As New World screwworm flies (Cochliomyia hominivorax, male pinned specimen shown here) spread northward again, researchers say two versions of a trap originally designed to catch tsetse flies in Africa are also effective for catching screwworm flies. The traps also preserve caught specimens better than traditional sticky traps, offering a promising boost to early-warning surveillance efforts. (Photo by Paul Langlois, USDA APHIS ITP, Bugwood.org)

By Andrew Porterfield

Andrew Porterfield

Beginning in the 1960s, the New World screwworm fly (Cochliomyia hominivorax) was eradicated in North America, first in the U.S. and then progressively southward to the Darien Gap in southern Panama by the early 2000s. It then remained endemic but controlled in every country in South America except Chile. However, recent outbreaks have been recorded across central America and Mexico, and in June 2026 the fly was identified in Texas and New Mexico.

Screwworm fly infestations, which involve the flies laying eggs in wounds of warm-blooded animals (usually livestock) and the subsequent flesh-eating activity of the larvae, could cost cattle producers $4.3 billion a year, and $10.6 billion in total economic losses, in the United States.

Pesticides have not been overly effective against the fly, and containment has been achieved by isolation of livestock hosts of the fly larvae and sterile insect technique (SIT), in which massive numbers of sterilized male flies are introduced into the population. Since females only breed once in their lifetimes, when they mate with sterile male flies, their eggs do not produce offspring. Historically, SIT has worked very well in controlling fly outbreaks. However, because of the very short lifecycle between egg laying and pupation, immediate identification and host animal isolation are essential.

The recent resurgence of the fly has called for better early identification of the fly, usually done through luring and trapping the insect. Recently, a research team from Uruguay and Brazil assessed the capabilities of three available fly traps: the commonly used vertical sticky trap (VST) and two versions of an Nzi trap (named for the Swahili word for “fly”). They found that all three traps worked equally well, and the Nzi traps had the advantage of preserving insect morphology for further identification. The results of their study were published last week in the Journal of Medical Entomology are included in a new, in-progress special collection of New World screwworm research in the Journal of Medical Entomology and the Journal of Economic Entomology.

As New World screwworm flies (Cochliomyia hominivorax) spread northward again, researchers say two versions of a trap originally designed to catch tsetse flies in Africa are also effective for catching screwworm flies. The traps, a standard and modified version of the Nzi trap (shown here), also preserve caught specimens better than traditional sticky traps, offering a promising boost to early-warning surveillance efforts. (Photo by Luisina Mezquita, Uruguay National Institute for Agricultural Research)

Led by Pablo Parodi, Ph.D., entomologist at the Uruguay National Institute for Agricultural Research, the researchers baited all three types of traps with the commonly used fly bait Swormlure-4 and set them in four types of sampling sites (forest edge, pastures, near bodies of water, and near livestock corrals), all in northern Uruguay. The traps were set during all four seasons of the year.

In total, 96 blowflies were collected, and 15 of them were identified as C. hominivorax. Seven flies were collected by the standard Nzi trap, five by the modified Nzi trap, and three by the VST trap. The difference was not statistically significant, which means all three trap types were equally capable of trapping the screwworm fly. “One of the most notable findings was that the Nzi and Nzi-modified traps accounted for nearly 50 percent of the total C. hominivorax captures,” Parodi says. “Furthermore, a significant advantage was that Nzi traps yielded well-preserved specimens, avoiding the severe morphological degradation caused by the entomological glue on the VSTs.”

As New World screwworm flies (Cochliomyia hominivorax) spread northward again, researchers are evaluating various kinds of traps for detecting the flies, a key step in surveillance efforts. Here, Gonzalo Escayola (left) and Pablo Parodi, Ph.D. (right), of the Uruguay National Institute for Agricultural Research set up a vertical sticky trap, a standard trap traditionally used in screwworm surveillance, for comparison with alternatives. They baited the traps with cotton soaked with the commonly used fly bait Swormlure-4. (Photo by Luisina Mezquita, Uruguay National Institute for Agricultural Research)

In addition, the researchers found a 14:1 ratio of females to males captured in the traps. This indicates that Swormlure-4 is working as a wound-mimicking attractant and a useful tool for monitoring wild populations of the fly. The low abundance of the screwworm fly was also in line with other studies in southern Brazil and Argentina.

The study, the first to evaluate Nzi traps on the screwworm fly (the traps were originally developed to trap tsetse flies in Africa), indicates its value as an effective monitoring tool against the insect pest. “Since Nzi traps perform competitively even in low-abundance scenarios and across diverse environmental ecotones, they provide an effective alternative for national surveillance networks,” Parodi says. “They allow the immediate recovery of high-quality specimens suitable for critical taxonomic and molecular identification. This is crucial for rapid response and early detection in border zones, especially given the insect’s recent northward expansion in the Americas.”

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He is based in Camarillo, California. Connect with him via LinkedIn or via email at [email protected].