The New World screwworm fly (Cochliomyia hominivorax), a parasitic pest known for its flesh-eating larvae, poses a significant threat to livestock, wildlife, pets, and, in rare cases, humans. Once eradicated from the United States in the 1960s, this devastating insect has recently reemerged in Central America and southern Mexico, prompting urgent action from the U.S. Department of Agriculture (USDA). To protect the nation’s $100 billion cattle and beef industry, as well as other vulnerable animals, the U.S. has revived a proven strategy: the Sterile Insect Technique (SIT). This blog article explores the U.S. plans to combat the New World screwworm fly with sterile males, detailing the biology of the pest, the history and mechanics of SIT, current initiatives, and the broader implications for agriculture and public health.
Understanding the New World Screwworm Fly
The New World screwworm fly is a metallic blue blowfly, roughly the size of a housefly, notorious for its obligate parasitic lifestyle. Unlike most fly larvae that feed on dead tissue, screwworm larvae burrow into the living flesh of warm-blooded animals, causing severe tissue damage, suffering, and often death if untreated. The scientific name Cochliomyia hominivorax translates to “man-eater,” reflecting its destructive impact. Female flies are attracted to open wounds—such as those from castration, dehorning, branding, or even minor scratches like tick bites—and lay 200–300 eggs on the wound’s edges. Within hours, the eggs hatch into larvae, which feed aggressively on living tissue for about a week before dropping to the ground to pupate. After roughly seven days, adult flies emerge, and the cycle repeats, with females mating only once in their 20- to 30-day lifespan, while males can mate multiple times.
Historically, the screwworm was a major scourge for U.S. cattle ranchers, particularly in the Southeast and Southwest, costing the industry an estimated $50–100 million annually in the early 1960s. The pest was eradicated from the U.S. by 1966 through a pioneering biological control method, but its recent northward spread from South America and the Caribbean has raised alarms. Since 2022, outbreaks have surged in Panama, Costa Rica, Nicaragua, Honduras, Guatemala, Belize, and El Salvador, with over 6,500 cases reported in Panama alone in 2023. By November 2024, the fly had reached Oaxaca and Veracruz in southern Mexico, just 700 miles from the U.S. border.
The Sterile Insect Technique: A Proven Solution
The U.S. plans to combat the New World screwworm fly with sterile males rely on the Sterile Insect Technique (SIT), a method first developed in the 1950s by USDA entomologist Edward F. Knipling. SIT exploits the screwworm’s unique reproductive biology: females mate only once, while males are polygynous. By mass-producing and sterilizing male flies using radiation (typically cobalt-60 gamma rays), then releasing them into the wild, the technique ensures that wild females mate with sterile males, producing unfertilized eggs that do not hatch. Over time, this reduces the wild population, potentially leading to local extinction.
SIT was first tested in 1954 on Curaçao, where 400 sterile males per square mile were released, eradicating the screwworm population in just 10 weeks. This success led to broader campaigns, eliminating the pest from the southeastern U.S. by 1959 and the entire country by 1966. The technique was later applied in Mexico (1972–1991), Central America (1988–2000), and the Caribbean, pushing the screwworm south to a biological barrier at the Darién Gap between Panama and Colombia. The Panama–United States Commission for the Eradication and Prevention of Screwworm Infestation in Livestock (COPEG) has since maintained this barrier by releasing up to 117 million sterile flies weekly.
Current U.S. Initiatives to Combat the Screwworm
The reemergence of the New World screwworm in Central America and Mexico has prompted a multi-pronged response from the USDA, with SIT at its core. On June 18, 2025, Agriculture Secretary Brooke Rollins announced a $8.5 million sterile fly dispersal facility at Moore Air Base in Hidalgo County, Texas, set to begin operations by the end of 2025. This will be the second such facility in the Western Hemisphere, complementing the existing COPEG facility in Panama. The Texas facility will breed and release millions of sterile male screwworm flies to mate with wild females, preventing the production of viable eggs. Additionally, the USDA is investing $21 million to convert a fruit fly breeding facility in Metapa, Mexico, to produce 60–100 million sterile screwworm flies weekly by July 2026.
The USDA’s five-pronged plan to combat the screwworm includes:
Stopping the Spread in Mexico: Enhancing sterile fly production and dispersal in Mexico, with close monitoring of animal health controls through in-person audits.
Protecting the U.S. Border: Strengthening surveillance and restrictions on live animal imports, which were suspended in May 2025 for cattle, horses, and bison from Mexico.
Maximizing U.S. Readiness: Developing emergency plans and stockpiling therapeutics for potential U.S. outbreaks.
Enhancing Sterile Fly Production: Establishing domestic production capacity in Texas to reduce reliance on Panama’s facility.
Stakeholder Engagement: Holding public listening sessions to gather input on eradication tools, production technologies, and innovative solutions.
The Texas facility is critical because the Panama plant, producing up to 117 million sterile flies weekly, falls short of the estimated 400–500 million needed to reestablish the Darién Gap barrier. The USDA’s goal is to flood affected areas with sterile males, ensuring that wild females are more likely to mate with them than with fertile males. Releases typically occur via aerial drops in rural areas, using temperature-controlled containers to disperse flies over targeted regions.
Why the Screwworm’s Return Is Alarming
The northward spread of the New World screwworm is driven by several factors. Increased cattle movement across borders, possibly carrying infested animals, has facilitated the pest’s migration. Higher temperatures due to climate change may also enhance fly development and survival, allowing the screwworm to thrive in regions previously too cold for sustained populations. Some scientists hypothesize that female flies may be adapting to avoid sterile males, though this remains speculative. The pest’s ability to infest a wide range of hosts—cattle, horses, bison, deer, dogs, and even humans—amplifies its threat. In Central America, 17 million cattle are at risk, while Mexico’s 34 million cattle and the U.S.’s 14 million in Texas and Florida alone face potential devastation. Human cases, though rare, have been reported, with eight infections in Mexico since April 2025.
The economic stakes are high. Before its eradication, the screwworm cost U.S. ranchers millions annually, and a reestablished population could cripple the livestock industry, disrupt food supply chains, and increase beef prices. Wildlife, such as deer and feral pigs, are also vulnerable, as they can cross borders unchecked, spreading the pest further. The 2016 outbreak in the Florida Keys, which killed significant numbers of endangered Key deer, underscores the risk to biodiversity.
Challenges and Criticisms
While SIT is environmentally friendly compared to widespread pesticide use, it faces challenges. Producing hundreds of millions of sterile flies is logistically complex, requiring precise rearing conditions to ensure larvae have adequate nutrients and females receive cues to lay eggs. Historically, fly factories used horse meat, honey, or a mix of dried eggs and cattle blood products to feed larvae. Sterilization must also preserve the males’ competitiveness, as wild females must accept them over fertile males. The current Panama facility struggles to meet demand, and the new Texas and Mexico facilities will take time to reach full capacity.
Geopolitical tensions add another layer of complexity. In April 2025, Secretary Rollins accused Mexico of restricting USDA aircraft to six weekly flights for sterile fly dispersal and imposing burdensome import duties on equipment, hindering eradication efforts. Mexico’s Agriculture Secretary Julio Berdegué has pushed back, arguing that fears of the screwworm are exaggerated and citing progress in surveillance. These disputes highlight the need for regional cooperation, as the screwworm’s spread is facilitated by cross-border animal movement.
The Broader Implications
The U.S. plans to combat the New World screwworm fly with sterile males reflect a broader commitment to protecting agriculture, wildlife, and public health. The success of SIT in the past—saving the U.S. livestock industry an estimated $20 billion and reducing beef prices by 5%—demonstrates its potential. However, the current outbreak’s scale and the pest’s proximity to the U.S. border underscore the urgency of scaling up production and coordination. The USDA’s investment in new facilities and its collaboration with state, federal, and international partners, including COPEG and the International Atomic Energy Agency (IAEA), aim to replicate past successes.
Beyond immediate eradication, the screwworm’s resurgence raises questions about long-term pest management in a warming climate. Higher temperatures could expand the fly’s range, potentially allowing it to overwinter in regions previously inhospitable, such as parts of the U.S. Southwest. Research into genetic sterilization methods, such as male-only strains, could reduce the number of flies needed for SIT, but funding for such innovations has been limited. Meanwhile, vigilance through surveillance, livestock inspections, and public awareness remains critical to preventing reinfestation.
Conclusion
The New World screwworm fly is a formidable adversary, but the U.S. plans to combat it with sterile males offer a scientifically grounded and historically effective solution. By leveraging the Sterile Insect Technique, bolstered by new facilities in Texas and Mexico, the USDA aims to halt the pest’s northward march and protect the nation’s livestock, wildlife, and food supply. While challenges like production capacity, regional cooperation, and climate-driven spread persist, the renewed focus on SIT reflects a proactive approach to a growing threat. As the U.S. ramps up its “fly factory” operations, the fight against the screwworm underscores the power of innovative science to safeguard agriculture and ecosystems from invasive pests.
For those in affected regions, reporting suspected infestations to state animal health officials or the USDA’s Animal and Plant Health Inspection Service (APHIS) is crucial. Producers should monitor livestock for signs of infestation, such as non-healing wounds or unusual behavior, and contact veterinarians for treatment guidance. With concerted effort, the U.S. can once again push back this “man-eater” and maintain its screwworm-free status.
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