The screwworm, a pest likely remembered only by our grandparents, is making headlines again. With a proper name of New World Screwworm and scientifically known as Cochliomyia hominivorax, this is an incredibly destructive fly that is appearing in places that it had been eliminated.
The New World Screwworm (also referred to as NWS) is native to the Americas. Historically, it was found from the southeastern United States through Mexico, Central America, and into South America. This parasitic insect thrives in warm climates and was once a common problem across this region. It’s high reproductive rate and ability to infest diverse hosts make it very dangerous. Screwworms primarily affect livestock (including cattle, horses, sheep, goats), wildlife, and occasionally humans.
The NWS is an ordinary-looking fly that lays its eggs in the wounds or moist tissues of animals. Unlike most fly maggots that consume decomposing tissue, screwworm larvae feed on living tissue, often leading to death in their hosts.
Myiasis refers to the infestation of living tissue by fly larvae. First, a female screwworm fly lays her eggs on an open wound or a moist body opening (like the nose or ears). The eggs hatch into larvae within a day. Next, the maggots burrow into the tissue, feeding on living flesh instead of only decaying tissue as most other maggots.
Without treatment, myiasis caused by screwworms leads to severe pain, swelling, infection, and often death. The wound enlarges, and infections are likely as the larvae consume tissue. This secondary infection allows bacteria to enter the wounds, worsening the damage.
While they are native in the southern US and were historically found throughout the Americas, we have kept them contained in the last several years to parts of the Caribbean and South America. But earlier this year in 2025, they were found in Southern Mexico.
Preventative efforts for livestock included insect repellants and screened enclosures to prevent infestations. These were difficult and time consuming. On affected livestock, ranchers applied insecticidal powders or sprays directly to infested wounds to kill the larvae. Additionally, stockmen often manually removed maggots from the wounds of their animals’ using forceps, a time-consuming and labor-intensive process.
Deer, particularly white-tailed deer in the U.S., were frequent victims. Infestations often occurred in wounds caused by fights or during the shedding of antlers. Screwworm infestations had devastating effects on white-tailed deer, with reports indicating up to 80% death rates in newborn fawns in some areas.
Humans were treated similarly to livestock. Maggots were manually extracted from wounds. Medical staff applied antiseptics and larvicidal ointments wounds to kill larvae and prevent secondary infections. In advanced cases, surgery was required to remove deeply embedded maggots and repair tissue damage.
Eradication efforts in the 1950s led to the development of the Sterile Insect Technique (SIT). Entomologists Edward Knipling and Raymond Bushland began developing the SIT at a USDA research station in Menard, Texas. SIT involves raising and sterilizing male flies using radiation. Sterile males, when released, mated with wild females, resulting in no offspring and a sharp population decline.
Implementation was in full swing by 1960 in Texas and most southwestern states using SIT. A very large facility in Mission, Texas, was established to produce approximately 200 million sterile flies each week!
The results of releasing the sterile male flies were hugely successful. It was a cost-effective method that was specific to only the targeted species, and sustainable. Sterile males were transported in planes equipped with automated release systems that dispersed the flies over targeted areas. This method allowed for coverage of vast, remote regions. Releases occurred weekly or bi-weekly, maintaining a steady presence of sterile males to outcompete wild males for mates.
Texas was declared free of NWS in 1964 and the entire US was declared free in 1966. The elimination of screwworms resulted in substantial economic savings for the livestock industry. Deer populations experienced significant rebounds. In Texas, the white-tailed deer population more than tripled compared to 1960 levels.
Ongoing efforts through the years had pushed them south to a permanent, barrier zone in Panama where control efforts still occur. However, late in 2024 the NWS was detected outside the barrier in southern Mexico.
By March of 2025, there were 369 confirmed cases, including six infected people, reported in the Mexican states of Oaxaca and Veracruz. As a result, in May of 2025, the USDA suspended imports of live cattle, horses, and bison from Mexico to help prevent entry into the US.
It should be noted that the USDA is investing $21 million to renovate a facility in Metapa, Mexico, which will then be able to produce 60–100 million sterile NWS flies weekly. These sterile male flies are being strategically released in affected areas to suppress wild populations.
In response to the increased public concern for this terrible pest, a webinar hosted by the Texas A&M AgriLife Extension Service will examine and help educate agricultural producers and the public about the New World screwworm.
This free webinar will cover the biology of the pest, prevention and treatment strategies, public health perspectives including pets, importance of New World screwworm awareness and prevention, current status, government response and surveillance programs as well as a question-and-answer session. To register, visit http://tx.ag/NWSwebinar.
Additionally, the Texas A&M AgriLife Extension Service in Angelina County is hosting a viewing of the webinar on Monday, June 9 starting at 2 pm. Attending this meeting in person will give producers 2 CEU credits towards their pesticide license. This will be a live, zoom meeting hosted from 2:00 – 4:30 PM at the Angelina County Extension office. The Angelian County Extension office is located at 2201 S. Medford Dr., Lufkin, Texas.
