The monarch butterfly’s status as an icon for pollinators and other beneficial insects is uncontested, but its population levels have been declining in recent decades.

By Harit Bal, Ph.D., and Frankie Stubbins, Ph.D.

Editor’s Note: This Entomology Today post is a sponsored article contributed by Bayer CropScience, a Gold Corporate Partner of the Entomological Society of America. The views presented in sponsored posts reflect those of partner organizations and not necessarily those of ESA. Learn more about Bayer and the ESA Corporate Partner program.

The life cycle and journey of the monarch butterfly is nothing short of amazing. Across multiple generations, monarchs make a 6,000-mile journey from Mexico in the spring to the U.S. and Canada in the summer and back to Mexico in the fall.

Every fall, millions of monarch butterflies (Danaus plexippus) travel thousands of miles from the central and eastern United States and Canada to central Mexican mountain tops, where they overwinter in oyamel fir forests. While most monarchs only live for two to six weeks, this migratory generation of monarchs will live for several months before they migrate back north to lay their eggs in early spring on milkweed plants.

The diet of monarch caterpillars consists solely of milkweed, making it essential to their survival. After the caterpillar pupates and becomes an adult monarch, it will then feed on a more diverse assortment of nectar plants. Four to five generations of adult monarchs will be produced on the northward journey that spans from spring until late summer. The final generation produced in late summer will be the one to make the return journey to Mexico during the fall. During this migration, the adults will feed exclusively on nectar. Unfortunately, the monarch butterfly population has declined over the past two decades.

At Bayer CropScience, we interviewed three of our scientists to find out more about monarch butterfly population decline, monarch research at Bayer, and how we can all contribute to monarch conservation efforts.

Tim Fredricks, Ph.D.

Chris Brown, Ph.D.

Peter Asiimwe, Ph.D.

Peter Asiimwe, Ph.D., is the Insect Bioassay Lead in Regulatory Science at Bayer CropScience based in Chesterfield, Missouri. In this role, he leads a team responsible for characterizing the activity of Bayer’s insect protection traits, to support environmental and food-and-feed safety assessments. He joined Bayer as an entomologist responsible for conducting field studies to understand the interaction of Bayer’s traits with beneficial non-target organisms. He obtained his Ph.D. and subsequently completed a post-doctoral fellowship in entomology at the University of Arizona.

Chris Brown, Ph.D., has been involved in ecological risk assessment for non-target arthropods, including the monarch butterfly, and genetically modified crops at Bayer CropScience (formerly Monsanto) since 2001. His work in industry prompted his involvement in the Entomological Society of America (ESA), but he also has ties to the organization based on personal entomology research interests, including a long-running survey of Missouri’s tiger beetles. His passion for insect photography led to being active in the ESA World of Insects Calendar committee, which he now chairs. He obtained a master’s degree in biology from Washington University and a Ph.D. in ecology and environmental science, with a focus on entomology and risk assessment, from Montana State University.

Tim Fredricks, Ph.D., has 10 years’ experience with Bayer CropScience and is responsible for coordinating projects and outreach directed to bees, monarch butterflies, and other pollinators or species of concern. He earned a Ph.D. in environmental toxicology from Michigan State University, where he conducted a large-scale field study on several bird species, examining the ecosystem for potential effects from exposure to industrial contaminants. Personally, he’s active in the Three Rivers Beekeepers Association and enjoys an active lifestyle with his young family centered around family activities, gardening, native landscaping, honey bee and fowl keeping, knitting, woodworking, hunting, fishing, cycling, running, and ice hockey.

Why does Bayer care about the monarch butterfly?

Fredricks: Monarch butterflies are an iconic species in North America and truly serve as an ambassador to habitat conservation for many species. Helping monarchs has other benefits, too, as good monarch habitat can have broad conservation and land-stewardship benefits, including supporting multiple other species of insects, birds, and mammals while improving soil structure, water infiltration, and decreasing field run-off to local water bodies. These benefits are all core to Bayer’s commitment to sustainability and biodiversity and deeply align with our farmer customers, who are some of the original and most passionate land stewards.

Brown: The only thing I’ll add to Tim’s comments is that I think we at Bayer have the same motivation to protect the monarch butterfly as so many others who share space with this charismatic species. In addition to appreciating things like its appearance in spring and its vibrant colors, the monarch provides opportunities to see and share textbook examples (literally!) of natural history right in our own backyard, like host-plant specificity, mimicry, chemical defenses, climate change, and insect migration, just to name a few.

The diet of monarch caterpillars consists solely of milkweed, making it essential for their survival.

The diet of monarch caterpillars consists solely of milkweed, making it essential for their survival.

Why is the monarch butterfly population declining?

Fredricks: The primary drivers affecting the health of the monarch population, according to the United States Fish and Wildlife Service’s (USFWS) Species Status Assessment Report for the monarch butterfly released in September 2020, include habitat loss and fragmentation throughout the monarch’s range (including feeding, breeding, and overwintering habitats); pesticide use, which can both destroy habitat but also expose them to insecticides; and a changing climate, which has intensified weather events and can have broad implications for population numbers (e.g., extreme drought in 2021 in the upper Midwest).

What does a status of “Warranted but Precluded” mean under the Endangered Species Act?

Fredricks: The “Warranted but Precluded” finding for the monarch butterfly by the USFWS in December 2020 acknowledges the current status and needs of the monarch butterfly population in North America, while also noting the additional species in the queue that have a higher priority level for listing actions, and thus the monarch decision and listing process is “precluded” by higher-priority listing actions. As such, the monarch butterfly is currently considered a candidate for listing under the Endangered Species Act, and the population status will be reviewed annually until a decision is made in 2024. The key for monarchs in the interim is that we continue to drive the “all hands on deck” approach to conservation to meet the 1.3 billion stems of milkweed goal to provide enough habitat to support a population level of 6 hectares of overwintering butterflies in Mexico.

What happens if the butterfly becomes listed as an endangered species?

Brown: That’s a great question and I think it’s on a lot of people’s minds. The short answer is we simply don’t know. The monarch butterfly, with its broad distribution and its ability to use a wide variety of habitats—say, your backyard or the edge of a farmer’s field—contrasts considerably from more typical listed species that are closely connected to specific habitats where conservation efforts can be focused. Additionally, there are quite a few ideas on what is causing the decline, so what protection of species with these unique characteristics looks like must be a challenge for USFWS.

We’ll have to wait and see, but for the time being we are supporting external conservation efforts as well as actively contributing to grassroots efforts in our own backyards and the managed prairie plantings on the Bayer sites in St. Louis. Just a couple of weeks ago, we were out collecting milkweed and other native plant seeds from our prairies at that Creve Coeur site in anticipation of having more host plants and nectar sources for monarchs next year. Hope springs eternal.

Tell me a little about the history of Bt corn and the monarch butterfly?

Brown: This was a formative experience for me, since I was just getting my start in regulatory considerations for Bt (Bacillus thuringiensis) crops, having spent most of my time prior in discovery and testing the efficacy of Bt crops. In fact, I still remember the moment when I first heard the news flash that Bt corn pollen could kill monarch butterflies. It was the summer of 1999 when John Losey and colleagues at Cornell had published the results of their experiments showing mortality of monarch larvae when they fed on milkweed leaves heavily dusted with Bt corn pollen.

Like many people at the time, I was very concerned given the iconic nature of the monarch butterfly, and I was under the common (mis)impression that any effect of a Bt protein on a beneficial insect was inherently a bad thing. Of course, it was also not realistic to think that the activity of Bt proteins was somehow strictly limited only to pest insects. I learned that the “effect” was just one part of the equation and that exposure was equally important in determining ultimate risk. The people involved in risk assessment of crop protection products knew well that a proper risk assessment included not just whether an organism was affected but also if the level of exposure under realistic conditions would translate to field-level effects.

It was fascinating to contribute to some of our own monarch butterfly testing and watch collaborative efforts toward gaining an understanding of the relevance of the Bt pollen effect in the headlines. The mobilization of government, academic, and industry scientists to take the deep dive into all facets of the story was truly impressive. They considered not only the effect of the Bt protein in the pollen on the monarch larvae but also aspects of field-level exposure like how much pollen actually accumulates on milkweed leaves at the margins of corn fields and spatial and temporal overlap of monarch larvae and anthesis in different geographies.

The conclusion was unequivocally that the commonly grown Bt corn products at the time had negligible risk to monarch butterfly populations, even when using conservative parameters. This was a great experience for me and others who were just getting their heads around non-target organism risk assessment for Bt crops. I’ve been applying those lessons ever since.

The diet of monarch caterpillars consists solely of milkweed, making it essential for their survival.

The diet of monarch caterpillars consists solely of milkweed, making it essential for their survival.

What research is currently ongoing at Bayer on the monarch butterfly?

Asiimwe: Bayer is taking a cross-functional, multi-layered approach to research on the monarch butterfly. When we develop a new technology for lepidopteran control, one of the initial aspects we look at as part of the screening process is sensitivity to monarch butterfly. These initial screening assays typically use Bt proteins as a test substance and provide semi-quantitative answers on whether we should expect to see activity on monarch butterflies.

In addition to that, we conduct tissue-specific diet bioassays where we test pollen from maize that has a lepidopteran protection trait to further quantify what that activity could look like in the field. We will then also conduct leaf-disk assays where we take pollen from maize and dust it on milkweed leaves, to mimic the realistic exposure scenario in the field. This step gives us a particularly good picture of what we can expect to see once this maize is deployed in the field.

We have also recently started conducting milkweed surveys in major corn-growing areas to understand what that exposure scenario is under current grower scenarios, relative to what it was several years ago when these surveys were last conducted. This work is performed by different various functions across Bayer, with each piece informing the next step. The combination of these assays and exposure assessments enables us to characterize what the risk scenario is prior to the commercialization of any lepidopteran control trait in maize.

Brown: To build on what Peter mentioned about the milkweed surveys, this information not only helps us with our exposure assessment but will also contribute to our understanding of where milkweed plants are on the landscape level to help with management practices. We have really appreciated the opportunity to get to the field with colleagues and do these surveys first-hand, especially during COVID times when in-person activities have been close to non-existent. Planning and conducting the surveys for ourselves and our partners also gives that ground-level perspective to better understand the real-world situation.

What can we do to help the monarch?

Brown: Tim mentioned it above—the “all hands-on deck” model. The widespread nature of the monarch butterfly provides a rare opportunity for everyone to be actively involved in conservation efforts for a species being considered for listing under the Endangered Species Act. Plant it and they will come, and milkweed plantings don’t have to be extensive. We’ve had monarch larvae most years on common milkweed plants in large pots on our back patio. Also, help us get a better understanding of where monarchs are and when by reporting observations on iNaturalist or Journey North, or volunteer for the Monarch Larva Monitoring Project.

Fredricks: Habitat is key! We as a human population don’t often consider how much of an impact our actions have had on our natural environment. Think about the areas you visit on a daily basis and what was likely there prior to a grocery store, gas station, school, highway, agricultural field, and the list could go on and on. Monarchs are a vagile species (wanderers) that need lots of small patches of quality habitat to reproduce and feed. This is great news for conservation since we can all have a positive impact on the species in our own backyards, schoolyards, and community spaces! Consider planting native nectar plants (i.e., flowers that bloom throughout the season) and including some milkweed species for monarch larvae to feed on in these places.

What’s your connection to the monarch butterfly? And do you have a favorite monarch butterfly moment?

Fredricks: We have pollinator habitat around our house, and in the fall my family keeps a butterfly net by the back door. When we spot a migrating adult monarch we’ll catch, tag, record data, and release the butterfly. It’s a great way for our family to learn about contributing to community science projects and how we can all get involved in conservation in our own yards!

Asiimwe: We always have some milkweed near the house, and the kids check on them frequently to see if they can find monarch larvae feeding. Having milkweed at home has also been a great way to expose my kids to other insects like the aphids and ants we typically find on our milkweed. These findings have spawned discussions about other entomological aspects like predation, mutualism, and symbiosis, which has been exciting and educational for them as well.

Brown: Obviously there’s something special about sharing monarch experiences with kids. I’ll never forget setting up a nearly ready monarch chrysalis in front of my young kids and watching the awe in their faces as the adult slowly emerged. They still talk about that. If I can add another moment, it would be seeing the monarchs on their overwintering ground in Michoacán, Mexico, in 2005. I’m an experiential learner and that gave me a lot more appreciation for the hardships they endure. I’ve never looked at them the same since.

Harit Bal, Ph.D., is a senior research entomologist and Frankie Stubbins, Ph.D., is a hemipteran research scientist, both at Bayer CropScience in St. Louis, Missouri. Stubbins is a 2021-2022 member of the ESA Diversity and Inclusion Committee. Email: haritkaur.bal@bayer.com and frankie.stubbins@bayer.com.