In the face of rising temperatures and increasingly favorable conditions for New World screwworm (NWS), livestock producers in the southern U.S. are anxiously awaiting winter's arrival. While not typically seen as a positive force in ranching, cold weather may be one of the last lines of natural defense against this destructive parasite's northward march.

"Certainly, winter will be our friend," says Chris Womack, a veterinarian and rancher from San Angelo, Texas. Historically, frost has driven the flies south, and this year, with NWS approaching the border, colder conditions may buy producers time.

NWS is highly temperature-sensitive. Adults don't thrive in environments below 59°F, and larvae struggle when soil temperatures drop below 46°F. Extended exposure to temperatures under 30°F can suppress populations altogether. However, the larvae's ability to burrow up to six inches into soil can sometimes shield them from surface cold, making duration and consistency of freezing conditions critical.

USDA entomologists and climate scientists are closely monitoring the situation. At the KnipliBushland U.S. Livestock Insects Research Laboratory in Texas, teams led by Kim Lohmeyer are using environmental modeling to project screwworm activity. Lohmeyer warns that climate shifts could allow the fly to expand its range significantly-north, east, and west-if winter fails to curb their movement.

Meteorologist Matt Makens supports this view. "This fly really loves 81.5°F," he explains. That's when adult activity and reproduction peak. At cooler temperatures, the life cycle slows from 21 days to up to three months, effectively reducing population growth. Conversely, seasonal monsoons, rainfall, and warm, humid air from the south promote expansion.

Makens has also observed that monsoonal wind patterns can transport adult flies hundreds of miles northward. The North American Monsoon-which delivered heavy rain to Arizona and New Mexico this summer-has played a key role in past outbreaks and remains a critical concern in surveillance and risk planning.

Even with active Sterile Insect Technique (SIT) programs in place, experts agree that weather remains the single most influential factor in limiting screwworm outbreaks. "If we could control the weather, we could control the fly," Makens says.

Loterm data supports this claim. In years with mild winters, the flies have migrated farther north and even overwintered in regions typically seen as safe. In contrast, strong cold fronts help establish a clear northern boundary. According to Derrell Peel, a livestock economist at Oklahoma State University, "One of the secrets to limiting screwworm spread is simply how cold the winter gets."

Texas rancher Wayne Cockrell, policy chair for the Southwestern Cattle Raisers Association, believes NWS's entry into the U.S. is likely. Still, he sees winter as a temporary buffer, potentially pushing serious threats into spring 2026.

Moisture also plays a role. Rain following dry periods, especially during peak temperature ranges, creates ideal breeding environments. Palmer Crop Moisture Index tools have even shown predictive potential, with outbreaks sometimes trailing elevated moisture levels.

Looking ahead, climate-informed management strategies will be essential. Understanding how temperature, rainfall, and seasonal wind flows intersect could determine the success or failure of future eradication efforts.

As Makens summarizes, "Weather patterns act as both a barrier and a bridge for screwworm activity. Knowing when that balance tips is essential."

Until then, livestock producers across the southern U.S. are counting on Old Man Winter to show up early-and strong.