06/05/2026
Ag News by Cary Sims
Screwworms: A Problem Our Grandparents Knew Well
Recent reports of New World Screwworm detections have many livestock producers a little concerned and paying close attention.
For today’s producers, screwworms may sound like something from a livestock history book. For their grandparents, they were a routine part of checking cattle.
Most stockmen today have never seen a screwworm infestation firsthand. That wasn't true for previous generations. There was a time when finding screwworms in livestock wounds was simply part of raising livestock.
While the recent detection has generated headlines, the New World Screwworm is anything but a new pest. Spanish explorers, settlers, and physicians were describing flesh-eating worms in wounds centuries before Texas existed.
Accounts from the 1500s and 1600s describe livestock, wildlife, and even people suffering from infestations throughout the Americas. They did not understand what insect was responsible, but they certainly understood the damage it caused.
Imagine trying to manage cattle when a simple scratch, tick bite, branding wound, or castration site could attract flies that laid eggs in living tissue.
For centuries, that was reality.
Most of us are familiar with blowflies around a dead animal. Those flies lay eggs on tissue that is already dead and decomposing.
Screwworms are different. They seek out living animals. The female fly is attracted to fresh wounds and deposits eggs along the wound margin. After hatching, the larvae begin feeding on healthy tissue.
As they feed, the wound enlarges, attracting even more flies and more egg-laying. Left untreated, animals can become severely weakened and may die from the infestation.
This is not simply a nuisance fly. It is a true parasite.
Although people had dealt with screwworms for centuries, scientists continued learning about the insect throughout the 1800s and early 1900s.
In 1858, the fly itself was formally described and identified by science. As research continued, entomologists began to understand something important: screwworm larvae were not feeding on dead tissue - they were actively attacking healthy tissue.
This discovery changed the rules. What had once been considered simply another fly became recognized as one of the most destructive livestock pests in the Western Hemisphere.
By the 1930s, scientists were studying its life cycle, reproduction, and movement in hopes of finding a way to stop it.
The turning point was two scientists with an unusual approach.
These forgotten heroes were Dr. Edward Knipling and Dr. Raymond Bushland. Both worked for the USDA and spent years studying screwworm biology. Their idea sounded almost ridiculous at the time. Instead of trying to kill every fly, what if they flooded the environment with sterile male flies?
Female screwworm flies typically mate only once during their lifetime. If a female mated with a sterile male, she would produce no offspring.
The theory was simple, yet the challenge was convincing anyone it would actually work. Many scientists doubted the idea. After all, releasing millions of flies to control flies sounds backwards.
Yet field trials proved the concept worked. Large numbers of sterile males were released, wild populations declined, and screwworm numbers began collapsing.
What followed became one of the greatest success stories in agricultural history. The sterile insect program eventually pushed screwworms out of the United States and much of North America. 1966 is the year that the United States was officially declared free from the New World Screwworm.
Eradication efforts later expanded through Mexico and much of Central America, creating a barrier that helped keep the pest from reestablishing itself in the United States.
For decades, ranchers enjoyed something their grandparents never had—the ability to raise livestock without routinely battling this devastating parasite.
The recent detection in South Texas serves as a reminder that some agricultural victories require constant vigilance.
Many of the problems we no longer think about today disappeared only because previous generations of researchers, livestock producers, and government agencies refused to accept them as permanent.
Fortunately, the recent detection occurred in South Texas and animal health officials moved quickly. That does not mean East Texas producers need to panic. It does mean livestock owners should pay attention to wounds on cattle, horses, goats, sheep, pets, and wildlife and report anything suspicious.
If you think you have an infection in livestock, call the Texas Animal Health Commission at 903-919-3748. If you see an issue in wildlife, call the Texas Parks & Wildlife Department at 512-389-4505.
For those looking for up to date information on the Screwworm, look at the following sites: Texas A&M AgriLife Extension Service - https://agrilifeextension.tamu.edu/new-world-screwwormOpens in new window. Texas Animal Health Commission - https://www.tahc.texas.gov/emergency/nws.htmlOpens in new window. USDA’s Animal & Plant Health Inspection Service - screwworm.govOpens in new window.
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Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is [email protected] in new window.
Educational programs of the Texas A&M AgriLife Extension Service are open to all people without regard to race, color, s*x, disability, religion, age, national origin, genetic information, or veteran status. The Texas A&M University System, U.S. Department of Agriculture, and the County Commissioners Courts of Texas Cooperating.
