08/23/2026
SCREWWORM, Pesticides and Mexican Cattle: The info that hasn't made the News
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The US is taking additional precautions as cattle imports from Mexico resume because of the threat of New World screwworm, a parasitic fly whose larvae feed on the living tissue of animals. An infected cow can carry the infestation in an open wound and potentially introduce the pest into the United States.
To reduce that risk, Mexican cattle are inspected for wounds and signs of screwworm before being moved. They may also be treated with an approved antiparasitic or insecticidal product, either by spraying or, in some operations, by passing the cattle through a treatment dip. The purpose is to kill screwworm larvae and other external parasites. (photos of cattle being sprayed below)
But the use of pesticides raises another question: What happens to the chemical afterward?
Pesticides applied to COWS are subject to rules governing the product, dosage and time between treatment and slaughter. That waiting period—known as a withdrawal period—is intended to allow chemical residues to fall to acceptable levels before the animal enters the food supply. For example, an FDA-authorized topical screwworm treatment in 2026 carries a 30-day slaughter withdrawal period for cattle when used according to its authorization.
At the U.S. border, USDA says every imported animal will receive a full inspection for signs of screwworm. How many employees does the USDA have at the border? Not nearly enough for this many inspections.
Ground beef does not undergo an inspection of the individual cow at the border. Instead, meat must come from eligible Mexican establishments operating under a food-safety system recognized by USDA, followed by inspection/reinspection by USDA's Food Safety and Inspection Service.
So there are really two separate concerns: preventing screwworm from entering the United States and ensuring that pesticide treatments used on cattle do not result in unacceptable chemical residues in the beef.
The important questions for consumers are what chemicals are being used, how they are applied, whether required withdrawal periods are followed, and how closely those requirements are actually verified.
