As New World screwworm (Cochliomyia hominivorax) has re-emerged in the United States, entomologists are researching new methods for eradicating it. A recent study finds the parasitoid wasp Tachinaephagus zealandicus to be a promising candidate for targeting the screwworm via biological control. Shown here, a T. zealandicus female deposits an egg in a larva of another blowfly species, Calliphora vomitoria. (Photo by Jitte Groothuis, Jittebug Ento Art, originally published in Geden et al. 2026, Journal of Medical Entomology)

By Jada Summerville

We may be able to control New World screwworm outbreaks threatening the U.S. with a parasitoid wasp no bigger than a grain of rice.

Led by researchers at the United States Department of Agriculture (USDA) in Florida and Texas, a study published in June in the Journal of Medical Entomology reports that secondary screwworm (Cochliomyia macellaria)—a fly species of the same genus as New World screwworm (Cochliomyia hominivorax)—is a suitable host for a parasitoid wasp Tachinaephagus zealandicus.

New World screwworm is a devastating parasitic pest typically found in South and Central America. The fly lays its eggs in the wounds of warm-blooded animals, such as cattle and other livestock. The subsequent larvae feed on live tissue, killing the animal if left untreated and posing risks to livestock populations.

The U.S. is one of the world’s largest producers of beef, generating roughly 12 million metric tons annually, with Texas leading the nation in production. Although New World screwworm was once eradicated in the United States, the fly’s recent re-emergence in Texas in June has jeopardized U.S. livestock and wildlife. To date, 59 cases of the parasitic fly have been confirmed, with 54 in Texas and five in New Mexico, according to USDA data.

Seeking a solution, the study authors explored the use of the parasitoid wasp T. zealandicus as a new method of biological control for screwworm infestations.

“Combatting New World screwworm requires a multi-pronged approach,” says Chris Geden, Ph.D., a research entomologist at the USDA’s Center for Medical, Agricultural, and Veterinary Entomology, who led the study. “Wasp releases could be helpful for treating localized hot spots where NWS populations are gaining a foothold.”

A Lethal Weapon

Because New World screwworm is a quarantine pest, it carries great risk to work with. Hence, the study authors utilized a proxy species, the secondary screwworm. The study examined whether T. zealandicus is effective in killing secondary screwworms as a natural biological control approach.

The biocontrol strategy the authors tested relied on the tiny parasitoid wasp targeting secondary screwworm larvae during their “wandering stage.” This is the final larval stage of the screwworm in which it drops from the animal host to burrow in the ground before pupating. The wasp injects its eggs into larvae that are more than double their size. The hatching offspring consume it from the inside out, killing the fly before it can mature into an adult.

The authors first tested if the secondary screwworm was susceptible to T. zealandicus. By exposing screwworm larvae to adult wasps, they found that the parasitoids killed the screwworms at rates similar to other fly species parasitized by T. zealandicus. When screwworm larvae were highly abundant at a 40-to-1 ratio, just five wasps achieved a maximum kill rate of almost half the population.

T. zealandicus has a broad range of suitable hosts to parasitize, providing competition that may leave screwworm less desirable in comparison. The authors then investigated the wasp’s preference for the screwworm compared to another known fly host, Sarcophaga bullata. The wasps did not prefer one fly over the other, showing screwworm is a suitable host for T. zealandicus.

Like many insects, T. zealandicus finds its prey through smell. Because wandering screwworm larvae burrow into the ground to pupate, different terrains might pose differing challenges to the wasps.

The authors found that providing clay or moist soil for larvae to burrow in did not prevent the wasps from hunting and parasitizing them. However, dry soil provided some protection from the wasps by allowing the larvae to thrash more effectively and escape the wasps.

Together, these experiments demonstrate that T. zealandicus is highly effective at killing secondary screwworms across various soil environments.

Death by a Thousand Cuts

While the U.S. and other countries control New World screwworm reproduction using sterile insect techniques, some flies slip through the cracks. This study suggests biological control approaches may offer a promising tool to add to the effort to eradicate New World screwworm from the U.S. and endemic countries.

“It fits into our approach of integrated pest management,” says Phillip Kaufman, Ph.D, a professor and head of the entomology department at Texas A&M University, who was not involved in the study. “It’s sort of like death by a thousand cuts. Everything you do takes a little bit more of the population away.”

Geden says additional studies are needed to determine if these results can be replicated in field settings and with the New World screwworm itself, rather than the proxy species.

“Our lab has developed methods for economical mass-production of T. zealandicus,” Geden says. “A critical next step would be to see whether mass-rearing and release of wasps reduces fly populations.”

Jada Summerville is a Ph.D. student in biomedical sciences at Albert Einstein College of Medicine. She studies synapse development in the nematode Caenorhabditis elegans. A participant in the National Association of Science Writers’ David Perlman Virtual Mentoring Program, she enjoys science outreach and communication and loves the use journalism as a way of reaching general and young audiences to spark an interest in science. Find Jada on LinkedIn or email: [email protected].

This article was originally published via the National Association of Science Writers’ Student Newsroom. Lightly adapted and republished with permission.