Monarch butterflies, with their striking orange and black wings, are some of the most recognizable insects in North America, but they are in trouble because their caterpillars can eat only the leaves of milkweed, a native wildflower that has increasingly disappeared. The problem is so extensive that the U.S. Fish and Wildlife Service submitted a proposal to list the monarch as a threatened species last December.

“This decision comes after a long process, including science-backed modeling that shows the likelihood of monarchs being extinct by 2080 ranges from 56 to 74 percent for the population east of the Rockies, and greater than 99 percent for the Western monarch population,” says Lead Conservation Ecologist Aster Hasle, of the Keller Science Action Center, at Chicago’s Field Museum. Additional modeling suggests that more than 1.3 billion stems of milkweed need to be added to the landscape to stabilize eastern population numbers. Much of that milkweed could be added in cities, Hasle points out.

In a 2024 study in the journal Frontiers in Ecology and Evolution, urban milkweed plants were monitored to learn what makes city gardens 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 Geographic Information Systems Analyst Karen Klinger of the Keller Science Action Center at the Field Museum, and the study’s lead author. 

Adding milkweed across all landscape types is a key strategy in stabilizing the monarch butterfly population. “In proposing listing the monarch as threatened, the Fish and Wildlife Service provided the flexibility to manage milkweed habitat, as needed, without concerns on impacting monarchs. They did this to encourage more milkweed habitat creation within the monarch’s breeding and migratory range,” says Hasle, a co-author of the study.

Monarch butterflies have one of the most unusual and demanding migratory patterns of any insect. The eastern population begins the year in Mexico and moves 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,” explains Klinger.

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

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. We took what we learned while searching for milkweed in residential areas and developed a community science project that became the basis of our 2024 paper.”

Klinger and Hasle trained more than 400 community scientist volunteers on how to monitor their milkweed plants 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 810 patches of milkweed in Chicagoland. The paper analyzed a portion of this data from 2020 to 2022. “We encouraged participants who had planters on balconies and 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 developed several theories about what makes for a successful milkweed garden. “There are several native species of milkweed, and we found that common milkweed (Asclepias syriaca) 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, older, more established milkweed plants did a lot better; they were more likely to see eggs than younger plants.” Having a variety of blooming plants was also important for monarchs to lay more eggs on milkweed, as it provided more nectar for the adults.

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

For more information, visit FieldMuseum.org/department/keller-science-action-center.

Top Five Ways To Support Monarchs

1. Plant native milkweed species, including common milkweed (Asclepias syriaca).
   
   
2. Plant a variety of nectar plants, ensuring blooming flowers from spring to fall.
   
   
3. Continue managing and adding to the milkweed garden year after year.
   
   
4. Share knowledge and support of monarchs and milkweed with friends and family.
   
   
5. Comment on the proposed rule to list the monarch butterfly as a threatened species at Regulations.gov/document/FWS-R3-ES-2024-0137-0001.

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