larvae burrowed into its umbilical area. It marked the parasite’s first confirmed US presence since 1966. Since then, federal officials have scrambled to outline a response, but the gap between what’s needed and what’s available is hard to ignore.
Screwworm flies are not ordinary pests. The females lay eggs in the open wounds or mucous membranes of living animals and people. Once hatched, the larvae use sharp mouths to tunnel through living flesh, multiplying by the hundreds, and will kill their host if the infestation goes untreated. While the threat to humans is considered low, the prospect of a livestock outbreak has cattle ranchers deeply unsettled about what it could mean for beef markets.
The centerpiece of the US response is the Sterile Insect Technique, a method that involves breeding screwworm flies in controlled environments, exposing them to radiation to render them sterile, then releasing them into the wild in enormous numbers. Since females only mate once, a sterile male is a dead end in the reproductive chain. The approach helped drive screwworms out of the US and Central America decades ago, when officials were deploying between 500 and 700 million sterile flies per week across the region.
The current numbers tell a different story. Combined facilities in the US and Mexico can produce roughly 100 million sterile flies per week. Officials say they need up to 600 million weekly to make a meaningful dent. Since the infected calf was discovered, four million sterile flies have been released by ground, alongside a weekly aerial release of four million that has been running since February.
US Secretary of Agriculture Brooke Rollins struck a confident tone regardless. “There is no reason to believe that this incursion will result in any sort of establishment of the pests,” she told reporters Thursday.
Cattle farmers in Texas are not so sure, with some openly accusing the Trump administration of failing to treat the threat with the urgency it deserves.
The politics around the outbreak have grown as quickly as the concern. Democrats have pointed to the dismantling of USAID, which ran a programme monitoring screwworm activity across Central America, as a factor in how the pest was able to advance northward unchecked. Texas Agriculture Commissioner Sid Miller went further, condemning what he described as “a slow, bureaucratic, and incomplete response that allowed the pest to advance unchecked through Mexico and reach American soil.”
Secretary Rollins, for her part, attributed the northward spread to “open border” policies and cartel-linked livestock smuggling, and said Mexico’s handling of the situation left “a lot to be desired.”
Officials have established a 20km control zone around the La Pryor site, implementing quarantines and movement restrictions in the area. Sniffer dogs, a specialised unit known as the “Beagle Brigade,” operated by Customs and Border Protection and the USDA, have been deployed at border crossings to detect the insects. A proposal to use insecticide traps was raised and then dismissed at Thursday’s press conference, with officials arguing the traps were ineffective and relied on chemicals linked to cancer risks in humans and wildlife.
The CDC reports that the current outbreak has produced 2,070 human cases so far.
The screwworm’s return is not entirely surprising to those who have been watching. After being pushed south of Panama’s Darien Gap in previous decades, the parasite began clawing back northward. Panama recorded a sharp rise in cases in 2022. By 2024, they had reached Mexico. Climate change, researchers note, may be expanding the range of a parasite that was once confined to tropical zones.
For now, officials are urging ranchers to keep wounds on their livestock covered, and asking the public to check themselves and their pets, and report anything suspicious immediately.
Source: BBC
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