Fall 2020

On Henry Mall

Skye Harnsberger, a graduate research assistant in the lab of entomology professor Claudio Gratton, leads research techs Michelle Chung BSx'21 and Chengkai Guo BS'20 out into a prairie to survey for monarch butterflies, eggs, and caterpillars at Lodi Marsh State Natural Area near Lodi, Wis., in summer 2019. Photos by Michael P. King


It’s a sweltering August day in 2019. The sun gilds the flowering prairies of southern Wisconsin. Entomology graduate student Skye Harnsberger and her research team park their pickup truck on the side of a back road, grab their clipboards, and trek into the waist-high grass.

As they tread a predetermined path, the team stays on the lookout for the distinct profile of milkweed plants. When spotted, the milkweed is examined and tallied. They repeat this simple process all summer long. It’s tedious work, but critical, because the data they collect could help save the monarch butterfly.

A monarch caterpillar clings to a milkweed flower at Lodi Marsh State Natural Area. 

Milkweed is essential for monarchs. It’s the only plant on which they’ll lay their eggs and the only one that their caterpillars will eat. But in the last two decades, urban development and extensive use of weed killers have depleted this habitat. Today, the number of adult monarchs may be insufficient to ensure that the species persists. And the loss of these butterflies could have implications for many other species.

“Monarchs connect many people to nature, and without these connections, people are less likely to be concerned enough to work to save natural systems,” says Karen Oberhauser, director of the UW Arboretum and a professor of entomology who serves as Harnsberger’s thesis advisor. “They also use habitats that are important to many other plants and animals. So while monarchs may not play a large role in ecosystems, protecting them will lead to increased engagement with natural systems and protected habitat that will benefit many other species.”

This is why Harnsberger is studying how and where to plant milkweed to attract the greatest number of monarchs. Her work is couched in an ongoing debate in conservation circles known as “SLOSS.” It stands for “single large or several small.” The unsettled question: Is it more effective to restore one large habitat or several small ones?

And it’s a crucial question in the context of monarch habitat. In the spring and summer, on their epic northward migrations — the longest of any butterfly species — the monarch seeks out milkweed as repositories for their eggs. What type of habitat configuration are they most likely to use during this critical rest stop?

Skye Harnsberger, foreground, works with Chengkai Guo and Michelle Chung, background, during a monarch survey at Lodi Marsh State Natural Area in summer 2019.

“You can imagine a large patch with thin milkweed might have different qualities to a monarch flying overhead than a large patch with really dense stands of milkweed or a small patch with lots of stems,” says Harnsberger. “There’s been some modeling on which [type] would be better for the monarch but no real boots-on-the-ground research yet.”

But Harnsberger and her team have put their boots on the prairie. Their study involves 30 sites, and they are categorized into four landscape configuration types with combinations of either large or small milkweed patches and either large or small amounts of surrounding grassland.

The surveys Harnsberger completed along with her team of four undergraduates during the summer of 2019 consisted of walking a transect, or survey line, at each site. They followed these lines for equal distances at each location and counted the number of monarch butterflies, monarch caterpillars, and milkweed stems they encountered on either side. Other characteristics the team noted include how many other flowering plants were available for adult monarchs to feed on and the different types of milkweed present (Wisconsin has 12 native species).

The data point to a few landscape preferences among monarchs. First, they seem to prefer fields with higher densities of milkweed and flowering plants. The size of the prairie, however, does not play a significant role in attracting monarchs. 

A monarch flies among milkweed and other prairie plants at Lodi Marsh State Natural Area.

The results also show that monarchs tend to like isolated prairie patches. Harnsberger found this most interesting because it means that patches with less surrounding grassland are more likely to entice monarchs. She describes this as an “if you build it, they will come” scenario for creating and maintaining monarch habitat. Whether a patch is surrounded by farmland, forest, or waterway, the monarchs will make their way to the milkweed.

This research will help land managers configure prairies to be more attractive to monarchs. For assistance, they can turn to the many federal agencies, nonprofits, and private organizations that dedicate funds to monarch habitat restoration designed to increase the butterflies’ declining population.

Harnsberger also points out that anyone, not just land managers, can take small steps to help monarchs. Their population is quickly declining, so any action is better than none at all.

“The number one thing the public can do for monarchs is to plant milkweed,” Harnsberger says. “As much as you can, wherever you can. The more stems, the better for monarchs.”

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