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

A Flesh-Eating Threat That Has Already Been Beaten Once Before

Few agricultural pests carry a name quite as viscerally alarming as the screwworm. The New World screwworm (Cochliomyia hominivorax) is a parasitic fly whose larvae feed on the living tissue of warm-blooded animals — and occasionally humans. Left untreated, a screwworm infestation can kill a healthy animal within days. For livestock farmers, wildlife managers, and border biosecurity officials, the reemergence of this pest in the Americas represents a serious and urgent threat.

The good news is that the United States has beaten this pest before. By the 1960s and 70s, a landmark eradication program had eliminated screwworm from the continental US entirely using a technique that sounds almost counterintuitive: flooding the environment with more flies. Specifically, with sterile ones. Now, as screwworm populations press northward from Central America, federal agencies are looking to scale that approach back up — and they're finding that doing so is far easier said than done.

What Is the Screwworm and Why Is It So Dangerous?

The New World screwworm is not your ordinary nuisance fly. Female screwworm flies lay their eggs in open wounds, in the navels of newborn animals, or even in the nostrils and mouths of healthy animals. Once the eggs hatch, the larvae — the screwworms themselves — burrow into living flesh, feeding and growing while causing progressively worse tissue damage. The name comes from the larvae's corkscrew-like burrowing motion as they drill deeper into wounds.

Livestock, deer, dogs, and other mammals are the most common victims, but screwworms are also capable of infesting humans. In regions where the pest is active without adequate control measures, losses to the livestock industry can be staggering. A single infested wound, if undetected, can kill an animal in under two weeks. For wildlife — including endangered species like the Florida Key deer, which was recently impacted by a local outbreak — the consequences can be even more ecologically significant.

The Sterile Insect Technique: Fighting Flies With Flies

The primary weapon the US has against screwworm is the Sterile Insect Technique (SIT), a biological pest control strategy that was pioneered largely in response to the screwworm threat itself. The concept is elegant in its simplicity: raise large numbers of screwworm flies in a controlled facility, expose them to radiation to render them sterile, and then release them into the wild in overwhelming numbers.

When sterile males mate with wild females, no viable offspring are produced. If enough sterile flies are released consistently, the reproductive rate of the wild population crashes. Over multiple generations, the local population collapses. Because the technique is species-specific and does not involve pesticides, it poses minimal risk to other wildlife or the broader ecosystem. It was used to eradicate screwworm from the United States in 1966, then from Mexico, and eventually from all of Central America down to the Panama-Colombia border, where a biological barrier has been maintained ever since.

The barrier works by continuously releasing sterile flies along a defined geographic corridor. As long as those releases are sustained, wild screwworm populations cannot gain a foothold north of the barrier. The moment releases falter — whether due to funding gaps, logistical disruptions, or production shortfalls — the barrier weakens and the risk of northward spread increases dramatically.

The Problem: The US Doesn't Have Enough Flies

Here is where the challenge becomes very practical, very fast. Producing sterile flies at the scale needed to maintain or expand a suppression barrier is an immense industrial undertaking. The USDA currently operates a screwworm rearing facility in Panama, but the capacity of that facility has limits. Producing hundreds of millions — potentially billions — of sterile flies per week requires significant infrastructure, consistent funding, technical expertise, and a stable supply chain for the specialized diet the larvae require.

Recent concerns about screwworm spreading beyond the Panama barrier and appearing in Mexico have put renewed pressure on the US to expand its production capacity. Officials and scientists have been explicit: the existing infrastructure is not sufficient to respond to a major outbreak or a significant northward expansion of wild populations. Scaling up is not just a matter of political will — it requires building or expanding physical rearing facilities, training staff, and locking in long-term budget commitments.

What a Scaled-Up US Response Would Look Like

A more robust American screwworm strategy would likely involve several parallel efforts. These include expanding or supplementing the Panama rearing facility, potentially establishing new production facilities on US soil, increasing aerial release operations over at-risk corridors, and investing in surveillance systems that can detect wild screwworm activity earlier and more precisely.

Coordination with Mexico and other regional partners is also essential. Screwworm does not respect borders, and any effective suppression strategy has to be managed at a hemispheric level. The USDA has a long history of binational cooperation on screwworm, and that framework would need to be both maintained and expanded to meet a growing threat.

Why This Matters Beyond Agriculture

The screwworm issue is about more than cattle and economic loss. Wildlife populations — including endangered and protected species — are vulnerable. The 2016 outbreak among Key deer in Florida demonstrated that even a small, isolated screwworm incursion can devastate a fragile population in a short amount of time. Rapid response capacity, backed by adequate sterile fly production, is the only proven tool that works at the scale needed.

The sterile insect technique is widely regarded as one of the most successful pest eradication programs in history. The science is proven. The strategy is established. What remains is a logistical and political challenge: producing enough sterile flies, fast enough, to stay ahead of a pest that has already shown it can adapt, spread, and cause enormous harm when left unchecked. The US has a plan. Now it needs the capacity to execute it.