Description

The New World screwworm fly, also called primary screwworm, is a devastating livestock pest that also parasitizes other warm-blooded animals, including humans. Adult New World screwworms are bright metallic green or greenish-blue flies, averaging about 6 mm in body length. The eyes are large, occupying most of the surface of the head, and are bright reddish-orange in color. Three vaguely visible black lines run the length of the upper thorax (mesonotum) against the metallic base color. The abdomen is entirely metallic. Adult flies are similar in appearance to a number of other flies in Louisiana, especially other members of the family Calliphoridae (blow flies).

Larvae of the New World screwworm are maggots, similar in appearance to other blow fly maggots. Their occurrence in healthy living tissue of animals is a critically important way of diagnosing New World screwworm larvae relative to those of other similar species. Fully mature larvae may reach 18 mm in length, and bear a number of dark spines arranged in a characteristic “screw” pattern. They are usually found in groups that hatch from a single egg mass. Individual screwworm eggs are about 1 mm in length. Egg masses are variable in size and may contain 10 to 400 eggs each. They are always deposited near wounds or body orifices of warm-blooded animals. Pupae are rarely seen without dissection because they are enclosed in the hardened, barrel-shaped remnants of the last larval molt (puparium).

A similar species and member of the same genus, Cochliomyia, is common in Louisiana. Adults and larvae of the secondary screwworm fly, Cochliomyia macellaria, are similar in size and physical appearance to the New World screwworm. Detailed examination of the hairs on the body and veins of the wing are necessary to separate the two species. Currently, only the secondary screwworm is known in Louisiana and is not considered a pest species. If New World screwworm is suspected, specimens should immediately be submitted to a qualified insect diagnostician or taxonomist for evaluation.

A similar species, with remarkably similar habits but belonging to a different genus of blow flies, occurs in the Eastern Hemisphere. The Old World screwworm, Chrysomya bezziana, also infests living animal tissue. It is found in tropical and subtropical areas of the Eastern Hemisphere from Africa to eastern Asia. Adults are similar in external appearance to New World screwworms, but the thorax lacks the characteristic dark lines.

Life Cycle

New World screwworm undergoes four developmental life stages, as with all insects having complete (holometabolous) metamorphosis. These include egg, larva, pupa and adult. Larvae of New World screwworm feed exclusively on living tissue of warm-blooded animals. This condition is called myiasis. Although mated female flies typically seek out suitable hosts close by, they are capable of traveling long distances (up to 78 miles or 125 km). Egg masses are deposited adjacent to open wounds, sores from insect or tick bites, or body orifices. Larvae hatch within 24 hours and begin burrowing into living tissue, often moving deep into the body. They mature after five to seven days. The constant body temperature of the warm-blooded host allows rapid development with little variation in duration. Larvae undergo three growth stages (instars), then exit the host and drop to the ground, where they pupate. Pupae remain underground for as little as one week during warm weather, up to four months during cold weather. After adults emerge from the puparium, they work their way to the surface and are able to mate after a few days. Adult flies may live for several weeks. They visit flowers and feed on nectar using their sponging mouthparts. Females also consume protein-rich exudates from lesions on infested animals to obtain nutrients for egg development.

Ecological Significance and Pest Status

The New World screwworm has a natural distribution in the Western Hemisphere from southern U.S. to Argentina. It has been largely eliminated from the northern parts of the range, but is showing signs of returning. The potential northern extent of New World screwworm is limited by winter temperatures, but Louisiana and other southern states along the Gulf Coast are well within potential areas of infestation. Infestations have been historically documented in Louisiana. Infestations may establish further north with continuing human caused climate change.

Earliest reports of New World screwworm in the U.S. date from the 1840s in the southwest. By the 1930s, the fly spread and established in southeastern states, with a notable outbreak in 1933 causing significant livestock losses in the region. Subsequent infestations were devastating in scope and severity. During high-density outbreaks, farmers reported that death of cattle could occur less than 10 days after initial detection of screwworms. In some cases, mortality rates of newborn cattle in infected herds reached 100%. Affected animals typically ceased milk production and suffered severe loss of body mass, with wounds generating a characteristic smell of rotting tissue in living animals. Treatment of infestations in cattle were impractical and expensive, requiring manual removal of all larvae and affected tissues, as well as monitoring treatment of secondary infections. While death typically resulted from sepsis due to infection, some extreme cases documented the ability of larvae to infiltrate the skull and consume the brain of animals.

Reliable statistics on historical instances of human infection by New World screwworm in the U.S. are unavailable. Known cases in other countries, although rare, appear largely limited to individuals working closely with livestock or living in rural areas. During 2025, the first case in over 50 years of New World screwworm infection in a human host in the U.S. was reported from a Maryland woman who had traveled to El Salvador. Endemic human transmission of New World screwworm in the U.S. has not been reported since the eradication effort that was completed during 1966.

Eradication Efforts, Past and Present

Initial control efforts largely focused on chemical pesticides and restricting movement of infected animals, but these efforts were limited in success. Raymond C. Bushland and Edward F. Knipling began work at the U.S. Department of Agriculture laboratory in Menard, Texas, during the 1930s to develop alternate methods of New World screwworm control, eventually resulting in the development of the Sterile Insect Technique (SIT). A form of autocidal (self-regulating) control, SIT disrupts the lifecycle of the pest. Male flies are reared in facilities and rendered sterile after irradiation with X-rays. When released, these sterile males mate with wild female flies. Because New World screwworm females are monogamous (mate only once during their lifetime), successive releases of sterilized males can quickly and dramatically impact the overall reproductive rate of wild populations. After a hiatus due to WWII, Bushland and Knipling developed techniques for mass-rearing facilities and refined methods of irradiation. The success of these efforts eventually resulted in complete eradication of New World screwworm from the U.S. by 1966.

In 1994, the Panama-United States Commission for the Eradication and Prevention of Screwworm Infestation in Livestock (COPEG) was established to continue eradication efforts. The sterile fly production facility, a 227,000 square foot facility in Pacora, Panama, has operated since 2006, producing billions of sterilized flies to maintain quarantine of New World screwworm. This coordinated, multinational effort eventually achieved the additional eradication of New World screwworm from southern Mexico to the Darien Gap in Panama. This successful eradication of New World screwworm from North and Central America established an effective biological barrier. With the exception of occasional incursions, the Darien Gap quarantine line remained relatively stable until 2023, when a nationwide outbreak of New World screwworm was declared in Panama. As of May 2024, New World screwworm had reestablished north of the Panama Canal, with reported human and animal cases from all countries in Central America and Mexico where the fly was previously controlled. In 2025, COPEG halted release of flies in other regions to concentrate on efforts to establish a new biological barrier to the north in the Isthmus of Tehuantepec, Mexico.

Despite the proximity of this major outbreak in Central America, New World screwworm is not currently reestablished in the U.S. Although rare, isolated outbreaks of New World screwworm in the U.S. have occurred, including a 1976 instance in Texas that caused an estimated $330 million in livestock losses. The most recent isolated outbreak of New World screwworm occurred in 2017 in Florida, resulting in 145 cases restricted to the Florida Keys. The majority of these cases were in the endangered key deer, as well as several household pets and a raccoon. No livestock were infected. If New World screwworm were to enter the U.S. in a more widespread outbreak, projections of losses are in the billions of dollars for the cattle and hunting/ wildlife industries. Currently, the USDA plans to supplement sterile fly production at COPEG (100 million flies per week) with construction of an additional facility at Moore Air Force Base in Edinburg, Texas, (30 million flies per week). A planned joint effort between Mexico and the U.S. to renovate a facility in Chiapas, Mexico, would increase capacity by an additional 100 million flies per week. These combined efforts are aimed at eventual reclamation of the biological barrier to the previous Darien Gap.

Current research efforts are focusing on physiological mechanisms of mate- and host-finding behaviors to discover novel ways of disrupting screwworm life history patterns. For example, a multinational group of researchers in Latin America is studying the genetics of host finding and foraging, focusing on the odor coreceptor “ORCO.” The closely related and native secondary screwworm (Cochliomyia macellaria) is a promising surrogate for research that could result in discoveries applicable to screwworm management in the future with the risk of accidental New World screwworm escape.

References

Anonymous. 2017. Final report for the APHIS Veterinary Services response to the 2016–2017 outbreak of New World screwworm (NWS) in Florida. USDA APHIS VS. 42 pp.

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Gutierrez, A.P., L. Ponti. 2014. The new world screwworm: prospective distribution and role of weather in eradication. Agricultural and Forest Entomology, 16: 158-173. (accessed Feb 03 2026).

Navarre, C. 2024. New World Screwworm. LSU AgCenter. (accessed 29 January 2026).

Paulo, D. F., A. C. M. Junqueria, A. P. Arp, A. S. Viera, J. Ceballos, S. R. Skoda, A. A. Perez-de-Leon, A. Sagal, W. O. McMillan, M. J. Scott, M. J. Scott, C. Concha, and A. M. L. Azeredo-Espin. 2021. Disruption of the odorant coreceptor Orco impairs foraging and host finding behaviors in the New World screwworm fly. Scientific Reports 11: 11379 (accessed 30 January 2026).

Unknown. 1959. “Screwworm Infestations Found in Mississippi and Louisiana.” Special Collections, USDA National Agricultural Library. (accessed 30 January 2026).

USDA. 2026. Screwworm publications 1830-1981. Screwworm Publications, 1830-1981. (accessed 30 January 2026).

Veterinary Medicine Extension. 2025. History of Screwworms in the U.S. Washington State University. (accessed 30 January 2026).

World Organization for Animal Health. 2019. New World screwworm (Cochliomyia hominivorax) and Old World screwworm (Chrysomya bezziana). Chapter 3.1.14 In: Terrestrial Animal Health Code. (accessed 30 January 2026).

World Food Prize Foundation. 2026. 1992: Knipling and Bushland. (accessed 30 January 2026).

Authors:
Brittany Owens
Chris Carlton

P4044
4/1/2026
The LSU AgCenter and LSU provide equal opportunities in programs and employment.

New World screwworm adult (Cochliomyia hominovorax); one of the sterile males released on Big Pine Key, Florida, during the 2017 outbreak. Photo by Judy Gallagher, iNaturalist

Adult secondary screwworm (Cochliomyia macellaria) adult, St. Gabriel, Louisiana. Photo by VanRemsen, iNaturalist

New World screwworm (Cochliomyia hominovorax) mature larva. Photo by Lesley Ingram, Bugwood.org