Sheinbaum Says US Wants Sterile Fly Results Before Cattle Border Reopens

Sonora and Chihuahua have registered zero screw-worm cases since the outbreak began in late 2025, but the US border for Mexican cattle exports remains closed. President Claudia Sheinbaum confirmed Monday that Washington is conditioning the reopening on measurable progress from the sterile fly release program, even in states where the pest has never been detected. The announcement extends a seven-month border closure that has cost Mexican cattle producers an estimated $2 billion in lost export revenue.

The numbers tell the story. Mexico exported 1.6 million head of cattle to the United States in 2024, generating roughly $3.5 billion for Mexican ranchers, according to the Agriculture Secretariat. That flow has dropped to near zero since the USDA detected screw-worm larvae in cattle at border inspection points and shut the border in December 2025. Sonora alone, which sends more than 300,000 weaned calves to Texas feedlots each year, has gone from exporting 25,000 head monthly to effectively zero.

Sheinbaum said during her morning press conference that the government is doubling down on the sterile fly strategy that worked in the 1970s and 1980s. The centerpiece is a production plant in Chiapas that breeds sterile male screwworms, which are released by aircraft to interrupt the pest's reproductive cycle. The same technique eliminated the screwworm from North America for nearly 40 years. But the current outbreak, first detected in cattle at Mexican entry points late last year, revealed weaknesses in the containment infrastructure that the original campaign established.

"We have states where the plague has not arrived and the border could be opened, but the United States wants it done under the sterile fly release protocol," Sheinbaum said.

The quote is worth examining. Sheinbaum frames the border closure as a matter of US protocol preferences rather than actual pest risk. But the USDA's position is straightforward: screw-worm larvae can travel on asymptomatic animals, and partial reopening based on state-by-state data would require a verification system that does not yet exist. The agency has not given a timeline for lifting the restrictions.

Mexico has dealt with screw-worm before. The original US-Mexico eradication campaign, which ran from the 1970s through the 1980s and cost roughly $750 million in today's dollars, pushed the pest south of the Isthmus of Panama. The current outbreak proves that eradication is not permanent without sustained investment. The Chiapas plant is designed to produce millions of sterile flies per week, but it is not yet at full capacity.

A new U.S.-developed technology that produces only male sterile flies is being evaluated by Cofepris, Mexico's health regulator, for domestic production approval. The new strain would eliminate the need to sort flies by sex before release, potentially doubling the plant's effective output without expanding its physical footprint. If approved, it could accelerate the eradication timeline significantly. That is a big if, regulatory approvals for biological control agents have historically taken 12 to 18 months in Mexico.

For ranchers in clean states, the wait is getting expensive. Juan Ochoa Valenzuela, president of the Sonora Cattlemen's Union, has publicly pressed both governments to find a middle ground. "The infrastructure is there," Ochoa said. "We have the fumigation equipment, the inspection stations, the veterinary personnel." Ochoa's frustration is understandable: the border closure is forcing Sonora ranchers to sell cattle at a 40 percent discount on the domestic market, according to industry estimates.

The cattle industry supports roughly 40,000 direct and indirect jobs in Sonora alone, according to a 2019 University of Sonora study. Each additional week of border closure compounds the damage through the supply chain: feed suppliers, veterinary distributors, trucking companies. The domestic market cannot absorb what the US market used to buy. Some ranchers are culling herds to reduce costs.

Sheinbaum expressed confidence in Mexico's ability to eliminate the pest again, citing the successful precedent. She is right about the precedent, but the current outbreak has spread farther in six months than the 1970s outbreak did in two years. The Chiapas plant needs to get to full production. The USDA needs to see results. And Sonora's cattle keep waiting in their pens.

Update July 14, 2026: The USDA has not issued a formal response to Sheinbaum's comments. The agency's Animal and Plant Health Inspection Service (APHIS), which manages the screwworm eradication program, has maintained a policy of not commenting on ongoing negotiations with Mexican agricultural authorities. The silence is consistent with the USDA's approach since the border closure: no timelines, no partial reopening frameworks, and no public commitments beyond the sterile fly program.

Mexico's Secretariat of Agriculture and Rural Development (SADER) has not released its own assessment of how long the eradication will take. The Chiapas plant's production capacity and the timeline for Cofepris approval of the male-only fly technology are not public. The lack of transparency makes it impossible for ranchers or investors to estimate when the border might reopen. They are operating blind, waiting for a government process that has no published milestones.