In part one of this article, John Maresh – a biologist and environmental specialist for the Texas Department of Transportation – related how a drive along Interstate 10 during Monarch Butterfly migration season got him thinking about ways to protect the winged creatures when traversing state roadways.

[Above photo by TxDOT]

That is not a just an academic concern, either. Not only are Monarchs and other butterflies critical to plant pollination, a federal research project may soon list them as an endangered species.

In December, the U.S. Fish and Wildlife Service is expected to release a status assessment that could list the Monarch as an endangered species. And that designation under the Endangered Species Act or ESA could have serious environmental consequences for all transportation projects that rely on federal funds.

“When any species is protected under the ESA, the protections are fairly strong,” Maresh said. “You can’t kill or harm or harass them, and when you disturb their habitat, you have to consider that, too.”

However, he explained that it is in state DOTs’ interests to take “conservation measures or other actions that would stabilize or reverse population declines and make more stringent ESA protections unnecessary.”

That’s why Maresh’s roadway curiosity could pay dividends. He helped lead TxDOT-sponsored research on how to protect the migrating Monarchs; work that culminated in a two-year trial project, from 2023 to 2024, to see if large roadside nets in strategic locations can encourage the Monarchs to fly above traffic.

The nets, called diverters, look like roadside netting next to golf courses or baseball fields, but shorter. The vertical diverters are anchored by poles about 10 meters apart and strung side-by-side for 100-meter stretches.

TxDOT set up two diverter locations – one in Crockett County along I-10 in West Texas, and the other next to the Lavaca Bay Causeway in Point Comfort, about 120 miles southwest of Houston. TxDOT installed the diverters in early October in 2023 and 2024 and takes them down when the migration ends in mid-November.

Scientists know the Monarch population is declining because they count the butterflies in Central Mexico each year by measuring how many hectares in the forests the butterflies occupy (one hectare equals 2.47 acres). In 2023, the space dropped nearly 60 percent from 2022 to just under one hectare, which translates to about 20-30 million Monarchs.

For comparison, the Monarchs took up more than six hectares in 2019 and more than 18 hectares in 1997. That translates to about 360 million to 540 million insects.

Maresh said research from Texas A&M University shows that anywhere from 1 percent to 5 percent of the Monarch population may end up on a vehicle windshield or front grill.

TxDOT’s method of determining how many butterflies the diverters saved is to “walk and count dead butterflies where the net is and where the net isn’t, and see if there’s a discernible difference,” Maresh said.

The 2023 migration to Central Mexico was very low, and that was reflected in the Texas diverter counts, Maresh said. “There were very few (butterflies) moving, and there really weren’t enough data points,” he noted. “They didn’t have a high enough number of roadkill either where the nets were or where they weren’t to have anything to say about it. The roadkill was very low.”

However, initial reports from the diverter sites indicate the 2024 migration is higher than last year, and more Monarchs have been sighted flying over the diverters, Maresh said.

“It does seem to be working…exactly as designed,” he said. “The butterfly is encountering the netting, raising up, maintaining their height, and flying over the roadway.”

So, will there be more diverters in TxDOT’s future? Maresh said it’s hard to say before all the data points are collected and studied.

“It’s to be determined,” he noted. “Does the data show it’s worth doing? Is it worth implementing on a wider scale? Is it worth doing more research?”

Even if the project is expanded, Texas roadside diverters alone won’t keep the iconic orange-and-black Monarchs from landing on the endangered list, he said. But it could be one piece of a comprehensive plan involving multiple agencies, groups, and individuals around the country.

“Owing to the complex ecology and widespread distribution of the Monarch, it is unlikely any single action could reverse the declines,” he said. “It would require collective action throughout the range and throughout the year.”

On a road trip one October afternoon about eight years ago, John Maresh stopped for lunch in Sonora, TX, and was greeted by massive, shimmering orange and black waves of Monarch butterflies on their annual migration journey.

[Above photo by Randy Jones for U.S. Fish & Wildlife]

Nearly all Monarchs east of the Rocky Mountains in the U.S. and Canada migrate in October to the forested mountains of Central Mexico; a trip that takes the butterflies deep through the heart of Texas. Generally, Monarchs west of I-35 funnel down through West Texas, with those east of I-35 flying south to the Texas Gulf Coast.

Seeing the butterflies’ migration “was really cool,” Maresh said. But when he resumed his journey west after lunch, things turned ugly.

“It was disconcerting to be driving down Interstate 10…and you might miss one Monarch and you would hit another,” he said. “It was kind of horrific to see all these monarchs getting whacked on the highway.”

Maresh, a biologist and environmental specialist for the Texas Department of Transportation, wondered as he peered through his dirty windshield: “Is this predictable? Is this a phenomenon? Does it happen every year?”

Monarchs are important pollinators and part of the wildlife food cycle. However, their population has dropped dramatically over the past several years because of habitat loss and climate change.

[Editor’s note: Many state departments of transportation across the country are engaged in ways to support pollinator habitats, as the video below by the Kentucky Transportation Cabinet exemplifies.]

But Maresh asked himself a more specific question: Are vehicular collisions also leading to the demise of the Monarch?

While preventing butterflies from becoming road kill is not a primary function of state DOTs, the consequences of not helping Monarchs thrive go beyond platitudes about environmental stewardship and could soon become a time-and-money issue for every state DOT in the country.

For, in December, the U.S. Fish and Wildlife Service is expected to release a status assessment that could list the Monarch as an endangered species. That designation under the Endangered Species Act could have serious environmental consequences for all transportation projects that rely on federal funds.

And that’s where TxDOT’s potential solution for helping Monarchs safely migrate across roadways could prove useful.

[Part two of this article will explore a two-year-long pilot project TxDOT is conducting to aid butterfly migration.]