The Fight Against Screwworms: How Sterile Flies Can Crash a Lethal Population

The New World screwworm, a flesh-eating pest that devastated livestock across the American South and Southwest for decades, has returned to the United States after a 60-year absence. In early 2025, the US Department of Agriculture confirmed the presence of the parasite in a calf in southern Texas. This reemergence, which experts had anticipated due to its recent spread north through Mexico, poses a serious threat to livestock and wildlife. To combat the outbreak, officials are turning to a tried-and-true biological control method: releasing vast numbers of sterilized male screwworm flies to crash the local population. However, the US currently lacks the capacity to produce enough of these flies to fully contain the threat.

Understanding the Screwworm Threat

A screwworm infection begins when a female fly lays her eggs in an open wound or bodily orifice of a warm-blooded animal, including livestock, pets, and even humans. When the eggs hatch, the larvae, or maggots, feed on the living tissue, causing severe pain, infection, and often death if left untreated. Unlike other fly larvae that consume dead tissue, screwworm maggots burrow into healthy flesh, making them particularly dangerous.

The economic and ecological stakes are high. Before its eradication, the pest killed hundreds of thousands of cattle annually in the United States. Its return threatens not only the livestock industry but also wildlife populations, including endangered species like the Key deer in Florida. The USDA has established a 12-mile quarantine zone around the infected calf in Texas and is conducting targeted releases of sterile flies from trucks, supplementing the ongoing aerial drops of 4 million sterile flies per week along the border.

The Power of Sterile Insect Technique

The sterile insect technique (SIT) is a form of biological control that exploits a key weakness in the screwworm's reproductive cycle: female screwworm flies mate only once in their lifetime. First developed by USDA researchers in the 1950s, SIT involves rearing large numbers of male flies, sterilizing them with radiation, and then releasing them into infested areas. When these sterile males mate with wild females, the resulting eggs are unviable, and no offspring are produced.

A Proven Biological Control Method

The technique was first successfully deployed on the Caribbean island of Curaçao, where it eliminated the screwworm population in just seven weeks, saving vital goat herds. Dr. Sally DeNotta, associate professor of veterinary medicine at the University of Florida, describes SIT as one of the most elegant examples of a completely successful biological control mechanism. By breaking the life cycle, the technique stops the population from reproducing and can lead to local eradication over time.

For decades, the Darién Gap, the dense rainforest between Panama and Colombia, served as a natural barrier reinforced by continuous releases of sterile flies to prevent the pest's northward spread. However, insects began breaching this barrier in 2022, leading to the current situation in Texas.

The Production Capacity Gap

Despite the proven efficacy of SIT, the United States faces a significant logistical challenge: it does not produce enough sterile flies. According to USDA secretary Brooke Rollins, an estimated 400 million sterile flies per week are needed to effectively beat back the screwworm population. Currently, the US can only produce about 100 million flies per week at its facility in Panama.

To address this shortfall, the USDA is investing in two major infrastructure projects. The first is a $21 million renovation of an existing fruit fly facility in Metapa, Mexico, which is expected to produce an additional 60 to 100 million sterile flies per week and become operational by summer 2025. The second, more ambitious project is the construction of a $750 million sterile fly facility at Moore Air Base in Edinburg, Texas, near the Mexican border. However, this facility will not be operational until November 2027, leaving a critical gap in capacity for the next two years.

The Path Forward

While the USDA has stated that the reemergence did not catch them off guard, the limited production capacity means that containing the outbreak will require careful management and rapid scaling. Dr. DeNotta warns that a single female fly can lay hundreds to thousands of eggs, and they are capable of traveling to new hosts, making it unlikely that the detected case in Texas is the only one. She expresses confidence that the US will eventually eradicate the screwworm again, but more cases are likely to emerge in the interim.

The situation underscores the importance of maintaining robust biological control infrastructure. The sterile insect technique remains one of the most effective and environmentally friendly methods for managing pests like the screwworm. However, its success depends on having the capacity to produce and deploy enough sterile insects at the right time and place. As climate change and increased global movement of goods and animals facilitate the spread of pests, investments in facilities like the one planned for Texas are not just reactions to a current crisis but essential components of a long-term biosecurity strategy.

For now, the USDA is racing to close the production gap while continuing its targeted releases. The coming months will be critical in determining whether the current efforts can prevent a wider outbreak and protect the nation's livestock industry from this costly and dangerous pest.