Monarch Butterfly Study Provides Recommendations for Habitat Development

Photos courtesy of the author.

The monarch is one of the most recognizable insects found in Illinois, it was designated the State Insect in 1975. Monarch butterflies are amazing creatures that are notable for several reasons. Two especially striking aspects of the species’ biology are the annual long-distance migration between overwintering areas in central Mexico and the breeding areas 3,000km to the north, and the steep decline observed in the numbers of monarchs in Mexico since the late 1990s. The steep decline in numbers has precipitated the species being considered for listing under the federal Endangered Species Act. The International Union for Conservation of Nature (IUCN) classified the monarch butterfly as endangered in 2022.

Efforts to help the monarch in the Midwest have concentrated on conserving, improving or creating suitable habitat for the species. Monarch butterfly habitat in the Midwest consists of open areas, both natural and human-dominated. Milkweeds, and close relatives, are the only thing that monarch caterpillars can eat, thus milkweeds are a major component of habitat quality. Adult butterflies need floral nectar for feeding, which is especially important at the end of summer for the generation of butterflies that do not lay eggs in Illinois, but fly south to overwinter in Mexico.

Monarchs are quite unique when you consider their size, migration, appearance and biology. Additionally, they are uniquely popular, which makes them a good representative for other Midwest pollinators. Establishment of monarch habitat, especially in the form of nectar plants, is helpful to a host of pollinators.

Conservation targets set for Illinois aim to establish 150 million additional milkweed stems by 2038. The Illinois Monarch Action Plan defines four sectors where monarch habitat action can be taken in the state, one of which is natural lands. The Illinois Department of Natural Resources (IDNR) is a key player in natural lands that contribute to monarch conservation because of the acreage owned or managed by IDNR across the state. Additionally, actions taken on IDNR natural lands have the potential to inform or guide the public and other organizations.

A team at the Illinois Natural History Survey, part of the University of Illinois, studied resources available to monarchs and other pollinators on IDNR properties. The objectives of the three-year study (2020-2022) were to determine what types of grasslands and management actions can contribute most to monarch conservation. We hoped our findings would inform decisions made by IDNR and other land managers when managing and restoring habitat for pollinators, and also assess how much different grasslands contribute to pollinator conservation.

We visited 49 grasslands on 29 IDNR properties to identify and count blooming plant species and milkweeds. The properties ranged from within a handful of miles to Kentucky, to within 10 miles of Wisconsin. Additionally, milkweeds were studied in lawn areas to understand how parts of IDNR land that may act more like gardens can contribute to monarch larval growth. More than 25,000 milkweed stems were checked for monarch eggs and larvae. Additionally, the team planted 3,200 milkweeds in grasslands and lawns to experimentally study monarch oviposition. Sites were visited multiple times each growing season to understand how floral resources and monarch habitat use differed between the early, middle and late growing season.

What Did We Find?

Looking at the blooming plants present we found that management, such as burning or woody species control, enhanced grasslands as pollinator habitat. For example, grasslands where prescribed burns had occurred had more, and a greater diversity of, flowers available to pollinators.

One interesting comparison was between the few remnant prairies included in our study and the other grasslands (restorations and old fields) on IDNR property. The blooming plants observed in prairies were more abundant and more diverse than in the other two habitats, except in the late summer. During late summer, restorations had the most flowers. However, the diversity of flowers was quite low, and digging a little bit deeper into the data showed that the advantage for restorations was driven by a single blooming species—Canada goldenrod. While Canada goldenrod does attract numerous pollinators its presence has some negative consequences. The species is aggressive and can quickly crowd out other native plant species. Plus, a diverse diet can be helpful for some pollinators, so having too much of a single flower may be problematic.

By far the milkweed species we observed most were the common milkweed and whorled milkweed. Common milkweed tended to be more abundant in the southern part of the state but was present in large numbers everywhere. Common milkweed was most abundant in old fields, moderately abundant in restorations, and the least abundant in prairie remnants, which tended to be sandy.

Whorled milkweed occurred mostly in the northern and western parts of the state, but caution should be used when evaluating monarch butterfly habitat based on this milkweed. We checked nearly 14,000 whorled milkweeds for eggs and larvae and found a monarch on 0.2 percent of the plants. Comparatively, monarch eggs or larvae were on 4 percent of the 10,000 common milkweeds examined. This suggests that every stem of common milkweed is approximately 20 times more valuable for monarch breeding than a stem of whorled milkweed.

Other interesting observations from our checks of milkweeds across the state included:

• 2022 was a bad year for monarchs in Illinois as about one-quarter as many eggs and larvae were found compared to 2020 and 2021.
• Grasslands with more milkweeds had more monarchs in them, but fewer monarchs per milkweed.
• Northern areas had more monarchs per milkweed, especially north of 40.5°N Latitude, about where Bloomington is located.

We also recorded the number of adult monarchs while studying the grasslands. In the early and middle parts of the growing season adult monarchs were sighted where there were more milkweeds. In the later part of the growing season more adults were in areas with more blooming plants. This finding lines up with what is known about monarch behavior when they are breeding versus preparing for migration. Also, we did see signs that the number of adults in 2022 may have rebounded by the late summer.

What We Learned

The study provides insights into how to enhance habitat for monarchs and other pollinators.

• Not all milkweeds are created equal. For the two species we observed most, whorled milkweed provides very little value to monarchs compared to common milkweed. The Illinois Monarch Action Plan goal of establishing 150 million milkweed stems across the state should not count whorled milkweed in the same way as common milkweed.
• Northern parts of Illinois are more productive when it comes to breeding habitat. All parts of the state are valuable when it comes to providing nectar resources.
• Monarch behavior and habitat use are affected by both milkweeds and nectar sources. One should not be overlooked at the expense of the other, as conservation goals sometimes concentrate only on milkweeds. An added benefit of enhancing nectar resources in grasslands is the positive effect on other pollinators.
• A goal for improving grassland restoration could be to boost the abundance of blooming plants early in the growing season, and to increase the diversity of blooming plants later in the season.
• Restorations appear to be succeeding in providing milkweeds for monarch butterflies.
• Boosting milkweed numbers would be most useful where densities are currently lower. We are likely seeing signs of ‘milkweed saturation’ in grasslands with many milkweeds.

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David Zaya is a plant ecologist at the Illinois Natural History Survey, University of Illinois. Much of his work centers on plant-pollinator interactions, with special focus on monarch butterflies and milkweeds, and traveling to every corner of the state while working for the Critical Trends Assessment Program. Other current scientific interests include rare plant conservation, plant phenology and invasive beetles.