Though it resembles the common housefly in size, and other blowflies in its appearance, Chochliomyia homnivorax exhibits a distinct behavior that ratchets up its health and economic risks far beyond those of its lookalikes. Commonly known as New World screwworm (NWS), this pest is categorized as a parasite because its larvae feed on healthy, living flesh rather than dead tissue or refuse. It’s this characteristic that turns the fly from nuisance to downright menace for animal welfare, the economy, and the food supply chain — a triple threat that could come into play should the recent NWS re-emergence in Mexico defy containment and resist eradication.
NWS, which thrives in warmer temperatures and thus can achieve stability in the tropical parts of North America as well as in Central and South America, has periodically spread and contracted throughout those regions. The flies are attracted to open wounds on warm-blooded animals, including dehorning, castration, ear-tagging, and umbilical cord wounds, as well as cuts and scratches and tick and insect bites. Only through a concerted effort in the 1950s and 60s was the screwworm’s presence in the U.S. eliminated, and an international effort into the 21st century helped beat the pest back to Darien Gap, the Panamanian land bridge between Central and South America. But in late 2024, Mexican officials notified the United States Department of Agriculture that NWS had made its way back into Mexico.
International endeavor
The Western Hemisphere’s efforts to contain NWS include what has proven to be the most effective route to eradication: the sterile insect technique (SIT). During a recent Texas A&M AgriLife webinar on NWS, Phillip Kaufman, a Texas A&M extension entomology specialist, weighed in on SIT. He noted that there are multiple techniques to reduce insect populations, but with screwworm, “eradication is the goal.” Of the sterilization process, he explained, “The pupae are irradiated, which breaks the chromosomes, and then the pupae are released into areas with known fly populations.” Ideally, the dispersal of these sterile male flies will overwhelm the wild population. Female NWS flies mate only once, then die off, so if there are more sterile males than fertile ones in a region, the bulk of the eggs laid in that population will be unfertilized.
After using SIT to hold the line at Darien Gap for years — a sterile fly breeding facility in Panama has been jointly funded by USDA and Panama’s Ministry of Agriculture Development — a nationwide screwworm outbreak was reported in Panama in 2023. The parasite’s range continued to expand: Costa Rica, Nicaragua, Honduras, and Mexico all reported cases in 2023 and 2024. Panama’s sterile fly production typically hovered around 20 million pupae per week to maintain the biological barrier, with the potential to scale up production fivefold in the event of an outbreak. However, Kaufman noted in the webinar that only half the number of flies generated at production facilities will be male.
Other sterile breeding facilities existed but were not online at the time of the reported re-emergence of screwworm, so the initial focus in 2025 was stopping the spread through border closing while identifying strategic areas in Mexico for sterile fly dispersal. In May, U.S. Secretary of Agriculture Brooke L. Rollins announced that live cattle, horse, and bison imports from Mexico were suspended — a move prompted by reports that NWS was within 700 miles of the U.S. border. “Once we see increased surveillance and eradication efforts, and the positive results of those actions, we remain committed to opening the border for livestock trade,” Rollins stated in a press release. At the end of June, USDA issued an announcement that selected ports would begin to reopen in July, but imports were halted again on July 9, when a new case of NWS was reported approximately 370 miles south of the U.S.-Mexico border.
Comprehensive approach
In between these announcements, USDA unveiled its multi-faceted plan, the “New World Screwworm Domestic Readiness and Response Policy Initiative.” The department’s priorities are:
1Stop the spread of NWS in Mexico. To achieve this, USDA infused $21 million into Mexico’s sterile fly production capabilities, which will in part be spent to upgrade an existing but inoperative facility in Metapa, Mexico. The department projects production numbers of 60 million to 100 million sterile flies from this facility, with completion anticipated in July 2026. Additional efforts center around communication and transparency among leaders and entities, and improving data collection and analysis, surveillance, and other containment and eradication methods such as traps and lures. Limitation of animal movement may also be implemented to stop NWS spread.
2Protect the border. While the above Mexico-based measures inherently contribute to the U.S.’s own protection, USDA is also planning for this side of the border. Outreach and education are planned to create a “barrier zone of vigilance” in Mexico-adjacent states and across the country, along with collaboration with state wildlife agencies and the National Veterinary Services Laboratories. Ports of entry, when reopened, will continue to step up live-animal inspections, and USDA will consider those reopenings and potential continued closings as the situation merits.
3Maximize readiness. Preparedness plans include facilitating emergency management plans among state and federal officials while continuing their training, managing inventory of treatment products, and confirming that regulation of such products is handled in a way that ensures access. USDA also stressed that it will “put preparations in place for deployment of existing employees and hiring of additional personnel” if emergency response is needed.
4Take the fight to the screwworm. By the end of the year, a sterile insect dispersal facility will be completed, a project that will help target the border and Northern Mexico. In addition, USDA is considering building a sterile fly production plant, proposed for Texas’s Moore Air Base where the dispersal facility is under construction. If built, the domestic production facility could contribute another 100 million sterile NWS flies to the eradication effort. Other options on the table for domestic production are modular rearing facilities, which could speed up construction and address surge capacity, USDA said.
5Innovate for continued success. Using the Agricultural Research Service, USDA will work on improving existing sterile fly production, trapping, and luring strategies, as well as exploring novel preventatives and next-generation treatments. USDA also plans on strengthening its partnerships with land-grant universities in the border states of Texas, Arizona, and New Mexico to promote outreach along with local training, surveillance validation, and trap deployment.
On guard and on the case
Texas, serving as ground zero for potential outbreaks, remains on alert. State-level efforts include a recent decision to use Swarmlure-5, a synthetic fly bait. In addition, multiple industry associations such as the Texas Association of Dairymen have teamed up to create the Screwworm Coalition of Texas, providing information and resources via their website, screwwormtx.org. Visit on.hoards.com/screwworminamericas for USDA updates on eradication efforts.
