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What is New World Screwworm?
New World Screwworm (NWS) has become a common talking point in the news today, with the government in the process of building a new facility to help combat the spread of this pest. But what are these creatures, where did they come from, and why are they such a threat? To understand that, we will start with a quick rundown of NWS’ biology, then look at the history of the battle against NWS, and then finally wrap up with how you can mitigate New World Screwworm risks on your ranch, combating this pest wherever it’s found.
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So, Why is New World Screwworm Such a Problem?
Biology
To understand just how dangerous this pest can be, we need to take a moment to examine its biology. This insect’s life cycle is what makes it such a danger to livestock, wildlife, and in some cases, even humans.
New World Screwworms are in fact, not worms, but the juvenile stage of a type of bot fly known scientifically as Cochliomyia hominivorax, often shortened to C. Hominivorax. While the adult flies tend to feed on flowers and decomposing carcasses, they are also known to ingest some wound fluids as well (Myiasis Caused by Old and New World Screwworms).
However, it’s not the adult flies that do the damage. The females look for open wounds or softer locations (such as around the eyes or nasal cavities) to lay their eggs. The eggs hatch rapidly in about 6-7 hours, or 12-24 hours according to other researchers, and then the horror begins.
The larvae rapidly burrow into the host, consuming the host animal’s flesh as they go. After around seven days of constant feeding, the larvae will exit the host and pupate in the ground. Depending on environmental conditions, the new adult fly will re-emerge from the soil between seven and fifty-four days later.
How Damaging is New World Screwworm?
The New World Screwworm maggots can cause extensive damage to livestock, and in rare cases, humans. As the first maggots in the infestation continue to grow and tear into the wound, new maggots hatch and begin feeding as well. This can cause the wound to continue to grow as more and more larvae begin feeding. Due to the constant damage, the wound also cannot heal properly, causing necrosis of the flesh and becoming an open pathway for serious infections. If left untreated, infested animals can die within weeks, often from the severe infection that follows.
Even if the animal is saved with larvicides and medical care, the damage is still extensive. Hides can be horribly scarred, and the animal may take a good while to recover before returning to previous production numbers. Damage to humans is also possible, as while rare and typically non-lethal (with medical attention), screwworm infestations are said to be extremely painful for the victim.
New World Screwworm in The United States
While NWS is currently registered as eradicated in the United States, it wasn’t always this way. The US has had several encounters with this creature, with efforts to combat it going back to the 1920s. To gain a better understanding of current control and eradication methods, we have included a brief overview of the history of the New World Screwworm eradication operation below. For a more in-depth history, at least up to the early 2000s, make sure to stop by the USDA’s National Agricultural Library. This web exhibit has a wealth of information on the battle against New World Screwworm, including images of NWS damage, dated public service announcement posters, and several old NWS training and PSA videos.
In the early 1920s, knowledge on screwworms was slim. It was well understood that some sort of fly was attacking livestock, but the exact species of fly and its lifecycle were not entirely understood. A similar species of blowfly, which used the carcass of a dead animal for its young, was often confused with the screwworm, leading to control techniques that were not always as effective. As you can see with this old USDA training video from 1920, some techniques were aimed at the already dead carcass of an animal, which worked more effectively for blowflies than screwworms. ( though it would remove any screwworm larvae still in the animal after it died.)
In the early 1930s, Dr. Cushing, in collaboration with Dr. Walter S. Patton, published their discovery that the New World Screwworm was a separate species from the blowfly. This discovery that one fly needed living tissue while the other focused on dead tissue helped scientists specifically study solutions for the more deadly screwworm instead. Shortly after, Raymond C. Bushland introduced an artificial blend that could be used to rear screwworms. This allowed for faster and more intensive research, as live animals were no longer needed for experimental fly rearing, which could be brutal to the animals.
Edward F. Knipling theorized near the end of the 1930s that a real solution to the problem would be the eradication of the fly, and to do it, they could use sterile male flies. Unfortunately, a sufficient way to easily sterilize the flies while keeping them fully able to compete with natural male flies was not available yet, crippling his theories’ actual effectiveness at the time.
As World War 2 hit, screwworm took a back seat to investigations into protecting US soldiers from foreign insect pests while overseas. However, NWS was not forgotten. It was in the 1950s that a new option appeared, one that could make Knipling’s theory a reality. Knipling happened onto an article by Dr. Hermann J. Muller, describing how he used radiation to sterilize fruit flies. Seeing the opportunity, Knipling jumped at the chance.
Utilizing nearby X-ray equipment from an army hospital, Knipling discovered he could sterilize the flies without any other serious damage. Even so, these newly sterilized males had to be capable of competing with natural males for the females. With his new production method available, Knipling began a series of field tests that quickly proved his sterile insect technique worked, eradicating natural populations of the flies. By early 1959, after several field tests and a full eradication attempt in Florida, the team found success, eliminating all natural screwworms in the Southeast. Reinfestations still occurred, but were quickly stamped out with the sterile insect technique.
As the 1960s began, the scope of the screwworm eradication campaign started to expand. With cooperation from Mexico, the eradication team launched the implementation of the SIT technique in the Southwestern United States and Northern Mexico, producing sterile flies to release over the combined area. However, to keep the flies from being reintroduced, rancher participation was required to identify new infestations and report them back to the eradication team. Extra scrutiny was also applied to animals being imported to make sure no screwworm passengers managed to migrate through the sterile fly blockade. By 1966, the United States was declared free of indigenous screwworms, though some cases were still found from migrating screwworms.
In 1972, due to a combination of warmer, wetter weather and heavy migration from the screwworm population further south, an extreme outbreak occurred in the Southwest United States. Texas alone, after having just reported only 444 cases the previous year, confirmed 90,000 cases of NWS infestation. In response, the USDA heavily intensified its public information efforts. However, the SIT technique was being called into question. With independent studies from entomologists W.G. Eden and Charles Lincoln, the government was assured the SIT technique would work, and the efforts were redoubled, with a new goal. to push New World Screwworm all the way back to the Isthmus of Tehuantepec, the narrowest point in Mexico. By 1977, Texas was down to just 39 confirmed cases.
With Mexico and the US working intensively to eradicate the pest, the United States reported its last case in 1982, though there were several incidents often attributed to infested imported livestock. A few years later, the original goal of the eradication commission was achieved, with the pest being pushed past the isthmus. As they moved forward, however, it was theorized that a location further south would make a better barrier than the isthmus, the Darien Gap in Panama. By 1991, the plans for this push were in place. By early 2000, only Costa Rica and Panama still had work to do to be completely freed of the New World Screwworm threat, and Costa Rica’s eradication was declared successful later that year. Finally, in 2006, Panama was also declared Screwworm-free.
In 2023, however, the determined New World Screwworm made a comeback. Managing to breach the Darien Gap, the pest began its rapid ascent further north. Now, in 2025, Mexico is reporting over 5000 cases this August. With such a rapid expansion, if nothing is done to re-contain the flies, the US could be contending with Screwworms affecting their livestock once again in the next few years.
So, What is the Current New World Screwworm Situation?
According to the COPEG commission, as of the writing of this article, and since the July 2023 outbreak, there have been just under 50,000 outbreaks past the Darien Gap barrier. Looking at the map on the right from the USDA (click the link for the USDA’s map which should hopefully stay up to date), the fly has managed to make its way all the way from Panama to Southern Mexico, along with a few sporadic territories in central Mexico in about two years. Now NWS has not made it to the US yet, but if nothing is done, the steady march northward will continue.
In response, the USDA announced the construction of a new sterile NWS fly production center in Texas near the US – Mexico border to increase production of sterile New World Screwworm males to help bolster the facility in Panama’s output. In addition, the USDA is investing 100 million dollars in NWS research to continue to innovate new ways to combat the threat. Finally, they are also investing in a team of “tick-riders” and detector dogs to deploy along the border to search for infested migrating wild animals, or potentially infested livestock coming across the border.
How Can We Combat New World Screwworm?
The USDA has begun working on preparing to combat the New World Screwworm threat. What can you do on your local ranch? To start, for anyone near the border or a location where livestock might be imported, inspecting your livestock for NWS infestations is important. This serves as both an early warning sign and a good practice to protect your livestock. If you happen to discover an infested wound on your livestock, pets, or a wild animal, contact an accredited vet to inspect the animal and provide care while extracting samples to be tested in the lab.
Additionally, should an infestation be detected in your area, the USDA has a few additional steps, such as postponing procedures that can create wounds on livestock and pets, and inspecting your vehicle for NWS flies before leaving the area to prevent the flies from hitching a ride to a new location. For more information, we have created this downloadable reference sheet, but make sure to keep an eye on the USDA’s page for the most up-to-date information on the screwworm threat.
While NWS is not a danger to US ranches yet, if its rapid advance across the Americas is not halted, it soon will become a common problem that ranchers and farmers will need to plan for in their risk management strategies once again. Rangeland insurance, while not able to help with NWS, can at least help put your mind at ease when it comes to other risks on the ranch. Stop by our Rangeland Insurance page to learn more about how our agents can help you build a risk management strategy tailored to your operation. Or, if you are ready to connect with one of our agents for a free quote, fill out our contact form to the right!