The US Has a Plan to Combat Screwworm — And It Involves Releasing Millions of Sterile Flies

A Flesh-Eating Pest Is Threatening US Livestock — And the Solution Is More Flies

It sounds counterintuitive: fight a fly infestation by releasing more flies. But that is precisely the strategy at the heart of the United States government's plan to combat the New World screwworm, a parasitic fly whose larvae feed on the living tissue of warm-blooded animals. With screwworm populations posing a growing threat to livestock across the Americas — and renewed concerns about the pest breaching US borders — federal agencies are doubling down on a decades-old biological control method known as the Sterile Insect Technique, or SIT. The challenge, however, is that the US currently lacks the production capacity to deploy it at the scale the moment demands.

What Is the New World Screwworm?

The New World screwworm (Cochliomyia hominivorax) is one of the most destructive livestock pests in the Western Hemisphere. Unlike common flies that feed on dead tissue, screwworm larvae — commonly called screwworms — burrow into the living flesh of their hosts. They feed aggressively, enlarging wounds and causing severe pain, tissue damage, and in untreated cases, death. Any warm-blooded animal is at risk: cattle, horses, deer, dogs, and even humans in rare cases.

Female screwworm flies are attracted to open wounds, fresh cuts, tick bites, or even the navels of newborn animals. Once eggs are laid, larvae hatch within hours and begin feeding immediately. Without prompt veterinary intervention, an infestation can kill a large animal within days. The economic toll on the livestock industry — particularly cattle ranching — is enormous, and the ecological consequences for wildlife populations can be equally severe.

The screwworm was eradicated from the United States in 1966 and from most of Central America by the early 2000s, largely thanks to the Sterile Insect Technique. Today, a biological barrier maintained in Panama prevents reinfestation from South America, where the pest remains endemic. But that barrier is under persistent pressure, and recent detections in countries previously considered screwworm-free have reignited alarm among agricultural officials and veterinarians.

How the Sterile Insect Technique Works

The Sterile Insect Technique is an environmentally friendly, species-specific form of biological pest control. The method involves mass-rearing screwworm flies in a controlled facility, sterilizing them — typically through exposure to gamma radiation — and then releasing them in large numbers into the target environment. When sterile males mate with wild females, no viable offspring are produced. If enough sterile flies are released consistently over time, the wild population is gradually overwhelmed and crashes.

The elegance of SIT lies in its precision. Unlike broad-spectrum pesticides, it targets only the screwworm species and leaves no toxic residue in the environment. It does not harm beneficial insects, predators, or the surrounding ecosystem. Over time, with sustained releases, local populations can be driven to local extinction — exactly what happened when the US eradicated screwworms from its territory in the mid-20th century.

For the technique to be effective, however, the ratio of sterile to wild flies must be overwhelmingly in favor of the sterile ones. Experts generally cite ratios of 10 to 1 or higher as necessary for meaningful population suppression. That requires producing and releasing staggering numbers of flies — sometimes hundreds of millions per week across a treatment zone — which is where the current bottleneck lies.

The Production Capacity Problem

The United States currently operates one primary screwworm rearing facility, located in Panama and managed jointly with Panamanian authorities through a long-standing binational program. While that facility has proven capable of maintaining the biological barrier in Central America, its output is insufficient to respond to a significant reinfestation event inside US borders or to rapidly expand coverage in an emergency scenario.

Experts and federal officials have acknowledged that scaling up production is the single most important step toward making the US response to screwworm threats more robust. Building or expanding fly-rearing facilities takes time, significant investment, and technical expertise. The infrastructure required — climate-controlled rearing rooms, specialized feeding systems, radiation sterilization equipment, and aerial dispersal aircraft — is not something that can be assembled quickly once a threat materializes.

The US Department of Agriculture has been examining options to increase domestic production capacity, including exploring new facility sites and modernizing existing operations. The goal is to ensure that enough sterile flies can be produced to flood any potential reinfestation zone before the wild population has time to establish itself.

Why This Matters Now

The urgency of the screwworm issue has intensified in recent years for several reasons. Climate change is expanding the range of suitable habitat for screwworm populations, potentially pushing the pest further north than historical models predicted. Trade and travel increase the risk of accidental introduction. And recent detections in parts of the Caribbean and northern Mexico have placed the pest uncomfortably close to the US border once again.

The livestock industry has significant financial stakes in keeping screwworms out. A single reinfestation event in Texas or Florida could cost the agricultural sector hundreds of millions of dollars in animal losses, veterinary expenses, and trade disruptions. Wildlife managers are equally concerned — screwworms devastated the endangered Florida Key deer population during a 2016 outbreak, underscoring that the threat extends well beyond domestic livestock.

The Path Forward

The science behind the Sterile Insect Technique is well-established and proven. The obstacle is not knowledge — it is scale. Investing in expanded fly-rearing infrastructure now, before a crisis occurs, is widely seen as the most cost-effective and strategically sound approach. Proactive capacity-building would allow the US to respond rapidly and decisively if screwworms breach the border, rather than scrambling to contain an outbreak after the fact.

Biological pest control through sterile insect releases represents one of modern agriculture's quiet success stories. The original screwworm eradication program remains a landmark achievement in applied entomology. Replicating and fortifying that achievement for the 21st century — with better facilities, faster production, and smarter logistics — may be exactly what stands between US livestock and one of the most destructive parasites in the hemisphere.