Can roadside habitat lead monarchs on a route to recovery?

Post written by Kyle Kasten, University of Minnesota Monarch Lab Researcher.

With monarch populations declining in large part due to habitat loss, conservationists have been searching for creative ways to create monarch habitat across all landscapes. In addition to restored prairies, pollinator gardens, and urban park habitat, roadsides provide potential for habitat restoration as well.

Roadsides comprise over 10 million acres of land in the U.S. In fact, in many states they are the largest holdings of public land. Additionally, most roadsides are part of a state-run management scheme, which means there is already a system in place to enact conservation-minded practices.

Recognizing this conservation potential, the University of Minnesota Monarch Lab sought to study the value of roadside habitat for monarchs. In the summer of 2015, Kyle Kasten, a researcher in the Monarch Lab, surveyed 212 roadside sites within a 250-mile radius of Minneapolis, Minnesota. This radius included parts of Minnesota, Iowa, Wisconsin, North Dakota, and South Dakota. Kasten collected density data on both milkweed plants and monarch eggs and larvae.

Over the course of the summer, Kasten identified seven different species of milkweed growing in roadsides. Common milkweed accounted for 97% of the milkweed found, but swamp milkweed, whorled milkweed, and butterfly weed were also present. At least one milkweed stem was present on 59% of the roadsides studied. Monarch eggs and caterpillars were also relatively common on roadsides. However, roadside egg and caterpillar densities were lower than garden and park sites surveyed as part of the Monarch Larva Monitoring Project.  

While it is possible that milkweed was planted intentionally in some of these roadsides, it is more likely these habitats occurred naturally. Milkweed’s ability to adapt and colonize roadsides suggests that best management practices, such as monarch-friendly mowing regimes, could have broad impacts. For information on how and when to mow, please reference the MJV’s Mowing Handout.

How much milkweed makes good roadside habitat?  In this study, the number of monarchs in a given area increased as the number of milkweed plants increased, but only up to a point.  This suggests a saturation effect, or a point at which adding more milkweed to a habitat will not necessarily increase the number of monarchs produced.  When looking solely at the effect of milkweed density on monarch density, the tipping point found in this study was 0.6 milkweed plants per meter squared, after which more milkweed did not result in more monarchs produced. This finding makes sense for monarch egg-laying behavior; females tend to spread out their eggs, typically laying a single egg per milkweed plant. This helps reduce risk of predation, competition, and disease. Thus, it is important to create and maintain well-balanced habitat that has milkweed as well as abundant nectar resources to maximize effectiveness.

For a more in depth explanation of this study, read the journal article published in November 2016 in the Journal of Insect Conservation. The Monarch Lab plans to continue roadside habitat research in the summer of 2017. Please direct questions or comments on this study to Kyle Kasten.

Map of sites sampled in 2015 roadside monarch butterfly habitat study.

Map of the area sampled during the 2015 roadside monitoring project (Kasten et. al, 2016).

The Monarch Joint Venture is a national partnership of federal and state agencies, non-governmental organizations, and academic programs working together to conserve the monarch butterfly migration. The content in this article does not necessarily reflect the positions of all Monarch Joint Venture partners. Header photo shows researcher Kyle Kasten sampling a roadside in Minnesota. Photo by the University of Minnesota Monarch Lab.

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