Research

The monarch butterfly is an iconic species because of its remarkable natural history. The eastern North American population of monarchs undergoes an amazing migratory journey over multiple generations. Adults overwinter at a few isolated mountains in one area in Mexico where they gather in the millions, completely covering the trees. As spring approaches, the butterflies fly north to Texas and the Gulf Coast states, breed, lay their eggs, and die. The caterpillars that hatch from those eggs eventually develop into new adult butterflies, which fly farther north where they breed, lay their eggs, and then die. The next generation then matures and flies even farther north where the process repeats — and as many as six generations can breed throughout one summer. The butterflies track the spring emergence and growth of milkweed plants, which is the only plant monarch caterpillars will eat. Milkweeds are perennials, so their roots remain alive year-round, but they grow new stems, leaves, and flowers each summer.

Monarchs have evolved an intimate relationship with milkweeds in a way that circumvents a defense mechanism of the plants against herbivores. Milkweeds produce toxic steroids called cardenolides in their sap that, when ingested, bind with cardiac muscle and stop insect hearts from beating, killing them. Monarchs, however, evolved the ability to mostly prevent these cardenolides from binding with their cardiac muscle, so they can consume the milkweed plants without harm. They accumulate the toxins in their tissue, effectively stealing the chemical defense of their food plant for their protection from predators. Milkweeds get nothing out of this relationship — Monarch butterflies do visit the flowers but they are too big to pollinate them — so milkweed pollination has to be accomplished by beetles and bees.

As summer wanes, the last generation of butterflies begins to fly south. Despite being several generations removed from their ancestors that overwintered in Mexico, and having never made the trip before, these butterflies manage to fly back to those few isolated mountains in Mexico for the winter. They won’t breed until the following spring, when they fly North again to Texas and the process starts all over again.

Since 1993, there has been a loss of many hundreds of millions of monarchs. There is no easy explanation for this because there are probably multiple factors involved, including climate change and loss of habitat in overwintering areas because of illegal logging. One additional hypothesized cause for monarch declines is a long-term decline in the abundance of their major food plant, the common milkweed. Flowers are not needed for the nectar — Monarch butterflies can feed on nectar from any number of flowers — but milkweeds are the only plant that the monarch caterpillars can eat and develop on.

There are two main causes for the decline in common milkweed. Like many of our native plants, common milkweed is killed by herbicides like Roundup. The use of Roundup has increased dramatically over the past three decades as new crop cultivars have been developed that are resistant to the herbicide (during the same time frame as the monarch population decline). Native habitat has also been lost as increased acreage is put into agricultural production or being developed commercially for other purposes. Overall, common milkweed has declined by 50-90%, with estimates varying with geographic area, time frame, habitat, and method of study.

In addition to other conservation efforts, like educating users about how to apply herbicides more conservatively and establishing no-mow zones along roadways, conservation agencies like the National Wildlife Foundation are growing, collecting, and distributing milkweed seeds in an attempt to restore common milkweed populations. While this is commendable, one potential problem with this approach is that common milkweed populations may have adapted to local environmental conditions. This phenomenon is termed “local adaptation.” Because of local adaptation, it may be important to use locally sourced seeds in common milkweed restoration efforts. Plants grown from non-local sources of seeds may not be adapted for the environment they’ve been introduced to, thereby hampering milkweed restoration efforts and monarch butterfly conservation.

The phenomenon of local adaptation is seemingly fairly common in plants, but the degree to which local adaptation occurs varies widely. The main goals of this research are to determine if local adaptation occurs in common milkweed, how strong local adaptation is if it occurs, and what particular environmental variables are causing it. This will better inform efforts to restore milkweed populations.

Ecological Society of America, Montreal, Quebec, Canada. August 2022. Common Milkweed, Asclepias syriaca, exhibits latitudinal clines in germination and early growth traits. (co-authors with E.K. Mohl, M. Reid, L. Sherman, M. Wood, A.C. McCall, S.E. Scanga, M.C. Fisher-Reid, H. Marella, D.E. Garneau, K.S. Whitney, K. Cipollini, P.E. Saunders, and K.N. Hopfensperger)

Society for Ecological Restoration: Midwest — Great Lakes Chapter, Willoughby, OH. April 2022. Defenses in herbivory show a latitudinal gradient in Asclepias Syriaca. (co-authors with B. Wagner, M. Newman, M. Bridges, A.C. McCall, E.K. Mohl, and S. Brauner)

Society for Ecological Restoration: Midwest — Great Lakes Chapter, Willoughby, OH. April 2022. Population latitude affects growth and senescence traits in Common Milkweed, Asclepias syriaca. (co-authors with M. Newman, B. Wagner, M. Bridges, A.C. McCall, E.K. Mohl, and S. Brauner)

Society for Ecological Restoration: Midwest — Great Lakes Chapter, Willoughby, OH. April 2022. Collaborative research tentatively supports local seed sourcing for the restoration of common milkweed, Asclepias syriaca, populations. (co-authors with S. Ronneberg, S. Dlamini, M. Reid, S. Rice, L. Sherman, C. Wilkens, M. Wood, A.C. McCall, S.E. Scanga, M.C. Fisher-Reid, H. Marella, D.E. Garneau, K.S. Whitney, K. Cipollini, S. Rasmussen, K.N. Hopfensperger, and E.K. Mohl)

Northeast Natural History Conference, Albany, ME. April 2022. Invited session paper (‘Collaborative Networked Research with Undergraduates through the Ecological Research as Education Network (EREN)’): MAREN: Leveraging EREN to Improve Milkweed Restoration. (co-authors with M.C. Fisher-Reid, H. Marella, J. Whalen, K. Cook, A. Vasapollo, W. Riendau, D. Andrade-Fonseca, K. Syliva, E. Mohl, A.C. McCall, M. Wood, L. Sherman, M. Reid, P.A. Saunders, S.E. Scanga, C. Danielson, D.E. Garneau, K.S. Whitney, K. Cipollini, S. Rasmussen, and K.N. Hopfensperger)

Society for the Advancement of Chicanos/Hispanics and Native Americans in Science (SACNAS), National Diversity in STEM Conference, San Juan, Puerto Rico. October 2022. Is there evidence of local adaptation in common milkweed? (co-authors with A. Muñiz, A. Collins, Z. Ding, J. Felker, N. Olidis, A.C. McCall, E.K. Mohl, and S. Brauner)

Peter O. Dunn, Insiyaa Ahmed, Elise Armstrong, Natasha Barlow, Malcolm A. Barnard, Marc Bélisle, Thomas J. Benson, et al. “Extensive Regional Variation in the Phenology of Insects and Their Response to Temperature across North America.” Ecology, 2023.