The Interstate 35 corridor, which stretches 1,500 miles between Texas and Duluth, serves as an important transportation thoroughfare in the country’s midsection — not just for cars and trucks, but for monarch butterflies, too.

Last month, transportation departments in six states — Minnesota, Iowa, Missouri, Kansas, Oklahoma and Texas — signed an agreement along with the Federal Highway Administration that calls for a collective effort to promote pollinator habitats along I-35, also known as the “Monarch Highway.”

The number of monarch butterflies has been in serious decline for the past two decades, plunging from 1 billion two decades ago to less than 60 million today. The U.S. Geological Survey said in a recent report that the butterfly is facing “quasi extinction.”

MnDOT Commissioner Charles Zelle is a huge fan of the multistate endeavor to restore those numbers.

“It’s really exciting that a multimodal transportation agency can actually be involved in something that the public might see as out of our scope of work,” he said.

“But [MnDOT is] a huge landowner, there are issues of beauty but also of practicality,” he added. “It’s really encouraging other states are involved.”

The idea behind the multistate pact is to increase the number of plants that provide refuge and food not just for butterflies, but also for other important pollinating insects.

In an amazing feat of nature, monarch butterflies born in late summer or early fall migrate to Mexico for the winter. Come spring, they return to the southern United States and lay eggs. Successive generations continue to migrate north along the I-35 corridor, ultimately to Canada, beginning the 2,000-mile trek anew as the season changes.

The monarch population has declined for a number of reasons, including the use of pesticides, natural predators and land development that wipes out vegetation on which butterflies feed.

The Monarch Highway is part of a program backed by President Obama, who formed a Pollinator Health Task Force in 2014. The group, which consists of representatives from government agencies and private entities, is crafting a plan to protect pollinator habitats nationwide, including I-35.

Sen. Amy Klobuchar, D-Minn., voiced her support of the effort by sending a letter to the U.S. Fish and Wildlife Service last year.

MnDOT will use existing resources to promote monarch-friendly habitats, said Tina Markeson, who works in the Office of Environmental Stewardship. An informal working group among various state agencies has formed, and information will be shared with other states as well.

“I don’t see this as a budget commitment, I see it more as an attitude commitment,” Zelle said.

Five years ago, MnDOT planted various butterfly-friendly plants along I-35 just north of Interstate 90 to prevent snow from drifting onto the highway. All told, the plants were spread along nearly three miles of MnDOT right of way, covering about 14 acres.

When the area was reviewed in the past year, MnDOT found that the plants had held up “remarkably well,” Markeson said. While the plants blocked snow, they also helped to shore up the soil and the roadway, preventing erosion.

And the butterflies liked them, too.

Going forward, MnDOT will use a variety of pollinator-friendly plants during new construction and maintenance projects. Because monarch butterflies lay their eggs on milkweed plants, that plant will figure prominently in the effort.

But, as Markeson noted, “an important thing to remember that while milkweed is necessary for the monarch larval stages, the butterfly stage needs a variety of nectar sources.”

So other plants will likely include golden alexander, wild bergamot, asters, blazing stars, prairie clover, vervain and coneflower. Grasses will include big and little bluestem, slender wheatgrass, Indian grass, prairie cordgrass and switch grass, she said.