This is the first in a new monthly series in which A&M-San Antonio faculty share their expertise to help explain the science, trends, and issues behind today’s headlines.
If you've been following the news lately, you've probably heard about the New World screwworm and the concern it's creating for Texas ranchers and the agriculture industry. While the name may sound familiar, many people aren't sure what a screwworm is, why it's such a threat, or how scientists are working to stop it.
To help answer those questions, Dr. Megan Wise de Valdez offers some scientific and historical context about this pest and why it's back in the headlines.
- The New World screwworm is not a worm at all—it's the larval, or maggot, stage of a fly. Adult female screwworm flies lay their eggs in open wounds on warm-blooded animals. After the eggs hatch, the larvae burrow into the wound and feed on living tissue, unlike most maggots, which feed only on dead tissue. After several days, the mature larvae drop to the ground, undergo metamorphosis in the soil, where it grows and develops, and eventually emerge as adult flies, beginning the life cycle again.
- Because the larvae feed on living flesh, screwworm infestations can cause serious disease and significant economic losses. As they feed, wounds become larger and deeper, increasing the risk of severe infection. In livestock, infestations can reduce weight gain, milk production and fertility and, if left untreated, may even be fatal. Wildlife, pets and, in rare cases, humans can also become infested.
- The United States and several Central American countries eliminated the New World screwworm between the 1960s and 1980s using the Sterile Insect Technique (SIT). The method involves releasing millions of sterile male flies to mate with wild females, which typically mate only once. Because those matings produce no offspring, the screwworm population gradually declines until it disappears. While SIT successfully eliminated the pest from the United States and much of Central America, it is a long-term strategy that can take months or even years to substantially reduce fly populations. During an outbreak, officials also rely on surveillance, livestock movement restrictions, wound inspections, rapid treatment of infected animals and prompt reporting to limit the spread.
- Screwworm infestations are treatable when detected early. Treatment involves removing all larvae from the wound, cleaning and treating the damaged tissue and providing continued wound care until the animal has fully healed. Early detection is critical because the larvae continue feeding on living tissue until they reach maturity, causing progressively more damage.
- The current spread of the New World screwworm does not mean the Sterile Insect Technique has failed. Since 2023, the pest has gradually spread north through Central America into southern Mexico, likely through the movement of infested livestock and wildlife. In response, animal health officials have expanded surveillance, increased sterile fly releases, restricted livestock movement from affected areas and begun building additional sterile fly production facilities to help prevent the screwworm from becoming reestablished in the United States. The U.S. Department of Agriculture has also funded research to better understand the screwworm and develop new methods of control that complement the Sterile Insect Technique.
Dr. Megan Wise de Valdez is the associate dean of the College of Arts and Sciences and a biology professor. She has taught about screwworms and the Sterile Insect Technique in both her Applied Entomology Biology of Disease Vectors courses.
To stay up to date on the latest news on New World screwworms, visit Texas A&M AgriLife Extension, part of the Texas A&M University System.
