Screwworm, the larva of a species of blowfly. Female screwworm flies lay their eggs in wounds of warm-blooded animals, especially cattle. When the screwworms hatch, they feed on the animal's flesh. Up to 400 screwworms may infest a single wound. Mass infestation with screwworms can kill an animal.

ScrewwormsScrewworms are the larvae of a species of blowfly.

Full-grown screwworms, about 2/3 inch (17 mm) long, drop to the ground and enter the pupal stage. In about 8 to 60 days, depending on the temperature, the adult flies emerge from the pupal stage. In a few days, they mate and the female seeks out a wound in which to lay her eggs.

In the past, screwworms have caused millions of dollars of damage to livestock in the United States. Since the late 1950's, however, a program in which sterile male flies are reared in the laboratory and then released to mate has resulted in female flies laying unfertilized eggs. This program has been successful in controlling screwworms in most parts of the United States, but occasional outbreaks still occur.

The screwworm is the larva of Cochliomyia hominivorax of the order Diptera, family Calliphoridae.