Monarch Butterflies How Urban Gardens Are Helping Them Thrive

Monarch butterflies, with their striking orange and black wings, are some of the most recognizable butterflies in North America. But they’re in trouble. Monarch caterpillars can only eat the leaves of milkweed, a native wildflower. As milkweed has disappeared, so have the monarchs, to the point that they’re at risk of extinction. Research shows that planting milkweed in home gardens can add significant monarch habitat to the landscape. In a new study in the journal Frontiers in Ecology and Evolution, urban milkweed plants were monitored to learn what makes these city gardens more hospitable to monarchs.

“In this study, we found that monarchs can find the milkweed, wherever the milkweed is, even if it’s in planters on balconies and rooftops,” explains Karen Klinger, a Geographic Information Systems analyst in the Keller Science Action Center at the Field Museum and the study’s lead author.

Monarch butterflies have one of the most unusual and demanding migration patterns of any insect. The eastern population of monarchs starts the year in Mexico and move up across North America in the spring and summer. “As they travel, they lay their eggs, and when those adults die, the next generation continues the migration northward. They will make it all the way to southern Canada, and at the end of summer, a new super generation is born that migrates all the way south and survives through the winter,” says Klinger.

Since it takes multiple generations of caterpillars to get the monarch population from Mexico to Canada each year, the monarchs rely on milkweed plants throughout their migration path. “There used to be wild milkweed growing along farmland in the Midwest, but now farmers use pesticides that kill the milkweed,” notes Klinger.

 Monarch populations have declined so much in recent years, they’ve been a candidate for endangered species status by the International Union for Conservation of Nature and the U.S. Fish and Wildlife Service. “If we don’t do anything soon, monarchs are going to be in serious trouble,” warns Aster Hasle, a lead conservation ecologist at the Field Museum’s Keller Science Action Center and a co-author of the paper.

Now, scientists are exploring if urban milkweed gardens are able to bridge this gap. Klinger was a co-author of a 2019 study led by Field Museum scientists that showed that even “concrete jungles” have room for milkweed plants in people’s yards, alleyways and rooftops.

“With our 2019 study, we found that a lot of the spaces where milkweed could grow was inaccessible to scientists—there was a lot of milkweed that we couldn’t account for,” says Klinger. “But we also found that there was a lot of enthusiasm among residents to plant milkweed and support monarchs. So based on that, we did a community science project that became the basis of this new paper.”

Klinger and Hasle worked with volunteers around Chicagoland to monitor milkweed plants in their yards and neighborhoods for monarch butterflies laying their eggs on the plants and caterpillars eating the milkweed leaves.

 Klinger and Hasle trained more than 400 community scientist volunteers on how to monitor their milkweed for monarch eggs and caterpillars and report back to researchers. Over the course of four years, the team collected 5,905 observations of monarch activity on the 810 patches of milkweed in Chicagoland. This paper analyzed a portion of this data from 2020 to 2022.

“We encouraged participants who had planters on balconies, who had planters on rooftop decks, and we saw some of the most amazing things,” shares Klinger. “There was one participant who had a planter set on the condominium roof that had five large caterpillars in one photo.”

Based on these observations, the researchers found several overarching trends about what makes for a successful milkweed garden. “There are several native species of milkweed, and we found that common milkweed was very prevalent in people’s gardens and was really key, both in terms of whether monarchs laid their eggs there and how many they laid,” reports Klinger. “Also, kind of surprisingly, that older, more established milkweed plants did a lot better—they were more likely to see eggs than younger plants.” In addition, having a variety of blooming plants was also key for monarchs to lay more eggs on milkweed, as it provided lots of nectar for the adults.

While monarchs are just one species of insect, they’re indicative of the big-picture health of the ecosystems they live in. “Because they cross this big landscape from Mexico to Canada, monarchs are an important indicator of what’s happening across a big area,” points out Hasle.

In July, Illinois Gov. J.B. Pritzker signed into law the Mobilizing Our Neighborhoods to Adopt Resilient Conservation Habitats (MONARCH) Act, which restricts homeowners’ associations from prohibiting native plantings and provides financial and technical assistance for establishing native and pollinator-friendly gardens.

Article provided by The Field Museum, located at 1400 S. DuSable Lake Shore Dr., in Chicago. For more information, visit Keller Science Action Center at the Field Museum at FieldMuseum.org/department/keller-science-action-center.