Part of a series of articles titled Picturing the Unseen.
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Monarch Butterfly Survival Depends on More than Milkweed
A recent study reveals how shade and plant type can support these iconic native pollinators.
By Michelle Boone
About this article
This article was first published online on March 25, 2026, as part of the Picturing the Unseen series.
U.S. Fish and Wildlife Service / Tom Koerner
What comes to mind when you think of a monarch butterfly? Do you picture their striking orange and black coloration? Are you enthralled by how they grow, form a chrysalis, and emerge a completely different looking creature? Do you revere their long-distance migration? In North America, there are actually two distinct migrating populations of these important native pollinators, divided by the Rocky Mountains. Both populations were recently proposed for listing as threatened under the Endangered Species Act.
But the western population, which overwinters along the California coast before dispersing north and east, is especially vulnerable. This is because, in the dry West, monarchs may face higher temperatures and longer heatwaves than they’re able to survive. Microhabitats, areas with different (in this case, cooler) conditions from the larger landscape, can thus be important refuges. This has implications for parks and other protected areas seeking to provide monarch-friendly “rest stops” during the butterflies’ long and arduous journeys.
"We wanted to know where and when monarchs were breeding in the West.”
In the September 2025 issue of Journal of Insect Conservation, a team of researchers, myself included, described a study, begun in 2017, to better understand the role of such microhabitats. “We have somewhat systematic counts of monarchs on the coast,” said Cheryl Schultz, the study’s senior researcher. “But,” she added, “very little is systematically known in the breeding range. We wanted to know where and when monarchs were breeding in the West.”
Boone and others. 2025. "Late summer western monarch survival is affected by shade environment and milkweed species." Journal of Insect Conservation 29:72.
Covering the Bases
The study covered five states: California, Nevada, Idaho, Oregon, and Washington. It focused on wildlife refuges and military bases. But the results are relevant to other natural areas, including national parks. The sites gave the team a mix of shaded and open areas to monitor.
“The military’s need to maintain open spaces for training, and disturbance regimes needed for butterfly conservation, are very compatible."
“Many butterflies need early successional habitats,” Schultz said. “The military’s need to maintain open spaces for training, and disturbance regimes needed for butterfly conservation, are very compatible.
The team visited each site about once per month during spring and summer 2017-2019, searching milkweed plants for monarch eggs, caterpillars, and pupae (chrysalises). Team members recorded the milkweed species, number of plants, and whether the milkweed was in the sun or shade. To investigate temperature variation within sites, they placed temperature loggers in open, sunny areas and shaded areas at two study sites: Beale Air Force Base in California and Umatilla National Wildlife Refuge in Oregon.
NPS / Michelle Boone
The data led to some surprising observations. There were more than twice as many monarch eggs and caterpillars, on average, on shaded milkweed plants than on those in open, sunny areas. This was true even when only a little bit of shade was available
Shaded areas were cooler during the day and warmer at night, so they may have buffered the caterpillars from extreme temperatures. But other reasons for these differences, like predation, couldn’t be ruled out.
Testing Different Microhabitats
To test our hypothesis that microhabitats were influencing monarch survival, we devised a field experiment at Umatilla National Wildlife Refuge in 2023. This site has ample shade and thousands of milkweed plants from two species: showy milkweed (scientific name: Asclepias speciosa) and narrow-leaved milkweed (Asclepias fascicularis).
NPS / Michelle Boone
NPS / Michelle Boone
I came into the study as a post-doctoral researcher fresh out of graduate school. My job was to set up the field study, collect data, and supervise the field crew.
We wanted to keep things simple for our study.
We set up study plots in four treatment areas representing different microhabitats that monarchs might encounter:
- Showy milkweed in full sun
- Showy milkweed in full shade
- Narrow-leaved milkweed in full sun
- Narrow-leaved milkweed in full shade
NPS / Kelsey King
Monarchs in the wild may encounter varying degrees of partial shade and many predators, but we wanted to keep things simple for our study. So we built enclosures out of wire fencing wrapped with mesh fabric to confine the monarch caterpillars to each uniformly shaded plot and to keep most predators out.
We first collected caterpillars from the wild then reared them in a greenhouse. When they were adults (butterflies), they mated and laid eggs, which hatched into tiny caterpillars.
We took the caterpillar offspring back to the field and placed them in the enclosures in each of the four treatment areas.
NPS / Michelle Boone
We put an equal number of caterpillars in each enclosure. Over the next 45 days, the crew kept daily records of how many caterpillars survived and how many made it to the adult stage. We removed the adult butterflies and measured them before releasing them back into the wild.
NPS / Michelle Boone
Beyond Host Plants
Our results were varied. Caterpillars on narrow-leaved milkweed in full sun were the most likely to survive to adulthood. They also grew the fastest and became larger adults with longer forewings. On the other end of the spectrum, monarchs on showy milkweed in full sun had the lowest survival, grew the slowest, and produced the smallest adults.
Boone and others. 2025. "Late summer western monarch survival is affected by shade environment and milkweed species." Journal of Insect Conservation 29:72.
Caterpillars in the shade on either milkweed species had moderate development time and survival. But the monarchs on narrow-leaved milkweed were still larger than those on showy milkweed in the same shady area. Our study shows that although the presence and type of milkweed are critical, good habitat goes beyond host plants. The amount of shade or sun is also a factor.
“If you have the ability to enhance or restore milkweed in the shade, it’s great...and if you don’t have shade, you can focus on different types of species.”
“In some cases," said Schultz, "if you have the ability to enhance or restore milkweed in the shade, it’s great...and if you don’t have shade, you can focus on different types of [milkweed] species.” Schultz believes this kind of adaptability and flexibility will help managers meet their monarch conservation goals.
About the author
Michelle Boone, PhD, is an entomologist and pollinator project manager with the National Park Service's Inventory and Monitoring Division and Colorado State University. Image copyright © Michelle Boone. Used by permission.
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Cite this article
Boone, Michelle. 2026. “Monarch Butterfly Survival Depends on More than Milkweed.” National Park Service, March 25, 2026. https://www.nps.gov/articles/000/psv40n1_monarch-butterfly-survival-depends-on-more-than-milkweed.htm
Last updated: March 25, 2026