USDA APHIS Expands Sterile Fly Dispersal Zone to Block Screwworm Threat

The United States Department of Agriculture's Animal and Plant Health Inspection Service (USDA APHIS) has shifted its sterile fly dispersal zone northward along the Texas-Mexico border, a tactical adjustment designed to strengthen the biological barrier against the approaching New World Screwworm threat.

The Texas Animal Health Commission announced the change last week, noting that while no screwworm cases have been detected within the United States, the modified dispersal area expands protection across more of South Texas. The adjustment comes as parasite cases in Mexico continue their gradual march toward the international boundary.

The Sterile Insect Technique Explained

USDA APHIS relies on a decades-old biological control method known as the Sterile Insect Technique (SIT). The agency breeds millions of screwworm flies, sterilizes them through radiation, and releases them by aircraft across targeted zones. When these sterile males mate with wild females, the resulting eggs fail to develop, collapsing the population over successive generations.

This approach eradicated screwworm from the United States in 1966 and eliminated a subsequent outbreak in the Florida Keys in 2017. The technique remains one of the most successful examples of biological pest control in agricultural history.

"The sterile insect technique is a proven tool for fighting the spread of NWS in combination with producer management practices, efficient wound treatment, and diligent response tactics," the Texas Animal Health Commission stated in its announcement.

Industry Support for NovoFly Technology

Coinciding with the dispersal zone expansion, the comment period for NovoFly—a genetically engineered screwworm control technology under EPA review—recently closed with substantial backing from agricultural organizations. Groups submitting supportive comments included the National Cattlemen's Beef Association, National Pork Producers Council, American Horse Council, Dairy Farmers of America, and the Entomological Society of America, among numerous state-level associations.

NovoFly represents a potential complement to existing SIT operations, offering another molecular tool in the screwworm defense arsenal.

Texas Officials Sound the Alarm

Texas Agriculture Commissioner Sid Miller has been vocal about the screwworm threat for months, urging ranchers, veterinarians, hunters, and pet owners to remain vigilant. In a statement issued last week, Miller praised USDA's decision to expand northward.

"USDA is making the right call by pushing sterile fly operations north," Miller said. "The New World screwworm threat is moving in that direction, and Texas isn't waiting around."

Miller characterized the parasite as a "nightmare" capable of devastating the state's livestock industry if it establishes a foothold. The screwworm's larvae burrow into living tissue, creating wounds that can kill affected animals within days if untreated.

Current Situation

According to the most recent data from USDA and CDC, Mexico and Central American countries have reported approximately 168,000 animal cases and 1,710 human cases since 2023. SENASICA, Mexico's agricultural health agency, reports that its new sterile fly production facility in Metapa, Chiapas, is 75 percent complete and scheduled to begin operations by late June.

USDA APHIS continues to adjust its dispersal boundaries based on scientific modeling and case tracking. The agency maintains that while infections have occurred within 400 miles of the border outside the current dispersal zone, these locations do not present elevated risk to the United States based on current trajectory analysis.

For the latest updates on screwworm monitoring and prevention, visit screwworm.gov.