Burrowing Trouble: Screwworms Edge Closer to Arizona

There have been many headlines around the country for months now discussing the issue of screwworm and the livestock industry. So, what exactly is happening when it comes to this pest? New World Screwworm (NWS, Cochliomyia hominivorax) is a small pest that if allowed to reach the United States can have a large impact, especially in the livestock and agricultural industries.

What is Screwworm?
New World Screwworm is a type of blowfly. It is named for its larval stage as the maggot burrow, or screw, into flesh as it feeds. This is what makes this particular fly and its larval stage unique and most serious. Most fly larva eat dead or decaying flesh, however NWS larva feed on living flesh. Infestation occurs when an adult fly lands on an animal with a cut or open wound and lays eggs. The eggs then hatch into larva that then continue to burrow into the wound eating as they go until they fall from the wound, transition into pupae, and eventually adult flies. This cycle can be as quick as 21 days. Meaning more eggs and more larvae.

Current History
Starting in the 1930’s, NWS became a huge issue in the livestock industry, specifically in the Southwest. In the 1950’s and 60’s, producers saw losses of $50-100 million annually due to NWS. Sterile male flies were found to combat the growing problem. Females only mate once, so when sterile males are released and mate with wild females, the result is a decrease in population. In 1966, NWS was eradicated from the United States using this method. From that point, eradication was targeted to push NWS south through Mexico and to be held at Panama’s southern border. Unfortunately, NWS has started to make its way back north. As of Sept. 21, 2025, the latest case was confirmed 70 miles from the US-Mexico border in Sabinas Hidalgo.

Why is it important?
Back to the question of why this is a big deal in headlines, the answer revolves around economics, as most things do. In Arizona, and other southwestern states, cattle are produced in wide-open and rugged landscapes. It takes a lot of time and labor to continuously monitor animals for infestations. If an infestation was to occur, those operations or even those infested states could be placed under quarantine and therefore, commerce would be reduced. Currently, the southern border is closed to all live cattle, horses, and bison to help prevent larvae entering on these animals. Many feedlots source cattle from Mexico, and without these imports some yards are sitting empty, meaning less beef supplied. For our cattle producers, it’s going to cost more to get steak on your plate, which in turn results in greater prices at the meat counter for you as a consumer.

Outside of the cattle and livestock industry, NWS can infest any warm-blooded animal. This means dogs, goats, chickens, horses, and humans can become hosts. Infestation can lead to larger wounds and even death of the animal. It is important to pay attention to your animals if they have open wounds, especially if NWS makes it across the border. At that point, animals that are showing signs of irritation, smell of decay, and have any maggot presence in wounds, cattle or not, need to be examined by a veterinarian.  For references and more information, please visit https://www.aphis.usda.gov/livestock-poultry-disease/cattle/ticks/screwworm

Yavapai County Livestock Program
The livestock program in Yavapai County provides education and assistance to livestock owners both big and small. Throughout the year, there are various opportunities to gain knowledge through workshops. In addition, please feel free to reach out to have one-on-one visits about your livestock and address any questions you may have. A Range 101 workshop will be held November 20th, 2025, at the Deep Well Ranch (8400 N. State Route 89 Prescott, AZ 86301). This workshop is aimed at new producers & agency personnel, and others interested in a primer on rangeland management. It will focus on core topics in rangeland management, forage communities, monitoring of these systems, and grazing. Lunch is sponsored by the Chino Winds and Triangle NRCDs. To register, visit https://forms.gle/8cZj9sEUWnEQvYbv8.

Katie H. Spanyers is an Assistant Agent with Yavapai County Cooperative Extension focused on livestock production in Yavapai County. For more information about Cooperative Extension and upcoming programming please visit the website https://extension.arizona.edu/local-offices/yavapai-county , sign up for our newsletter, or contact us directly at:
Prescott Office
840 Rodeo Dr Building C
Prescott, AZ 86305
928-445-6590
Camp Verde Office
2830 N Commonwealth Dr Suite 103
Camp Verde, AZ 86322
928-554-8999
Office hours at both locations are M-Th, 9-12, 1-4