Fly Deadly To Livestock Resurfaces

By the Editorial Team · Monday, August 31, 2026

In June 2025, the United States confirmed the first New World screwworm fly infestation in decades, affecting livestock across 18 counties in Texas and Arizona and prompting the USDA to invest over $1 billion in eradication efforts. Although no wild flies have been trapped so far and the current sterile male release program shows promise, experts warn that improved detection methods are crucial to prevent a potentially rapid resurgence that could threaten the livestock industry.

In early June 2025, the United States reported the first case in decades of the New World screwworm fly (Cochliomyia hominivorax) infesting livestock in South Texas. Since then, 46 additional infestations have been confirmed in cattle, sheep, goats, and several dogs across 18 counties in Texas and Arizona. The U.S. Department of Agriculture (USDA) has launched a large-scale response, allocating over $1 billion to efforts aimed at controlling and eradicating the pest’s resurgence.

The New World screwworm fly is a parasitic insect whose females lay eggs in open wounds or living flesh of animals, causing severe tissue damage that can be fatal if untreated. Prior to successful eradication programs in the mid-20th century, the fly was endemic to Texas and surrounding regions, leading to massive losses estimated at $200 million annually for ranchers.

USDA’s current containment strategy heavily relies on releasing sterile male screwworm flies, which mate only once in their lifetime. Females that mate with sterile males fail to produce viable offspring, thereby reducing the fly population over time. However, the breeding process for sterile flies also produces sterile females, which are unintentionally released alongside males. Weekly, approximately 100 million sterile flies are distributed throughout Texas and near the U.S.-Mexico border; officials expect this number to increase to 500 million per week by mid-2026.

Despite extensive trapping efforts—including more than 128 screwworm-specific sticky traps and some 8,000 exotic fruit fly traps—the USDA has not caught any wild screwworm flies to date. The traps, coated with swormlure, a synthetic blend of chemicals mimicking decomposing flesh, aim primarily to monitor fly distribution rather than directly kill them. More than 80,000 non-screwworm flies have been captured and identified, but none confirmed as wild screwworms. This has led USDA officials to conclude that the number of fertile wild flies currently present in the U.S. is very low.

Experts generally agree with the USDA’s assessment and acknowledge that the eradication program remains on the right track geographically. Nonetheless, entomologists warn that even a small number of undetected wild flies could lead to rapid population increases, given the screwworm’s prolific egg-laying capacity. The source of the outbreak is believed to be linked to detections in neighboring Mexico since November 2024, prompting heightened surveillance along the border, including the use of drones and helicopters to identify infested wildlife.

Due to the outbreak, live cattle imports from Mexico were suspended for over a year, although they partially resumed in August 2025 with quarantines and monitoring in place. Measures include establishing quarantine zones within the United States each time a new case is detected.

One significant challenge lies in the limitations of current monitoring tools. Swormlure, which simulates the odor of rotting flesh, may be less attractive to screwworm flies than the scent of living wounds where they typically lay eggs. This discrepancy has prompted renewed research into improving trap baits and detection methods. In response, the USDA has pledged $105 million to 40 research projects aimed at enhancing eradication techniques.

The screwworm fly was largely eradicated from the U.S. by the late 1960s through sterile male release programs, and then from Central America by 2006, except for a small area in southern Panama. However, decades of reliance on sterile fly releases meant that trap technology and monitoring methods have seen limited innovation. The USDA recently rehired one of its few remaining screwworm specialists to help augment these efforts.

As the USDA continues its campaign, officials emphasize the importance of early detection and swift containment to prevent a broader outbreak that could have severe economic and agricultural consequences.

This article was reported by synthesizing coverage from multiple independent news outlets to present a balanced, unbiased account. Read about our editorial standards. Spotted an error? Let us know.