New World Screwworm Fly

Cochliomyia hominivorax — the New World screwworm fly — is one of the most economically and historically significant livestock parasites in the Americas, and the centerpiece species of one of the greatest triumphs of modern integrated pest management. The fly lays 100-400 eggs in clusters at the edge of open wounds on living warm-blooded animals (cattle, horses, sheep, goats, deer, and occasionally humans). Eggs hatch in 12-24 hours and the larvae burrow INTO living tissue, feeding on the host's flesh in screwing motions (the source of the common name). A single screwworm infestation can kill an untreated 500 kg steer in 7-14 days; before eradication, the species caused an estimated $400 million per year in US livestock damage. The species was the target of the first successful Sterile Insect Technique (SIT) program. USDA scientist Edward F. Knipling developed the SIT concept in the 1930s-1940s: mass-rear male flies in factories, sterilize them with gamma irradiation (which damages reproductive cells but leaves males fully able to mate), release them in the wild in such overwhelming numbers that they outnumber wild males, and let them mate with wild females. Females mate only once; mating with a sterile male produces no offspring; the wild population collapses. The 1958-1959 USDA mass-rearing facility at Mission, Texas was the first industrial-scale insect mass-rearing facility in human history (producing 50-500 million sterile flies per week). Screwworm was eradicated from the southeastern US by 1966, the entire US by 1982, Mexico by 1991, Central America by 2000, and most of Panama by 2006. The current barrier zone is at the Darien Gap between Panama and Colombia, where 30-50 million sterile flies are released weekly to prevent re-incursion from South America. The SIT methodology developed for screwworm is now used worldwide against tsetse flies, fruit flies, mosquitoes, and other pest species. Screwworm eradication is regularly cited as one of the most successful pest control programs in modern history.