Winter or colder temperatures — it’s not something you typically hear livestock producers anticipating or praying for. But this year, as the threat of New World screwworm continues to inch closer to the U.S.-Mexico border, winter can’t come soon enough.
“Certainly, winter will be our friend,” says Chris Womack, a veterinarian and rancher from San Angelo, Texas. “Historically the flies were pushed south with frost.”
USDA Agricultural Research Service entomologist Kim Lohmeyer agrees we need winter to come fast. Lohmeyer serves as the laboratory director of the Knipling-Bushland U.S. Livestock Insects Research Laboratory in Kerrville, Texas, where she and her colleagues use modeling to know when and where NWS may show up in the U.S. They are partnering with Lee Cohnstaedt and his team at the National Bio and Agro-Defense Facility in Manhattan, Kan., to study the life stages of NWS and its susceptibility to temperature and weather.
“With the climate changes and weather patterns we have now, if this fly gets here, it can go a lot further north, a lot further east and a lot further west,” Lohmeyer says. “It’s something to keep an eye on.“
The Winter Win
So, what exactly does “winter” mean? Lohmeyer says NWS are fairly cold sensitive, so several days of sustained cold temperatures in around 30°F would be enough to suppress NWS.
The NWS’s ability to survive is restricted to locations where low temperatures are regularly above freezing. Ideal adult fly activity occurs at 77°F to 86°F and relative humidity of 30% to 70%.
“This fly really loves 81.5°F,” explains meteorologist Matt Makens. “The research shows that’s when they’re friskiest.”
Although the Sterile Insect Technique (SIT) remains a primary control mechanism, Makens says studies have made it clear that eradication success, outbreak intensity and re-emergence potential are closely tied to weather and climate.
“From seasonal wind patterns to monsoonal moisture, numerous environmental conditions shape the life cycle, spread and population dynamics of this parasite,” Makens explains. “Understanding these weather-related drivers is critical not only for control, but also in considering how long-term temperature trends have altered the geographic boundaries of screwworm viability since the major outbreaks of the 1950s and early 1960s.”
During a recent Cattlemen’s Call podcast Makens said weather is one of the main forces of spreading NWS.
“If we could control the weather, we could control the fly,” he stresses.
Research indicates larval and pupal development cannot proceed below approximately 58°F and halts above 110°F. The optimal temperature for adult survival and reproduction lies around 81.5°F, a range common in tropical and subtropical climates like Mexico and Central America. NWS do not survive in regions with cold winters, though they may spread into these areas during the warmer months.
Impact of temperature on NWS:
- Reduced activity: Adult screwworm fly activity is limited when temperatures are below 59°F.
- Larval vulnerability: Pupae are vulnerable to soil temperatures below 46°F. Sustained maximums hotter than 95°F can also be limiting — extreme heat reduces adult fly activity and increases mortality in larvae exposed to hot surface soils.
- Life cycle: While the entire life cycle can be completed in warm conditions, it takes longer in cooler, more temperate environments.
“One of the secrets to historically, the northern limit of screwworm, was basically a matter of how cold the winter got. That sort of limited how far it got,” says Derrell Peel, Oklahoma State University Extension livestock marketing specialist. “In years when it was warmer, it would go farther. It would overwinter farther north, but in general, that’s going to set the upper limit, or at least it does if it’s not under control in any other way. That will limit its northward, northern movement, for sure.”
Fly Season
David Anderson, Texas A&M professor and Extension specialist, says, historically, the fly season is April to October.
“If we get a really cold winter, that helps us,” Anderson adds. “It buys us some more time. But things are warmer now than it used to be. Fly season may not be that April to October anymore.”
NWS’s typical life cycle lasts about 21 days in warm weather and slightly longer in cooler climates. According to Texas A&M AgriLife Extension Service, NWS fly life cycle is highly sensitive to temperature:
- In warm (more than 80ᴼF daytime highs) and tropical conditions, the full life cycle may be complete in two to three weeks.
- In more temperate conditions, the life cycle may take three to four weeks.
- In cooler conditions, the life cycle may take up to two to three months.
It is important to understand that the flies do not die at these lower activity temperatures, but prolonged exposure to these temperatures can reduce populations or active infestations.
Texas rancher Wayne Cockrell says NWS’s entry into the U.S. is inevitable but suggests winter and colder weather might temporarily delay the spread until next April or May. Cockrell serves as the Southwestern Cattle Raisers Association director and chair of the cattle health and well-being policy committee.
“We should be within 60 days hopefully when cold weather helps in two-thirds of the state,” Cockrell explains.
Rainfall and Monsoon Factors
Makens says while temperature defines where screwworms can survive, rainfall and moisture influence when and how intensely they can thrive. Outbreaks often follow moderate to heavy rainfall by improving conditions for larval survival.
He explains tools like the Palmer Crop Moisture Index (CMI) have shown promise in predicting screwworm risk, with higher-than-normal CMI values sometimes preceding population spikes.
When rainfall coincides with favorable temperatures, screwworm activity tends to increase. In contrast, hot and dry conditions tend to suppress survival and reproduction.
“One of the most striking climate connections is the role of seasonal monsoon winds in transporting screwworms over long distances,” Makens explains. “In multiple outbreak years, adult flies were documented migrating northward into Texas and the desert Southwest via the North American Monsoon (NAM) — a seasonal pattern that delivers moisture to northwestern Mexico and the U.S. Southwest from June through September. During the summer, prevailing winds shift from west to a more humid, southerly flow, creating favorable conditions for fly migration from central and southern Mexico into their northern states and, at times, into the southern U.S. The NAM this year had an early start and gave significant rainfall to parts of Arizona and New Mexico. The NAM typically calms by early fall.”
In the 1960s, these wind-assisted movements allowed the temporary re-establishment of breeding populations even after local eradication. In some years, migration spanned hundreds of miles, connecting source populations in northern Mexico to re-infestations in Arizona and New Mexico.
“This atmospheric ‘conveyor belt’ remains a critical concern in surveillance strategies for livestock-producing regions of the Southwest,” Makens says. “The 2025 monsoon was undoubtedly a factor in the most recent northward migration of NWS.”
If NWS should cross the border, it will be key for producers to coordinate management practices with weather patterns expected for their region.
“Ultimately, weather patterns act as both a barrier and bridge for screwworm activity,” Makens summarizes. “Knowing how and when the balance tips is essential to preventing the return of one of the industry’s most damaging parasites.”
Here’s to hoping Old Man Winter decides to arrive quickly and help buy livestock producers and government leaders more time to prepare to battle NWS.
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