The US Plan to Combat Screwworm: Why More Flies Are the Answer

The Flesh-Eating Threat Making a Comeback

It sounds like something out of a science fiction horror story, but the New World screwworm is a very real and very dangerous parasite that has long haunted livestock producers and wildlife managers across the Americas. The screwworm fly — Cochliomyia hominivorax — lays its eggs in open wounds on warm-blooded animals, and the larvae that hatch then burrow into living flesh, causing severe, often fatal damage if left untreated. Now, as the pest creeps closer to the United States border, federal agencies are scrambling to put a robust defense in place. Their weapon of choice? More flies — just of the sterile variety.

What Is the Screwworm and Why Is It So Dangerous?

The New World screwworm was once eradicated from the United States in the 1960s through a decades-long effort that stands as one of the most successful pest control campaigns in agricultural history. But the threat never entirely disappeared. The fly remains endemic in parts of South America and has periodically re-emerged in Central America and the Caribbean, raising alarm among U.S. livestock producers and conservation officials.

Unlike ordinary fly species that feed on dead tissue, screwworm larvae are uniquely destructive because they actively feed on living flesh. A single infestation can kill a deer, a cow, or even a domestic pet within days if not treated. Wildlife populations — including endangered species — are especially vulnerable, as they have no human caretakers to notice and treat wounds. The reintroduction of screwworm into U.S. soil would represent a devastating blow to both the agricultural economy and native ecosystems.

The Sterile Insect Technique: Fighting Flies With Flies

The cornerstone of the U.S. strategy against screwworm is a method known as the Sterile Insect Technique, or SIT. The approach is elegantly simple in principle: raise enormous quantities of screwworm flies in controlled laboratory environments, sterilize them using low doses of radiation, and then release them into the wild in massive numbers. When sterile males mate with wild females, no offspring are produced. Over successive generations, the local screwworm population collapses.

This method was first pioneered in the 1950s by American entomologists Edward Knipling and Raymond Bushland, whose work eventually led to the continental eradication of the screwworm from the United States, Mexico, and much of Central America. A permanent eradication barrier has since been maintained in Panama, where a joint facility continuously produces and releases sterile flies to prevent reinfestation from South America.

The technique has several important advantages over chemical pesticides. It is species-specific, meaning it targets only the screwworm without harming beneficial insects or pollinators. It leaves no toxic residue in the environment. And because it works by manipulating the pest's own reproductive biology, it does not trigger the development of resistance in the same way that chemical treatments can.

The Problem: The US Can't Make Enough Flies

Here is where the plan faces its most significant practical obstacle. While the Sterile Insect Technique is proven and effective, it is also enormously resource-intensive. Producing tens of millions — or even billions — of sterile flies requires sophisticated rearing facilities, large quantities of protein-based larval diets, precise climate controls, and complex radiation equipment. The United States currently has limited domestic capacity to generate the volume of sterile flies that would be needed to respond to a serious screwworm incursion.

The main production facility supporting North American eradication efforts has historically been located in Tuxtla Gutiérrez, Mexico, operating as a joint U.S.-Mexico program. While that partnership has been instrumental in holding the line for decades, it also means U.S. response capability is partially dependent on foreign infrastructure — a vulnerability that policymakers are now taking seriously. Should an outbreak occur on U.S. soil, ramping up production quickly enough to contain it could prove enormously difficult without expanded domestic facilities.

What the US Is Doing to Scale Up

Federal officials and agricultural agencies are now actively working to increase domestic sterile fly production capacity. The goal is to build a more resilient supply chain for sterile insects that does not rely entirely on a single facility or cross-border logistics. Investments in new rearing infrastructure, research into more efficient diet formulations, and improvements to radiation sterilization protocols are all part of the broader push.

There is also growing interest in improving aerial dispersal methods — the systems used to drop sterile flies from aircraft over target areas. Distributing flies uniformly across large, often remote landscapes is a logistical challenge in its own right, and better dispersal technology could dramatically improve the cost-effectiveness of future eradication campaigns.

Why This Matters Beyond Livestock

The urgency of the screwworm issue extends well beyond cattle ranches and hog farms. Wildlife biologists have raised serious concerns about the impact a screwworm resurgence could have on endangered species such as the Florida Key deer, which is already under pressure from habitat loss and climate change. Any animal with a wound — including humans in extreme cases — is technically susceptible.

• U.S. livestock industries could face billions of dollars in losses in the event of a widespread outbreak.
• Endangered wildlife populations with small geographic ranges are particularly at risk of rapid decimation.
• Cross-border trade in cattle and other livestock could be severely disrupted, with economic ripple effects across the supply chain.
• The cost of response rises exponentially the longer an outbreak goes uncontrolled, making early investment in production capacity far more economical than reactive containment.

A Proven Solution That Needs Stronger Infrastructure

The science behind combating the New World screwworm is not in question. The Sterile Insect Technique has worked before, and when applied at sufficient scale, it works reliably. The challenge facing the United States today is not a question of scientific knowledge but of industrial and logistical readiness. Building the fly-rearing infrastructure to match the scale of a potential threat is an investment in biosecurity that experts argue is both overdue and urgently necessary.

In the fight against screwworm, the answer is indeed more flies — but getting there requires sustained funding, strategic planning, and a commitment to protecting both American agriculture and native wildlife for generations to come.