U.S. Fish and Wildlife Service

Can the Monarch Highway Help Save a Butterfly Under Siege?

The population of North American monarch butterflies has plummeted from 1 billion to 33 million in just two decades. Now, a project is underway to revive the monarch by making an interstate highway the backbone of efforts to restore its dwindling habitat.

Interstate 35 lies at the heart of a vast circulatory system, one of the massive transportation arteries that enable Americans to move long distances quickly. The highway also cuts through the heart of the eastern monarch’s central flyway, which produces the vast majority of brilliant orange and black butterflies that undertake one of the world’s most grueling insect migrations.

En route from as far away as southern Canada to their wintering grounds in steep, fir-clad slopes northwest of Mexico City, monarchs must fly through numerous metropolitan areas strung along the 1,568-mile river of asphalt, including Minneapolis-St. Paul, Kansas City, and Dallas-Fort Worth. Once a vast expanse of prairie, today the I-35 corridor not only bisects cities and suburbs but also passes through the Corn Belt, an ever-expanding patchwork of corn and soybean monocultures laced with the herbicide glyphosate. According to Chip Taylor, director of Monarch Watch and a biologist at the University of Kansas, the resulting loss of monarch habitat has been “tremendous.”

Interstate 35, which stretches from Minnesota to Mexico, lies in the heart of the monarchs' migration route.

Interstate 35, which stretches from Minnesota to Mexico, lies in the heart of the monarchs' migration route.  

During the winter of 2013-2014, only about 33 million monarchs made it to their Mexican mountain sanctuaries, a staggering drop from the estimated one billion recorded in 1996. A study published last year concluded that if current trends continue, there is a “substantial probability” that the number of wintering butterflies will fall so low that a single storm could virtually wipe them out, dealing the migration a fatal blow.

To stanch the losses and safeguard the migration’s future, in 2015 and 2016 a pollinator task force formed at President Obama’s request released reports that detail a major new strategy to rebuild the butterfly’s wintering population, mostly through aggressive habitat restoration in natural areas, along roadways, on utility right-of-ways and farms, and in parks, gardens, and schoolyards throughout a broad swath of land that runs for 100 miles on either side of I-35.

At the center of the plan is the effort to rebrand the interstate as the “Monarch Highway,” creating habitat on the roadway’s verges while employing educational materials such as signs to make motorists aware of the plight facing the monarch and other critical pollinators. In a 2015 white paper, Taylor sketched out a five-year plan for the massive restoration project, including two planting options and projected costs. In one option, two 10x100-foot plots of “plugs,” or small plants, would be installed on each mile of interstate, at a total cost of some $7 million. In the other, one acre per mile of roadway would be restored using seeds, at a total cost estimated anywhere from $2.8 million to $5 million. The plantings would include a variety of species favored by bees, butterflies, and other pollinators, including milkweeds, the only food source for monarch caterpillars. Over time the habitat islands would expand as an annual, late-season mowing dispersed the seeds. 

Planted with non-native grasses and mowed regularly, highway borders have been considered anything but wildlife friendly.

An interstate highway, with its hurtling vehicles and constant din, might seem an unlikely refuge for a declining species. Planted with non-native grasses, mowed regularly, and doused with pesticides for decades, highway borders have been anything but wildlife friendly. But the makeover now being considered by proponents of the Monarch Highway could transform roadside dross into gold for critical pollinators.

The concept is increasingly capturing the imagination not only of highway planners but biologists and conservationists around the globe.  Monarch Joint Venture (MJV) — a partnership of more than 50 federal and state agencies, universities, and non-governmental groups that works to study and protect the species — hopes that the Monarch Highway will “serve as a national model for native pollinator habitat restoration along transportation byways.”  It is also the biggest test of the “habitat highway” concept anywhere in the world to date.

The Monarch Highway, says May Berenbaum, head of the Entomology Department at the University of Illinois Urbana-Champaign, “is both uplifting and daunting — uplifting because if it’s successful for monarchs it will also benefit other pollinators and it can be a model for other species here and elsewhere in the world, and daunting because there’s still so little known about the right-of-way way of life.” One of the biggest unknowns is the number of pollinators, particularly peripatetic monarchs, that will be killed by vehicle collisions.

Until the benefits and potential perils are better understood, some scientists believe proponents of roadside pollinator habitat, especially supporters of the Monarch Highway, should proceed with caution. “The idea of creating habitat is great because pollinators are in trouble,” says University of Georgia biologist Andy Davis. “I just don’t know if the science that supports roadside habitat for them is really there yet.”

In January 2016, the Federal Highway Administration (FHA) published a set of best management practices to guide transportation officials in the creation of roadside pollinator habitat. Two “transportation summits” have been held to bring state transportation department staffers up to speed on the concept. In May of last year, the FHA and the six states along I-35 signed an agreement furthering their commitment to the effort. Last month the Monarch Highway logo was unveiled.

Existing research provides no clear answer on whether habitat highways are likely to be successful.

This fall, Taylor, working with the Oklahoma Department of Transportation, plans to install habitat strips at two sites along I-35 in the state and monitor the health of the plantings and their ability to support monarchs and other pollinators. If all goes well, by fall of next year, he says, “we’ll have proof of concept, and I think we can get a lot of buy-in.” State transportation officials are doing everything they can to facilitate the Monarch Highway, says Taylor, but “there isn’t going to be much federal and state money.” Individuals, corporations, and advocacy groups “will have to drive this thing,” he says.

Existing research provides no clear answer on whether habitat highways are likely to be successful. In a review of the scientific literature published two years ago, researchers noted that most studies of the ecological impact of roads have focused not on insects and other invertebrates but instead on birds and mammals, particularly large ungulates such as deer and moose, and endangered felines like the Florida panther.

In the first study to examine highway habitat for monarchs specifically, published last year in the Journal of Insect Conservation, University of Minnesota researchers found that some 60 percent of the 212 roadside stretches they studied within a 250-mile radius of Minneapolis contained milkweeds. They discovered that the butterflies utilize the plants, but the numbers of eggs and larvae on the milkweeds were lower than those at what lead author Kyle Kasten calls “5-star habitats” with lots of pollen and nectar sources, such as parks and backyard gardens.

A monarch butterfly in a field near downtown Minneapolis. 

A monarch butterfly in a field near downtown Minneapolis.  Kyle Daly/USFWS

“It’s tough to put our finger on why,” says Kasten, although he speculates that one possible reason is the caterpillars are not as healthy as they could be due to vehicle pollution and pesticide drift from farms. The University of Georgia’s Davis believes that the deafening noise along busy roadways could be another factor.

While conservation biologists have sought to keep other animals as far away as possible from speeding cars and trucks, cultivating highway habitat for pollinators has garnered considerable support. Availability is a big part of the appeal. In their study, the University of Minnesota scientists noted that roadsides “comprise over 10 million acres of land in the U.S., and in many states they are the largest holdings of public land.” In highly developed agricultural and urban areas, they added, “roadsides may provide the only semi-natural habitat available for milkweeds.”

Roadside habitat can only rebuild declining species if population increases surpass increases in roadkill.

There have been eight published studies examining collisions between vehicles and insects of any kind. The most emphatic point of agreement among these highway mortality studies, in which researchers walk so-called transects along designated roadsides on a regular basis collecting dead insects, is that further research is needed.

Roadside habitat can only rebuild declining species if population increases surpass increases in roadkill. Much of the optimism about habitat highways derives from a few papers that indicate that restoring native vegetation on roadsides does increase butterfly abundance and, just as important, that the percentage of individuals lost is relatively small. In a 2013 paper, for example, scientists walking 60 transects in southern Poland estimated that 6.8 percent of the butterflies they recorded were dead on the road. However, the Polish researchers also pointed to what they called a “conservation dilemma”: While higher-quality habitat boosts butterfly abundance, it also increases the total number of butterflies killed.

Whether the mortality rate on roadside habitat is higher for long-distance migrants like the monarch than their more sedentary kin is a matter of disagreement. As May Berenbaum has pointed out, despite the monarch’s “disconcerting habit of playing in a few thousand miles of traffic” during its annual autumn migration, roadkill was overlooked in the still-pending 2014 petition to the U.S. Fish & Wildlife Service to list the butterfly as a threatened species, an otherwise lengthy litany of afflictions the butterfly faces. Berenbaum was a co-author of the sole paper to assess the impact of vehicle collisions on monarchs. In that paper, published in 2001, she and her colleagues estimated that the number of monarchs killed along roadways in the entire state of Illinois during one autumn week alone may have exceeded 500,000 individuals. The peak of mortality was observed in mid-September, in the midst of the monarch’s southward migration.

Based on these findings, Andy Davis did some back-of-the-envelope calculations in a blog post to try to determine the total number of monarchs killed by vehicle collisions in the central flyway each year during the fall migration. He came up with roughly 25 million. “That’s a staggering number,” he says, especially considering that in recent years as few as 33 million monarchs have made it to Mexico. 

Some conservationists say restoring habitat on every available landscape in the I-35 corridor is vital for the monarch.

Monarch biologists have been engaged in a heated debate of late over whether the major cause of the eastern monarch’s steep decline is the dramatic loss of milkweeds in the butterfly’s breeding range or an as-yet-unidentified factor killing them during their return trip to Mexico. Davis believes that road mortality is “potentially a huge problem that we are overlooking.”

Scott Black, co-chair of MJV and executive director of the Xerces Society, discounts the role of road mortality in the butterfly’s demise. It may be more of a factor in Mexico, he says, but “there isn’t a lot of evidence that roadkill is one of the major causes of the decline of the monarch” in the U.S.

Given the grim numbers of butterflies arriving in Mexico, proponents of the Monarch Highway believe that the benefits more than justify the costs. Black points out that the state of the monarch migration is so dire that although restoration on the I-35 roadsides “is an important piece of the puzzle, it’s not enough.” Like many monarch advocates, he believes that an “all-hands-on-deck approach” to habitat restoration on every available landscape throughout the I-35 corridor “is vital for the preservation of the future of the monarch migration.”

 The plight of the monarch butterfly is emblematic of an even bigger conservation story — the devastating loss of living space for native plants and animals. So much wildlife habitat has been usurped by humans that the survival of a wondrous animal migration depends at least in part on how well a butterfly that weighs little more than a paper clip can tolerate one of the modern world’s most lethal environments.