Return of the New World screwworm

Do you ever have a flashback memory from a college course you took years ago? Well, recently I did from a class I took 40 years ago — entomology at Iowa State University. I remember dissecting a giant grasshopper, but I can't say that I can still identify the 20 insect orders that were required for the course. But one thing I vividly remember was the instructor going on and on about how entomologists had eliminated a devastating livestock pest called the New World screwworm. They accomplished this feat by releasing sterile males in the wild. Since female screwworm flies only mate once they were able to eliminate the population overtime by inundating an area with sterile males. Well, elimination has been successful for 60 years, but last week it was found again in Texas, renewing concerns among livestock producers and animal health officials. To understand their concern, one needs to understand the life cycle of the screwworm. The New World screwworm adult is a parasitic blow fly that appears harmless at first glance, but its larvae pose a serious threat to warm-blooded animal. Adult screwworm flies are about three-eighths of an inch long and are easily recognized by their metallic blue or green bodies, dark stripes across the thorax, and bright orange eyes. Although the adult fly itself does not bite or feed on flesh, it is the larval stage that makes this insect so dangerous and economically damaging. The life cycle of the New World screwworm begins when an adult female fly searches for a warm-blooded host with an open wound or moist body opening. Attracted by the scent of blood and body fluids, the female deposits clusters of approximately 200 to 300 eggs around cuts, scratches, tick bites, surgical sites, or natural openings such as the nose, ears, eyes, or mouth. Within 12 to 24 hours, the eggs hatch into larvae, called maggots. Unlike ordinary maggots that feed on dead or decaying tissue, New World screwworm maggots aggressively attack healthy, living flesh. The maggots burrow headfirst into the tissue using a twisting, screw-like motion that gives the insect its name. As they feed and grow, the wound becomes increasingly larger and deeper, producing severe pain, bleeding, foul-smelling discharge, and extensive tissue destruction. The maggots remain embedded in the wound for approximately five to seven days while passing through several stages of development. During this time, affected animals often become weak, irritated, and restless. Secondary bacterial infections commonly develop within the damaged tissue, and untreated infestations can eventually kill the animal. Once fully developed, the maggots leave the wound and fall to the ground, where they burrow into the soil and enter the pupal stage. During this resting period, the insect transforms into an adult fly. Depending on environmental conditions and temperature, the pupal stage usually lasts at least one week. After emerging from the soil, the adult fly mates and begins the cycle again. Female flies typically mate only once during their lifetime, then search for another suitable host on which to lay eggs. Under warm environmental conditions, the entire life cycle can be completed in roughly three weeks, allowing populations to increase rapidly when left uncontrolled. These populations use to cause havoc in the southern U.S. during the early twentieth century. Ranchers had to constantly inspect herds for wounds, remove larvae by hand, and apply chemical treatments. The pest hurt not only beef production but also sheep, goats, wildlife, pets, and occasionally humans. During the 1930s and 1940s, researchers raised massive numbers of screwworm flies in laboratories, sterilized the males with radiation, and released them into infested regions. Because female screwworm flies mate only once, any mating with a sterile male produced no offspring. Over time, the wild population collapsed. The screwworm was first eliminated from Florida and the southeastern states during the 1950s, then pushed westward through Texas and the Southwest. In 1966, the U.S. officially declared itself free of screwworm populations. After eradication inside the United States, eradication efforts continued through Mexico and later into Central America until a biological barrier zone was established on the Panama/Colombia border. The barrier was maintained by releasing millions of sterilized male flies weekly. The barrier was breached when screwworms were being detected north of the barrier zone in 2022. It has been surmised that the breach was caused by supply chain issues at the Panama sterile fly factory and reduced surveillance during the COVID-19 pandemic The screwworm rapidly moved north through Central America reaching northern Mexico by December 2025. U.S. agriculture agencies had been following its movement during this time and suspended live cattle imports from Mexico. It was only a matter of time that the screwworm returned to the U.S. now having been detected in Texas at four different locations. However, the U.S. Department of Agriculture has not been complacent. They have implemented six steps in preventing further spread of the screwworm and eventually eliminating it again in the U.S. These steps include 1) mass-producing and releasing sterile screwworm flies, 2) strengthening surveillance and border defenses, 3) eradication partnerships with other countries, 4) federal funding to expand sterile fly production facilities, develop innovative management strategies, and increase surveillance efforts; and 5) launched a screwworm information site at https://www.aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/stop-screwworm. If the screwworm continues to spread in the United States, overall cattle production would likely decline further. Available market supplies are already at a 75‑year low, driven in part by restrictions on cattle imports from Mexico and prolonged drought conditions that have reduced herd sizes across the Plains. In response to an outbreak, animal health officials may implement quarantines and restrict livestock movement, which can disrupt normal marketing channels and limit producers' ability to move cattle to feedlots or sale barns. At the same time, increased death loss and reduced animal performance would further shrink the available cattle supply. Together, these pressures could tighten beef supplies, push prices higher, and contribute to increased volatility in the cattle market. To prevent this, different agencies will strive to once again eliminate the New World screwworm in the U.S. Bottom line: don't expect beef prices to drop anytime soon in the grocery store. Ed Lentz is agriculture and natural resources educator with Ohio State University Extension — Hancock County.