WEATHER UPDATE
WEATHER’ S RELATIONSHIP TO THE SPREAD OF NEW WORLD SCREWWORM
By Matt Makens Atmospheric Scientist
I originally wrote about this subject in August 2025, and given recent developments, it seems to be a good time to revisit the weather’ s role in the spread of New World screwworm. On June 3, 2026, USDA confirmed the first NWS case in the continental U. S. in more than 60 years.
The NWS was largely eradicated from the U. S. in the 1960s. Yet, in recent years, it reemerged as a concern for American producers as the populations dispersed northward from endemic zones. Although the Sterile Insect Technique( SIT) remains a primary control mechanism, studies have made it clear that eradication success, outbreak intensity and reemergence potential are closely tied to weather— a relationship that has now been borne out with the confirmed arrival of NWS in Texas and New Mexico.
From seasonal wind patterns to monsoonal moisture, numerous environmental conditions shape the life cycle, spread and population dynamics of this parasite. Understanding these weatherrelated drivers is critical.
Temperature thresholds can dictate screwworm development. Research indicates larval and pupal development do not proceed below approximately 58 ° F and stop above 110 ° F. The optimal temperature for adult survival and reproduction is around 81.5 ° F, a temperature range
Historically favored temperature zones( not accounting for precipitation and animal transport).
common in its primary endemic zones, such as Mexico and Central America.
Winter temperatures remain one of the most effective natural barriers to NWS spread. Not only will cooler weather slow adult activity, but pupae are also killed when soil temperature consistently falls below 46 ° F. Winter survival is unlikely in an average year across much of the U. S., and historically, year-round populations have been confined to southern Florida, southern Texas and parts of New Mexico.
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, which improves conditions for larval survival. Tools like the Palmer Crop Moisture Index( CMI) have shown promise in predicting screwworm risk, with higher-than-normal CMI values( wetter) sometimes preceding population spikes. A tool that will be invaluable during the seasonal monsoon this summer, and then during El Niño’ s wetter correlation with the southern U. S. in fall and winter.
However, rainfall alone is insufficient. Collectively, precipitation timing, duration and temperatures are equally important. When rainfall coincides with favorable temperatures, screwworm activity tends to increase. In contrast, hot and dry conditions tend to suppress survival and reproduction. The former is more likely in the months ahead for the southern and southwestern U. S.
One of the most striking climate connections is the role of seasonal monsoon winds in transporting screwworms over long distances. 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 northern Mexico and the southern U. S.
In the 1960s, these windassisted movements allowed the temporary reestablishment of breeding populations even after local eradication. In some years, migration spanned hundreds of miles, connecting source populations in northern Mexico to reinfestations in Arizona and New Mexico. This atmospheric“ conveyor belt” should be a key consideration in surveillance strategies for livestock-producing regions of the Southwest.
Looking Ahead: The 2026 Monsoon Season and ENSO Outlook
With NWS now confirmed in Texas, the trajectory of the 2026 monsoon season takes on new urgency. The current ENSO outlook adds a layer of complexity: a weak La Niña that prevailed through winter 2025 – 2026 has faded, and El Niño conditions have taken over, potentially strengthening into a strong or very strong El Niño by year’ s end. The relationship between El Niño and the NAM is not straightforward. El Niño tends to disrupt the mechanism that drives traditional monsoon moisture into Arizona and New Mexico, sometimes delaying the onset and shifting moisture corridors eastward. Another potential delay: there is a correlation between drought in the Southwest and the timing of the heaviest monsoonal rain— later in years with dry soil like we have this year.
From a temperature standpoint, the summer and fall of 2026 are forecast to be average to hotter-than-average across the southern tier, consistent with El Niño. This matters for NWS in two ways: while extreme heat above the mid-90s can suppress adult fly activity, warm overnight lows— increasingly common across southern Texas— accelerate larval and pupal development. El Niño may relate to another issue— a reduced chance of winter mortality, with less frequent bouts of cold reaching the southern border.
From a weather standpoint, there are critical factors associated with our upcoming El Niño that could help the screwworm population for quite some time.
See monthly maps for weather and NWS relationships, first freeze data, etc., at www. makensweather. com / nws.
16 JULY 2026 www. NCBA. org