Understanding the New World Screwworm - Biology, Identification, and Prevention

Background

The New World screwworm (NWS; Cochliomyia hominivorax) is a parasitic blow fly that is native to the subtropics (Figure 1). Like many flies, NWS is a pest of livestock, companion animals, and wildlife. Unlike most blow flies whose larvae feed on carrion, these flies lay their eggs on living tissue where their larvae develop. The name "screwworm" stems from the behavior of the fly larvae, or maggots. Larvae burrow into tissue, which can lead to nonhealing wounds, secondary infections, and often death if left untreated.

In the late 1950s and early 1960s, NWS populations were found across Texas and Florida. To help combat population spread, several researchers developed what is known as the sterile insect technique (SIT) (Valdez-Espinoza et al. 2025).

SIT involves producing sterile male NWS and releasing them, allowing them to mate with females; the females then lay eggs that never hatch. This effort was extremely successful, pushing NWS out of the United States by the mid-1960s, then out of Mexico in the 1980s, and finally they were pushed as far as the Darién Gap in Panama in the 1990s (Valdez-Espinoza et al. 2025). However, before this success was achieved, Texas alone lost $329,598,005.00 in livestock (goat, sheep, and cattle) revenue and screwworm mitigation casts by 1976 (USDA APHIS Veterinary Services 2025). As such, the United States remains on high alert, as the risk that the NWS pose to the livestock industry and wildlife is immense.

Quick Reference

Instar—Growth stage between molts in insect larvae. NWS larvae go through several instars while feeding in wounds.

Larva (larvae)—Immature stage of insects, often called maggots; NWS larvae burrow into animal tissue.

Larvicides—Insecticides that target larval stages; used in wound treatment.

Pupa (pupae)—Dormant stage between larva and adult; NWS pupae develop in soil.

Sterile insect technique (SIT)—Pest control method using sterilized male flies to reduce populations.

Lifecycle

Adult male NWS feed primarily on flower nectar and live for 3 to 4 weeks, whereas adult females live for about 1 to 3 weeks and feed on decomposing animals or liquid from animal wounds (Thomas and Mangan 1989; World Organisation for Animal Health 2013). Females that have mated are attracted to wounds as small as a tick bite or a scrape from barbed wire. Females can travel about 6 to 12 miles looking for a place to lay their eggs (Costa-Júnior et al. 2019; World Organisation for Animal Health 2013). On average, females lay 100 to 300 eggs that typically hatch in 6–8 hours, and the larvae feed for 5 to 7 days in living tissue (Costa-Júnior et al. 2019; World Organisation for Animal Health 2013) (Figure 2). The larvae then enter the soil to pupate and remain there for approximately 7 to 10 days before emerging as adults (Costa-Júnior et al. 2019; World Organisation for Animal Health 2013). In this pupal stage, NWS cannot survive in soil temperatures below 46°F (Thomas and Mangan 1989; World Organisation for Animal Health 2013). Adults prefer temperatures between 77 and 86°F with a relative humidity of between 30 and 70% (Thomas and Mangan 1989; World Organisation for Animal Health 2013).

Identification

NWS larvae can be difficult to identify, as those of many species often resemble each other. Proper identification relies on the presence or absence of dual internal breathing tubes, which are easiest to observe in more mature, third instar larvae. In general, NWS larvae are about 17 mm in length, with spines that look almost screw-like (Figure 3). Confirmation of the species should only be performed by a trained professional. Specimens from the deepest part of the wound should be used for identification, as other species of fly larvae feed on the dead tissue surrounding the wound. Adult flies of interest can be photographed and emailed to the Texas A&M AgriLife Extension Service at screwworm@ag.tamu.edu. Samples can also be collected and sent to the National Veterinary Services Laboratories for laboratory identification (for more information, please see the APHIS SOP for Detection of NWS in Animals [PDF]).

Adult NWS have additional characteristics to aid in proper identification. NWS are metallic with three distinct stripes that run from behind the fly's head. There are many other metallic blow flies present in the United States that play a vital role in natural decomposition. However, unlike NWS, these other blow flies only lay eggs on dead tissue and pose no threat to living animals. Species like the secondary screwworm (Cochliomyia macellaria) may look similar; however, NWS stripes begin behind the head with a shorter middle stripe, whereas the stripes of the secondary screwworm are more even (Figure 4).

Range as of July 2025

The NWS is native to the tropical and subtropical regions of the Western Hemisphere. Historically, NWS were found throughout the southern United States, Mexico, Central America, and parts of South America, as well as the Caribbean islands (Figure 5). The Darién Gap between Panama and Colombia serves as a permanent barrier zone with ongoing SIT-based control. As of 2025, NWS remain endemic in parts of South America and have re-emerged in Central America to southern Mexico. Although unlikely to survive in the climates of the northern United States due to temperature constraints, the fly remains a serious threat in warmer regions.

Signs

When NWS larvae hatch, they burrow into the tissue of their host with sharp, hook-shaped mouthparts. This action causes tissue damage, which is compounded as additional eggs hatch and more larvae begin feeding on the same wound (Figure 6). If left untreated, infestations can be deadly. NWS larvae can also feed on mucous membranes, such as those found in the nostrils. Signs livestock producers should look out for:

• Wounds with visible larvae, often foul-smelling, in areas such as:
  • Navels of newborns
  • Ear tags and brands
  • Scratches from fencing
  • Tick bites
• Excessive head shaking
• Lethargy or restlessness
• Excessive licking of wounds

What Happens Next

As of September 21, 2025, the NWS has been reported in Sabinas Hidalgo, located in the state of Nuevo León, and as of July 9, 2025, the USDA Animal and Plant Health Inspection Service (APHIS) has suspended the importation of live cattle, bison, and equines originating from Mexico through U.S. ports of entry along the southern border (USDA 2025a).

To increase NWS surveillance, detection, and eradication efforts, the USDA, in collaboration with APHIS experts and their counterparts in Mexico, is set to begin a phased reopening of the southern ports starting with Douglas, Arizona (USDA 2025b). It is unlikely that NWS will reach Pennsylvania due to an unfavorable climate. With that said, it is important to keep the following factors in mind:

International travel: Any farm visitors who have traveled internationally to regions in which NWS are present should wait at least 5 days after returning before visiting livestock operations (healthyagriculture.org/hfha-blog/international-travel-livestock-diseases/).

Animal purchases: Producers should be extra cautious about sourcing livestock from southern states or any animals with recent travel history to affected areas. Purchased animals should be inspected before travel and monitored before integration.

Show animals: Animals returning from shows/events near areas with NWS could pose biosecurity threats. These animals should be quarantined and monitored before travel and before they integrate back

Due to this fly's wide host range, including wildlife, livestock, pets, and humans, typical insecticide control approaches are challenging. Despite this, historical control methods for NWS are still effective. Preventative measures are a priority, such as treating wounds promptly and monitoring herds consistently. If an infestation occurs, topical larvicides such as permethrin and cleaning/removing maggots from the wound can be methods of control. Livestock producers or owners who suspect an NWS infection should contact their veterinarian to evaluate the animal or herd.

Report Suspicious Cases

For Pennsylvania cases, immediately isolate any suspected animals and contact the following:

• Your local veterinarian  or PA Game Commission 
• Pennsylvania Bureau of Animal Health and Diagnostic Services: 1-717-772-2852
• U.S. Department of Agriculture APHIS Area Veterinarian: 1-717-540-2763
• Penn State Extension: 1-877-345-0691 or Extension website - Contact Us

To prevent unintentional spread, avoid transporting any suspected animals until advised.

Resources

• Livestock and Poultry Disease, Animal and Plant Health Inspection Service, U.S. Department of Agriculture.
• National Agricultural Library, U.S. Department of Agriculture, The New World Screwworm.
• New World Screwworm, Animal and Plant Health Inspection Service, U.S. Department of Agriculture.
• Obligatory Myiasis-Producing Flies of Animals, Merck Veterinary Manual.

Acknowledgments

We would like to thank Dr. David Taylor, Adjunct Professor emeritus at the University of Nebraska–Lincoln, for his expert insights on the NWS and feedback on this article.

Prepared by Paige S. Alexander, Ph.D. student in the Department of Entomology, and Dr. Erika T. Machtinger, associate professor of entomology.

References

Costa-Júnior LM, Chaves DP, Brito DRB, Santos VAFD, Costa-Júnior HN, and Barros ATM. 2019. A review on the occurrence of Cochliomyia hominivorax (Diptera: Calliphoridae) in Brazil. Revista Brasileira de Parasitologia Veterinária 28(4): 548–562. doi.org/10.1590/s1984-29612019059

Thomas DB and Mangan RL. 1989. Oviposition and wound-visiting behavior of the screwworm fly, Cochliomyia hominivorax (Diptera: Calliphoridae). Annals of the Entomological Society of America 82(4): 526–534. doi.org/10.1093/aesa/82.4.526

USDA. 2025a. "Secretary Rollins Suspends Live Animal Imports Through Ports of Entry Along Southern Border, Effective Immediately." Press Release No. 0105.25. 

USDA. 2025b. "USDA Announces the Phased Reopening of Southern Ports for Livestock Trade." Press Release No. 0150.25. 

USDA APHIS Veterinary Services. 2025. "Historical Economic Impact Estimates of New World Screwworm in the United States." United States Department of Agriculture. 

Valdez-Espinoza UM, Fadda LA, Marques R, Osorio-Olvera L, Jiménez-García D, and Lira-Noriega A. 2025. The reemergence of the New World screwworm and its potential distribution in North America. Scientific Reports 15(1): 23819. doi.org/10.1038/s41598-025-04804-9

World Organisation for Animal Health. 2013. "Technical Disease Cards: Screwworm."