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New World Screwworm (NWS) remains a looming threat for U.S. livestock producers as it continues its northward expansion. Positive cases of the pest — which had previously been eradicated in most of Central America — have been reported in Oaxaca and Veracruz, Mexico, around 700 miles from the U.S. border.
If allowed to spread to the U.S., NWS could cost millions in production losses, according to the Animal and Plant Health Inspection Service (APHIS). “The last time this devastating pest invaded America, it took 30 years for our cattle industry to recover,” said Secretary of Agriculture Brooke Rollins in an X post. “This cannot happen again.”
Eradication Protocols
Eradication through the sterile insect technique remains the most effective method of control for NWS, according to APHIS. This method uses radiation to create sterile male flies which are released in areas with established NWS populations.
The sterile male flies mate with females who then lay unfertilized eggs. Because female flies typically mate once in their lifetime, NWS populations progressively reduce once sterile males are introduced.
The Panama-United States Commission for the Eradication and Prevention of Screwworm (COPEG) oversees the mass production of sterile NWS flies in eastern Panama. The USDA reports current production and release of sterile flies is at max capacity, with up to 44 flights a week releasing 100 million sterile flies.
While eradication efforts are costly, the National Cattleman’s Beef Association reports NWS cost the U.S. livestock industry more than $100 million annually during the 20th century.
Identifying New World Screwworm
Adult NWS flies have orange eyes, metallic blue or green bodies, and three dark stripes down their back. They are similar in size to the common housefly. The larvae are tapered on both ends, and appear pale in color with encircling bands of dark spines.
NWS can affect livestock, pets, wildlife, birds, and humans. The larvae feed on live flesh, burrowing into open wounds and mucous membranes. They are visible by the third day of infection.
Livestock suffering from NWS infestations may have reduced appetite; exhibit signs of pain or irritation, including frequent headshaking; or have a foul odor. Rapidly enlarging wounds or the presence of maggots in a wound are also cause for concern.
If you suspect NWS on your farm, contact your veterinarian, state animal health official, and the APHIS office immediately.
USDA
New World Screwworm Timeline
May 29, 2025
USDA announces $21 million investment to renovate an existing fruit fly production facility in Mexico. The renovation will allow for the production of 60-100 million additional sterile NWS flies each week.
May 22, 2025
American Farm Bureau reports NWS has moved beyond the containment threshold.
May 15, 2025
The STOP Screwworms Act is introduced in the U.S. Senate.
May 11, 2025
NWS is found 700 miles from the U.S. Mexico border. USDA restricts the southern land border to live cattle, bison, and horses following the continued, rapid spread of NWS. The suspension is to be evaluated every 30 days.
Feb. 25, 2025
APHIS shifts sterile fly dispersals to Mexico.
Feb. 1, 2025
APHIS and Mexico agree to comprehensive inspection and treatment protocols. Animal imports through the southern border resume.
Dec. 16, 2024
APHIS announces $165 million in emergency funding to protect U.S. livestock and increase efforts to control NWS spread in Mexico and Central America.
Nov. 22, 2024
U.S. receives reports of NWS in Mexico and closes the southern border for live animal trade.
2023
APHIS reports unprecedented levels of NWS in Panama.
Oct. 3, 2016
NWS is detected in the Florida Keys, but is quickly controlled through the sterile insect technique.
2006
The U.S. and Panama establish and maintain a NWS barrier zone, preventing the insects’ northward expansion.
1986
Mexico eradicates NWS using the sterile insect technique.
1966
The U.S. eradicates NWS using the sterile insect technique.
1950s
Radiation is found to be an effective sterilization technique. The sterile insect technique is developed, tested, and deployed.
1930s
The first significant losses due to NWS are documented. The USDA and southeastern states develop the Screwworm Educational Program.
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