As the New World Screwworm Returns, Scientists Unite to Fight Back

Insect pests like flies are a nuisance, but some, like the New World screwworm, have the potential to devastate cattle, a serious and costly bane to the agriculture industry. The New World screwworm is a type of blowfly that sniffs out and lays eggs in the wounds of living tissue, often livestock and companion animals. When the larvae emerge, they eat the surrounding flesh before falling out to mature in the soil, causing more injury that attracts more blowflies to the site in the meantime.

Before mitigation efforts, New World screwworm led to more than $200 million in livestock losses every year in the US alone.¹ While strategies to eliminate New World screwworm populations have worked well in the past, today, researchers are exploring new genetic technologies and multidisciplinary approaches to better combat this devastating pest.

Sterile Insects Kept New World Screwworm at Bay

The New World screwworm is endemic to the warm regions of the Americas. To combat this devastating fly, scientists took advantage of the insect’s sexual behavior.^2-4 Male blowflies mate aggressively, while female blowflies only mate once. By sterilizing large numbers of flies to release them into the wild, researchers discovered that they could drive out the blowflies by causing large numbers of unsuccessful pairings.⁵

“It was a brilliant idea,” said Maxwell Scott, an entomologist at North Carolina State University. Indeed, the US in coordination with Mexico and, later, central American countries drove the New World screwworm out of their territories down to Panama in the 1960s. There, a production facility has continued to churn out up to 100 million flies a week to maintain a barrier between Panama and Colombia. “So, it can be very effective, but it is a numbers game. You've got to outnumber the fertile flies in the area,” Scott said.

However, in 2022, the barricade broke. Researchers said that it was likely a combination of many factors: Varying land use had introduced more cattle into the area, the COVID-19 pandemic caused a decrease in the number of people available to conduct investigations, and the number of flies released into the area wasn’t increased in sufficient time when early cases were detected.

“We have to take these factors into account, and the long-term management will require coordinated international action rather than isolated local responses,” said Andrés Lira-Noriega, an ecologist studying the interaction between species, including New World screwworm, and their environment at the Institute of Ecology.

The blowfly gradually migrated north. “[Mexican officials have] done a great job slowing it down, but there's just not enough flies. One hundred million sounds like a lot, but when you've got to really outnumber the fertile males to stop the population from increasing, that's not enough,” Scott said.

On June 3, 2026, the United States Department of Agriculture (USDA) announced the first case of New World screwworm in the US in a young calf in Texas. Since then, USDA in collaboration with other agencies in both the US and Mexico have launched responses to track, identify, and mitigate cases of New World screwworm. A new facility to produce up to 300 million sterilized flies per week is under construction in Texas, and a different facility is being retrofitted for this task on a smaller scale in Mexico.

While these rapid responses are a commendable example of cooperation and outbreak management, this incident has also provided researchers with an opportunity to consider how the current approaches could be improved.

One Health Approaches Can Better Address Complex Problems

Chad Cross, a One Health researcher at Texas Tech University, said that, while the sterile insect technique strategy is solid for managing the New World Screwworm, the current outbreak highlights some limitations in the approach. He said that relying on one facility to produce the necessary flies was less than ideal, and that even if it’s not feasible to constantly maintain multiple facilities, having procedures to rapidly increase production early in an outbreak could help.

Additionally, he said that implementing a One Health or multidisciplinary approach could bring more experts to the table to improve surveillance and mitigation. “It is fundamentally a way to de-silo how we approach problems in science,” Cross said. “So instead of looking at it [as] a single basic science pathway towards one part of an answer, we try to develop an integrated set of ways to investigate a problem.”

For example, as a coleader of the New World Screwworm response at Texas Tech University where he directs the team’s research efforts, he said that the group has brought in both veterinarians and physicians for educational opportunities. Since these flies can lay eggs in both animal and human tissues, the team wants to make sure that the medical professionals are ready to spot and treat new cases of New World Screwworm. At the same time, they are working with environmental scientists to track cases and develop models to predict where these flies may be most likely to migrate to, which can inform where field personnel need to be sent with interventions.

Lira-Noriega said that this is important because researchers cannot physically monitor every location, so predictive models help identify where resources are most needed to respond efficiently. For example, he and his team investigated the factors that influenced where New World Screwworm would likely migrate after the 2022 breach of the Panama barrier.⁶ He said that, so far, the spread of the fly has matched his and his team’s predictions, but he added that they found many more areas that have suitable conditions for this fly. He thinks that surveillance and mitigation efforts should extend to these regions as well before cases emerge.

“Historically the sterile insect technique has been really useful and is today probably one of the most promising approaches, but definitely it has to be combined with surveillance, rapid response, [and] regional cooperation,” he said. He added that integrating existing tools, such as geospatial information, reported cases, and the distribution of livestock can help public health officials monitor outbreaks in real time and prioritize responses. Lira-Noriega also said that incorporating newer molecular surveillance technologies could further improve surveillance.

Cross and his team are working on just those types of solutions. They are collaborating with biochemists and engineers to develop better field tests to detect New World Screwworm, including lateral flow tests to detect the blowfly’s DNA from the wounds of infected animals and strategies to identify the presence of the fly from water trap samples using environmental DNA testing.

Male Only Flies: The Next Evolution in the Sterile Insect Technique

Another area that researchers have explored improving is in the sterile insect technique itself. Currently, when facilities produce flies, they raise both male and female insects because differentiating them on the scale of millions is not feasible. This leads to raising and releasing female blowflies that don’t largely aid in the elimination strategy, essentially wasting resources.

“It's enormously expensive, so that's why people like myself have been searching for more efficient solutions,” Scott said. He and his team developed a male-only line of New World Screwworm flies to overcome this challenge. The researchers use a transgene under the control of tetracycline that is spliced with an intron so that a lethal protein is only overexpressed in female larvae and embryos.^7-9

The Environmental Protection Agency recently chose one of the team’s lines to undergo field testing in Panama. If successful, Scott’s fly line could be approved for use in the US, and with additional regulatory approval, in Mexico.

Switching to male-only flies, Scott said, would allow facilities to produce and release more flies that are directly relevant to the mitigation strategy. Additionally, he said that since male flies can be sterilized more easily than female flies, the producers can use lower doses of radiation to perform that task. This reduction could help keep these flies fitter for mating.

Looking further ahead, some researchers are exploring avoiding radiation all together. Instead, they hope to use CRISPR-Cas9-based gene editing to render the flies sterile. Scott said that this strategy would be cheaper. However, researchers currently struggle to grow these colonies at scale, since the approach involves many genetic crosses.

One of these strategies or another novel approach could soon get some additional help in coming to fruition. Earlier in the year, the USDA opened a grant application for, in part, novel strategies or enhancements of existing approaches with new technology to improve the control of the New World screwworm.

“I suspect we'll find some really neat and inventive molecular techniques being posited as potential solutions to help with the issue,” Cross said.

Today, researchers have the tools necessary to combat the New World screwworm, but by working together and implementing new strategies and better technology, scientists and other stakeholders can stay one step ahead of this deadly pest.

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