screwworm

screwworm, any of several North and South American blow fly species named for the screwlike appearance of the larva’s body, which is ringed with small spines. Screwworm maggots are serious pests that attack livestock, deer, and other animals, including humans. The true screwworm, or New World screwworm (Cochliomyia hominivorax), and the secondary screwworm (Callitroga macellaria) develop in surface wounds of domestic and wild animals and occasionally of humans. Each female deposits 200–400 eggs near an open wound or mucous membrane. The larvae burrow into the tissue and, when mature, drop to the ground to pupate. Severe infestations (myiasis) may kill the affected animal, sometimes in as little as one or two weeks.

The sterilization of male screwworm flies has been successfully used in attempts to control screwworms. Edward Fred Knipling, a pioneering American entomologist, developed this efficient, pesticide-free method of insect control with colleague Raymond Bushland in the mid-1900s. Given that each female mates only once, Knipling theorized that screwworms could be eradicated by sterilizing male flies in large numbers, then releasing them to mate with fertile females; the result, he thought, would be that no fertilized eggs would be produced and the numbers of the insect would drop dramatically. He and Bushland began sterilizing New World screwworm flies with an old army X-ray machine shortly after World War II, and in 1954 sterile flies were dropped on the Caribbean island of Curaçao. In only nine weeks the screwworm population virtually disappeared. Similar results were soon achieved across North America, and the technique was later used to control outbreaks of other pests, including the tsetse fly in Africa.

This control method virtually eradicated the parasite from the U.S. and Mexico in the 1960s and 70s, and throughout much of Central America in the following decades. To slow the encroachment of the pest from South America into Central and North America, the Panama-U.S. Commission for the Eradication and Prevention of Screwworm (COPEG) was formed in 1994 to create and maintain a buffer zone along the border of Panama and Colombia. To this end, the U.S. Department of Agriculture (USDA) has frequently and routinely dropped millions of sterilized male screwworm flies along the Darién region in Panama, many of which were produced at the sterile fly production plant that opened in Panama in 2006.

Despite these major national and international efforts, outbreaks have continued to arise. In 1976, for example, Texas experienced an outbreak that affected hundreds of thousands of goats and sheep, and more than 1.4 million heads of cattle, causing significant economic harm to ranchers and the livestock industry. A 2016 outbreak in the Florida Keys affected around 10 percent of the population of endangered Key deer (Odocoileus virginianus clavium); the outbreak was quickly contained. In 2025 many countries throughout Central America and Mexico reported hundreds or thousands of New World screwworm infestations, and efforts were renewed to reestablish eradication.