As social media fills with maps and predictions about how far New World screwworm might spread this summer, many livestock producers are asking the same question: When will it get to me?
According to Cassandra Olds, Kansas State University Extension entomologist, that’s the wrong place to start. “I think everybody’s kind of bracing for when will it get to me, and I don’t think that we really know,” she says.
She argues producers should focus on how the pest moves and what management steps can slow it down or keep it out of their herds in the first place.
Meteorologist Matt Makens explains weather patterns act as both a barrier and bridge for screwworm activity. For producers, that may be the most important line to keep in mind through the rest of the warm season.
How Temperature and Rainfall Affect Screwworm Survival
Weather does not just influence screwworm activity — it helps determine where the pest can survive, reproduce and spread.
Makens explains temperature thresholds can dictate screwworm development. Pupa development cannot proceed below about 58°F and halts above 110°F, while adult survival and reproduction are most favorable around 81.5°F. That means much of the southern U.S. is warm enough during summer to support at least temporary fly survival and movement. But heat alone does not tell the full story.
“While temperature defines where screwworms can survive, rainfall and moisture influence when and how intensely they can thrive,” Makens adds. In other words, many areas may be warm enough, but not every area will be equally favorable.
For producers trying to judge risk, moisture may be the deciding factor.
“Outbreaks often follow moderate to heavy rainfall by improving conditions for larval survival,” Makens says. “When rainfall coincides with favorable temperatures, screwworm activity tends to increase.”
In contrast, hot and dry conditions tend to suppress survival and reproduction. That is an important distinction for livestock producers who may assume the hottest areas automatically face the greatest danger. Makens suggests the higher-risk zone is really where warmth and wetness overlap — especially where recent rainfall has improved soil moisture and where livestock are exposed through wounds or other openings attractive to egg-laying flies.
Makens stresses, “The fly can’t live without water and dry soil is no good for their larvae.”
That means producers should watch not only temperatures, but also rainfall patterns, wet soils, humidity and any shift toward a more tropical summer setup.
Screwworm Migration Patterns: The Role of the North American Monsoon
Weather can do more than create suitable habitat — it can also physically help move the pest northward.
“One of the primary ways to move the fly, other than animal transport, is the weather,” Makens says, pointing specifically to the North American Monsoon, describing it as “a dominant source of migration.”
That monsoon pattern generally develops from late spring into summer and shifts winds into a more southerly, moisture-rich flow across Mexico and into parts of the Southwest and southern High Plains. He adds wind-assisted movements created an atmospheric “conveyor belt” that, in past outbreak years, helped reconnect infestations in the Southwest with source populations farther south.
For producers, that means concern should not be limited to locations immediately adjacent to current infestations. If winds, moisture and temperatures line up, the pest could move well beyond the border region.
The broader weather pattern matters, Makens summarizes, but so does what is happening in each pasture and pen. Screwworm is most likely to move north not in a uniform wave, but in jumps — following corridors where heat, moisture and wind align.
Why We Can’t “Draw a Line” to Where Screwworm Goes Next
Olds says predicting screwworm migration with precision goes well beyond a simple climate map.
“To actually predict, you need fairly sophisticated models that take into account the number of eggs that a fly can lay, the number of flies in the population, how far a fly can move, animal movements and things like that,” she explains. “Realistically, it’s incredibly difficult to predict how fast it’s going to spread.”
Historic data gives rough guidance on how far the flies themselves can move.
“We have some data, right, like 0.75 to 1.25 miles a day is average dispersal in previous outbreaks,” Olds notes. “But that really depends on conditions and how big your population is.”
Landscape also matters. Olds says NWS flies don’t like to cross bodies of water, so major rivers and other barriers can slow natural spread. But those biological and landscape limits are only half the story.
How Livestock Transport Spreads Screwworm Outbreaks
In addition to natural fly dispersal, livestock movements can pick the parasite up in one region and plant it in another.
“To predict where it’s going to go really depends on whether you’ve got fly movement and animal movement, or a combination of both,” Olds summarizes.
In other words, trucks can leapfrog flies. That’s one of the biggest reasons no one can honestly tell a producer in Kansas, Oklahoma or farther north that “their turn” will come in a certain month or year.
“I don’t think it’s as easy as, yeah, Kansas will get it next year, or Oklahoma will get it next year, or next month,” she stresses. “The only way we know is once it starts behaving — once we can start watching it in Texas — then we can make predictions about how it may spread further out.”
How Producer Behavior Can Speed Or Slow Screwworm Spread
Human decisions might matter as much as weather and biology.
Olds warns failure to report infestations and comply with control measures could dramatically accelerate spread. That puts producers in the position of front‑line risk managers. Reporting and allowing treatment might be inconvenient, but Olds says it’s also the best way to keep the parasite from marching across the map.
Making Sense of Social Media Maps
One viral graphic, pictured above, created by Beverly Thomas of Texas Tested Seeds & Plants attempts to predict the potential NWS range based on climate suitability and historical data.
Thomas admits she is not an expert: “I’m just reporting what’s happening and going by historical references.”
Her post says: “The threat is divided into two distinct biological zones…where the fly can live *permanently*, and how far north it can march during the warm months. The ‘Summer Dispersal Zone’ — NWS can reach Kansas, Missouri, Tennessee and further north via wind/livestock. The ‘Overwintering Zone’ where NWS could become permanently established in South/Central Texas & Gulf Coast areas due to mild winters. Winter is the ultimate limiting factor because screwworm pupae can’t survive hard, prolonged soil freezes. However, the winters of the 2020s are significantly milder than those of the 1950s when eradication began. The new reality is that the permanent, year-round survival zone is no longer confined safely to Southern Mexico. South Texas, the Lower Rio Grande Valley and the Gulf Coast could now be treated as year-round establishment zones. Many entomologists believe that warmer winter trends will allow the permanent NWS overwintering line to push into Central Texas and across the deep Southeast (Louisiana, Mississippi, Alabama, Georgia and Florida).”
Thomas’ map predicts:
- Critical/Active Regions: South Texas, Lower Rio Grande Valley. High humidity and thick brush provide ideal habitat. The parasite can easily establish a year-round lifecycle here if not eradicated quickly.
- High Risk Regions: Central/East Texas, Gulf Coast, Coastal Louisiana, Southern Florida. These areas are at high vulnerability for permanent establishment. Mild modern winters mean soil temperatures rarely drop low enough for long enough periods of time to kill burrowed pupae.
- Moderate Risk Regions: Northern Texas (Dallas/Panhandle), Oklahoma, Arkansas. These areas have a high vulnerability for summer infestation. While winter freezes will reliably clear out populations annually, unchecked spring/summer migrations could trigger devastating seasonal outbreaks.
Olds reviewed the map and says it is directionally reasonable, but producers should understand what it is — and what it’s not.
“If you have a suitable climate and a way for the fly to get there, a population can establish,” she says. “Suitable climates are different than what they were historically because we’ve got changing climate conditions.”
She notes climate change and cattle movement mean today’s “at‑risk” zone may be broader than the historical endemic area.
“A large part of the United States is at risk. We know what it was historically. This may have changed because climate conditions are different now,” she says. “I don’t think this map is super alarmist, it’s probably close enough, as much as we can be at this early stage.”
For producers, that means maps can illustrate potential climate and geographic risk, but they don’t provide a day‑by‑day forecast for when screwworm will show up at the ranch gate.
Practical Risk Management: What Producers Can Do Now
Since no one can reliably answer “when,” Olds recommends reframing the question: How do I reduce the risk of introduction and spread in my herd and region?
She highlights three practical steps to reduce risk:
- Control Movements Out of Infested Areas. “The management of risk is making sure we aren’t having cattle leave infested areas that have not been inspected because that’s the fastest way to get it to seed somewhere else,” she explains.
- Inspect New Arrivals. “Monitor any cattle that you receive to make sure they don’t have it, so you’re curbing also the infestation,” she adds. That means working with veterinarians, checking animals closely — especially any wounds — and asking questions about origin and treatment history before cattle are commingled.
- Track Confirmed Cases Through Official Channels. Rather than relying only on social feeds, Olds points producers to official reporting: “Watch the screwworm.gov website because they are going to report every single case, so then you’ll be able to gauge month by month, it’s moving here.”
She says following those steps allows producers to distinguish between natural fly expansion risk and introduction risk. For livestock producers in the Texas counties surrounding current outbreaks, she says the priorities are heightened vigilance and wound monitoring.
The Takeaway: Manage Risk Now, Don’t Wait for Certainty
The science isn’t yet able to offer producers a neat line on a map or a firm date on the calendar. But Olds’ message is that uncertainty about timing does not mean inaction.
While NWS’s exact migration path will depend on climate, flies, cattle movements and human behavior, producers can act now: Report cases, cooperate with control efforts, tighten biosecurity on cattle movements and stay current on official updates.
Or, as Olds puts it, instead of focusing on when, focus on: “What am I doing today to keep it from getting here at all?”
