The United States Releases Millions of Flies over Panama Every Week
This is an agreement between both countries to eradicate a deadly disease from the continent. The health operation, active for two decades, is maintained as a model of international cooperation.
Every week, U.S. aircraft drop more than 14.7 million sterile flies over the Panamanian rainforest to curb the screwworm, a key operation to protect the U.S. livestock economy. In an operation almost invisible to the public, the United States government deploys more than 14.7 million sterile flies each week along the Panama-Colombia border. The measure seeks to contain the New World screwworm, a parasite capable of devastating livestock production and which, despite decades of control, is a health threat to North America. The operation is part of a policy maintained for almost twenty years by the Panama-United States Screwworm Eradication Commission (COPEG).
From a plant located in Pacora, on the outskirts of Panama City, screwworm flies are bred and sterilized and then released from aircraft into a strip of jungle where the continent narrows and becomes a natural barrier to entry into the rest of the hemisphere. Considered eradicated in the United States in 1966, the pest returned in 2016 when Florida veterinarians detected its presence in Key West deer. The discovery sparked a national alarm. The screwworm, whose larvae invade open wounds in animals and people, causes mutilations, serious infections, and economic losses that, according to the Department of Agriculture (USDA), could exceed US $1.3 billion annually if the parasite manages to re-enter the country.
Since then, health surveillance has been intensified and funding has been renewed to maintain the biological barrier in Panama, with the United States covering most of the US$15 million annual cost of the operation. The strategy of releasing sterile flies was born in the 1950s, pioneered by entomologist Edward F. Knipling. The technique, first applied in 1954 in Curaçao, involves breeding millions of flies in the laboratory, sterilizing them with radiation, and releasing them in high-risk areas. Since females only mate once, contact with sterile males interrupts the reproductive cycle. Today, this practice continues at the COPEG plant, where the larvae feed on a mixture of blood, milk, and powdered egg before being irradiated. Once they become adult flies, they are dispersed from military aircraft over the Panamanian jungle.
Screwworm Outbreak in Central America
Since 2023, Central America has faced the worst outbreak of the New World screwworm in more than four decades. Panama went from detecting an average of 25 cases annually to recording more than 6,500 in just one year, according to the USDA’s Animal and Plant Health Inspection Service (APHIS). The pest has rapidly spread to Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador, and Mexico, and has managed to overcome the historic biological barrier established in Darién, which for decades has contained the spread of this threat to regional livestock. Given this scenario, APHIS announced on February 26, 2025, the transfer of its sterile fly dispersal strategy to two strategic locations in Mexico, the northern limit of the current outbreak. The operation will continue from centers outside the country to ensure the constant release of sterile insects and curb the spread. The comprehensive plan is based on three pillars: the sterile insect technique, animal movement control, and an intensive surveillance and education campaign. The USDA allocated $109.8 million to strengthen this operation.
