Each fall, millions of monarch butterflies cross Texas on their 3,000-mile journey to the mountains of central Mexico. Because Texas sits at the center of the monarch migratory route, the state plays a vital role in the species’ survival both during the northbound spring migration and the southbound fall migration. The timing and success of this remarkable natural event depend heavily on habitat, wind patterns, and roadside vegetation.
At Texas A&M University, researchers Robert Coulson, Ph.D., and postdoctoral research associate James Tracy, Ph.D., are studying the ways geography, weather, and roadside habitats shape these migration paths. Their goal is to keep these routes viable and support the recovery of monarch populations, which have declined by more than 80% since 1990, according to the U.S. Fish and Wildlife Service. Their work, conducted in partnership with the Texas A&M Transportation Institute and the Texas Department of Transportation (TxDOT), uses science-based mapping and modeling to reduce risks to monarchs and improve pollinator health across the state.
Mapping Migration Risks
Their research began in 2016 with funding from the Texas Comptroller’s Office. Coulson and Tracy documented monarch activity along major Texas highways, revealing that vehicle collisions were a major contributor to monarch mortality. Between 2018 and 2021, supported by TxDOT, the team drove thousands of miles each fall, stopping regularly to collect butterfly remains, take GPS readings, and compile data on where deaths occurred.
By combining field observations with topographic information, vegetation mapping, and wind data, they created detailed models showing how landforms and weather patterns influenced monarch flight. The findings revealed that monarch deaths were heavily concentrated in specific areas where terrain and wind funneled butterflies toward roadways. Two major flyways emerged: a central Texas corridor from Wichita Falls to Del Rio, and a second path tracking the Gulf Coast. In some locations, nectar-rich wildflowers and roosting trees near highways pulled butterflies closer to traffic, while structures like bridges and causeways narrowed their flight paths.
Early modeling suggested that monarch-vehicle collisions in Texas may account for a significant portion of overwintering losses, highlighting the need for refined migration data and long-term mitigation strategies.
Designing Safer Crossings
To address these concerns, TxDOT partnered with Texas A&M University to explore practical methods for reducing butterfly collisions. The resulting collaboration became one of the nation’s first efforts to link ecological research directly with transportation management.
Engineers at the Texas A&M Transportation Institute, including Darlene Goehl ’88 and Carl Bierman, developed and tested mesh “flight diverters” designed to encourage monarchs to fly higher over busy roadways. The prototypes were tested at the Texas A&M-RELLIS campus to ensure they met highway safety standards and could withstand strong winds. Coulson and Tracy evaluated how effective these diverters were at redirecting monarchs above traffic height.
Two field test sites were selected: one along Interstate 10 near Ozona in West Texas and another on the Lavaca Bay Causeway along the coast. At the West Texas location, early results showed the diverters reduced monarch deaths by roughly 50% during the fall migration. Results on the coastal causeway were more variable due to shifting wind conditions, offering insights into further design refinements. Researchers are now considering permanent, year-round mesh barriers in high-risk locations.
Understanding Habitat Interactions
The study also examines how roadside vegetation shapes monarch behavior. Because milkweed—the only plant monarch caterpillars can eat—and native wildflowers often grow along highway corridors, these areas can unintentionally draw butterflies toward traffic. By mapping where these plants occur alongside migration patterns, the team has advised TxDOT on potential adjustments to mowing schedules and vegetation management that can improve habitat quality while reducing risk.
Monarchs serve as an indicator species for broader pollinator health. Their population trends reflect the overall condition of ecosystems that support bees, flies, beetles, and other pollinators vital to agriculture, rangeland productivity, and native plant reproduction. A strong monarch migration signals a healthy environment, while population declines point to larger ecological challenges.
Applied Research With Statewide Impact
Coulson and Tracy’s work shows how applied research can shape conservation and policy. Their findings are already influencing how TxDOT and other agencies incorporate pollinator safety into roadside design and maintenance. Their surveys have documented not only monarch losses but also thousands of bees and other butterflies affected by traffic. This information helps transportation planners integrate ecological considerations into future projects.
A Migration Worth Protecting
Each fall, Coulson and Tracy return to the field to track the monarchs moving through Texas. Their ongoing data collection and modeling help identify new trends and understand how changing land use, weather patterns, and ecosystem shifts may alter future migrations.
Because monarchs connect ecosystems from Canada to Mexico, Texas continues to serve as the heart of their journey. The researchers aim to strengthen this connection through science-driven partnerships with state agencies and local communities, ensuring that one of North America’s most extraordinary migrations remains protected for generations to come.
