Cochliomyia
Cochliomyia is a genus in the family Calliphoridae, known as blowflies, in the order Diptera. Cochliomyia is commonly referred to as the New World screwworm flies, as distinct from Old World screwworm flies. Four species are in this genus: C. macellaria, C. hominivorax, C. aldrichi, and C. minima. C. hominivorax is known as the primary screwworm because its larvae produce myiasis and feed on living tissue. This feeding causes deep, pocket-like lesions in the skin, which can be very damaging to the animal host. C. macellaria is known as the secondary screwworm because its larvae produce myiasis, but feed only on necrotic tissue. Both C. hominivorax and C. macellaria thrive in warm, tropical areas. They are considered an agricultural pest of farm animals and the United States has partnered with Mexico and Central American countries to eradicate the fly via the sterile insect technique.
Characteristics
Adult
In general, all Diptera have three body regions, three pairs of legs, one pair of forewings used for flight, one pair of halteres which are modified hindwings, and one pair of antennae.New World screwworm flies share many characteristics with the common house fly. When keying out a dipteran specimen, it is important to note first whether bristles on the meron are present or absent. All species in the family Calliphoridae have bristles on their merones, plumose arista, and well-developed calypters. Both C. macellaria and C. hominivorax are metallic green to bluish green in major coloration, with setae on the dorsal surface of the stem vein, orange gena, pale white anterior spiracles, filiform palps, and three black longitudinal stripes on the notum of the thorax. The species C. macellaria has pale setulae on the fronto-orbital plate outside the row of frontal bristles, while C. hominivorax has dark setulae on the fronto-orbital plate outside the row of frontal bristles. The female C. macellaria has a yellowish basicosta, while the female C. hominivorax has a brown basicosta. C. macellaria is in length. C. hominivorax is in length.
Larvae
The larvae of both C. macellaria and C. hominivorax have cylindrical bodies tapering anteriorly with 10 or more robust spines around the spiracular area, incomplete peritremes, an indistinct or absent button, and bands of small spines on each segment. The C. hominivorax larvae have distinctly pigmented tracheal trunks. C. macellaria larvae do not have pigmented tracheal trunks; they have spines in a V shape on the anal protuberance and no oral sclerite. The mature third instars of both species' larvae can reach a length of.Life cycle
The general life cycle of Cochliomyia is similar to other Diptera in that they are holometabolous. The four stages are egg, larva, pupa, and adult. This entire life cycle lasts an average of 21 days under optimal conditions, and can last as long as three months in colder climates. Females only lay eggs once in a lifetime and can lay 100 to 400 in a clutch. Females usually lay their eggs on the edge of an open wound. Warm and moist conditions are the perfect combination of home and food source. The nasal, oral, or anal areas of a host are especially prone to Cochliomyia oviposition.Larvae hatch about 12–21 hours after the eggs have been laid, and are cream colored. C. hominivorax larvae dive head-first into whatever food source is nearest, and burrow deeper, eating into live flesh if available. This results in a pocket-like lesion that causes severe pain to the host. C. macellaria larvae only feed on the necrotic tissue of a wound. After five to seven days, the larvae drop and move away from the food source to pupate. The larvae burrow into the topsoil, beneath leaves or garbage, and begin their pupation. The pupa is a dark brown color. This stage can last from seven days at a warm temperature to as long as two months if the weather is much colder. Adults emerge and spend a day or two finishing maturity. Adults of C. hominivorax breed only once in their lifetimes. Sexually mature adults breed 3–4 days after emerging from the pupa. Males mature rapidly and spend their time waiting, eating nearby vegetation, and nectar of flowers. Females, however, are predatory and feed on the fluids from live wounds. Females can fly long distances to find a mate. Adult flies of this species live around two to three weeks.
''Cochliomyia hominivorax''
Biology
The primary screwworm, C. hominivorax, is a parasitic species, whose larvae are renowned for eating and infesting the flesh of living organisms, primarily warm-blooded animals such as cattle and other livestock. Their larvae cause myiasis, an infestation of maggots in lesions or other wounds and injuries that the host animal may have. Flystrike may occur due to farming processes as branding, castrating, dehorning, and tailing of the host animals. These processes, along with barbed-wire cuts and flea bites, lead to myiasis in the host animal. The navels of newborns can also be the sites of infestation. C. hominivorax tends to reproduce only on the flesh of a living host. Unlike most other maggots, these maggots attack and consume healthy living tissue along with decaying tissue. The larvae are responsible for their common name, the screwworm, because they possess small spines on each body segment that resemble a screw's threads. After the larvae hatch, they dive into the wound and burrow deeper, perpendicular to the skin surface, eating into live flesh, again resembling a screw being driven into an object. The larvae then continue to feed on the wound fluids and the animal's tissue.Elimination programs
North and Central Americas
The sterile insect technique was proposed by scientists Edward F. Knipling and Raymond C. Bushland, and was rapidly adopted by the United States Department of Agriculture in 1958. The technique centers on a unique reproductive handicap that prevents female C. hominivorax flies from mating more than once. The scientists reasoned that if it were possible to clinically sterilize and release huge numbers of males as breeding time approached, fertile males might be outcompeted. The majority of female flies would lay sterile eggs. Irradiating the males was used for sterilization.Because the agricultural industry was losing millions of dollars annually due to treatment and loss of fly-struck animals, this solution was quickly approved for testing. It was first applied on a large scale in Florida in the 1950s, due to the severity of the problem there and the state's unique island-mimicking geography, which allowed for relative isolation of the Florida C. hominivorax population. The eradication of Florida's primary screwworm population was completed in 1959. The program was then applied throughout the southern United States. Eventually, it was adopted throughout much of Mexico in 1972 and parts of Central and South America. The primary screwworm was completely eradicated from the southern United States in 1966, from Mexico in 1991, and from Central America in the early 2000's, down to the Panama-Colombia border. Livestock there continue to be vulnerable, however, and strict laws regarding animal inspection and reporting of suspected infestations remain in place.
A new screwworm infestation in the Florida Keys was first reported in October 2016, which was mostly in the Key deer population, along with five confirmed infestations in domestic animals. In response, the United States Department of Agriculture released over 80 million sterile flies from 25 ground release sites on twelve islands and in the town of Marathon.
There is an ongoing program between the USDA and Panama, the Comisión Panamá–Estados Unidos para la Erradicación y Prevención del Gusano Barrenador del Ganado to prevent the return of screwworm to countries where it has been eliminated and further extend prevention efforts into Colombia.
In November 2024, screwworm was detected in farms in Mexico, causing the USDA to halt imports of cattle, horses, and bison from there. Concern about possible further spread led the USDA to announce that it will spend up to $750 million on a new elimination program including an $8.5 million, 300 million fly per week production facility at Moore Air Force Base in Edinburg, Texas, which will likely take 2-3 years to build. The USDA is also partnering with Mexico by funding $21 million of a new $51 million, 100 million sterile fly per week production plant in southern Mexico near the Guatemala border projected to open in mid 2026. The only facility currently producing sterile flies is in Panama; that facility produces 100 million per week.
North Africa
The first, and to date the largest, documented infestation of C. hominivorax myiasis outside of the Americas occurred in North Africa from 1989 to 1991. The outbreak was traced to a herd of sheep in Libya's Tripoli region, which began suffering screwworm attacks in July 1989; over the following months, the myiasis spread rapidly, infecting numerous herds across a 25,000 km2 area. Eventually, the infested region spanned from the Mediterranean coast to the Sahara Desert, threatening the more than 2.7 million animals susceptible to C. hominivorax that inhabited the area. More than 14,000 cases of large-scale myiasis due to the C. hominivorax species were documented. Traditional control methods using veterinary assessment and treatment of individual animals were insufficient to contain the widely dispersed outbreak, so the United Nations Food and Agriculture Organization launched a program based on the sterile insect technique. About 1.26 billion sterile flies were produced in Mexico, shipped to the infested area, and released to mate with their wild counterparts. Within months, the C. hominivorax population collapsed; by April 1991, the program had succeeded in eradicating C. hominivorax in the Eastern Hemisphere. This effort, which cost under US$100 million, was declared to have been among the most efficient and successful international animal health programs in UN history.The North African outbreak both provided proof of the sterile male technique's efficacy and led to numerous enhancements in its implementation; after 1991, it entered into use across parts of Central and South America. However, the inaccessibility of some areas that the fly inhabits, language differences, and the need for constant vigilance have slowed the eradication of this species.