Roughly every week from May through October, you can find Cincinnati Nature Center volunteers carefully inspecting milkweed plants in gardens and prairies throughout Rowe Woods and Long Branch Farm & Trails. They are searching for the eggs, caterpillars, and pupae of the Eastern Monarch butterfly (Danaus plexippus), which relies on milkweed (Aesclepias spp.) for reproduction. If there are Eastern Monarchs reproducing here, the milkweed plants are where we will find their offspring.
Of the more than 750 species of butterflies in the United States, the Eastern Monarch is likely the most well-known and most recognizable. The Monarch’s notoriety is multi-faceted: their bold orange and black markings are striking; their range extends throughout most of the US; and their unique multi-generational migration is an awesome and mysterious natural phenomenon. These beloved insects hold ecological and cultural significance in their own right, and they function as an “umbrella species”; efforts made to conserve and restore Eastern Monarch habitat also aids other species who share that habitat.
The Infamous Decline
Another reason many people know about Eastern Monarchs is because of the rapid and news-worthy population declines of the past few decades. It is estimated that the Eastern Monarch population has declined by >80% since the 1990s. These declines are observed in the overwintering population of Monarchs, which are found in the mountainous oyamel fir forests of central Mexico. These overwintering Eastern Monarchs began their lives much further north, in the Upper Midwest and Ohio River Valley, hatching from eggs laid on milkweeds in the late summer. They feed on the milkweed and go through several larval instars as yellow-black-and-white striped caterpillars, then pupate and emerge as adult butterflies. However, influenced by environmental cues such as shortening days, senescing milkweed, and cooler temperatures, this generation of Monarchs have immature reproductive organs. Rather than remaining in the north to feed and reproduce, as their parents, grandparents, and great-grandparents did, these autumnal generation will instead migrate hundreds of miles south to congregate in the oyamel fir forests.
Researchers use the area of the forests that is occupied by the overwintering Eastern Monarchs as a proxy for their population size. Since tracking began in the early-mid 1990s, sizeable declines have been observed, which has rightfully spurred concern and further investigation. The average population estimate from the past decade (winter of 2013/2014–winter of 2023/2024) is approximately 55% lower than the average population estimate from the proceeding 20 years. The overwintering population during the winter of 2023/2024 (0.9 hectares) was only ~5% of the recorded high of 18.19 hectares observed in 1996/1997. On a local level, the Monarch Monitoring volunteers at Cincinnati Nature Center saw 95% fewer Monarch eggs, larvae and pupa than they had in 2023, which was already a lower-than-average year since monitoring began at the Nature Center in 2015.
What is driving these declines?
Thanks to the efforts of professional researchers and citizen scientists, we have evidence to support a few hypotheses. The abundance and distribution of milkweed (for reproduction) and other nectaring plants (for feeding) are likely to limit the size of Eastern Monarch populations. The availability of this habitat has decreased substantially in the past 25 years. Land-use change and development has resulted in the loss and fragmentation of prairie habitat. The adopted use of herbicide-resistant row crops has eliminated milkweed from growing in crop fields, particularly in the Upper Midwest where the majority of Eastern Monarchs spend their summer months. Herbicide used to manage “weedy” areas where milkweed grows, such as roadside ditches, may also contribute to an overall loss of summer habitat.
Weather conditions and extreme events also appear to play a large role in Eastern Monarch population size, even when habitat is available, and adverse effects of extreme weather can occur anywhere along the migration route. Warmer-than-average temperatures can draw Eastern Monarchs further north at inopportune times, which can make them vulnerable when colder temperatures return. Late spring or early fall freezes can result in mass mortality. Drought lowers their reproductive success. Most of the dramatic population crashes in the last 30 years are associated with some type of extreme weather.
The recent population counts and monitoring results certainly seem bleak for the Eastern Monarch, and they undoubtedly face threats related to habitat loss, climate change, and pesticide use. The data from the last 3 decades demonstrate that eastern monarch populations fluctuate—sometimes dramatically—from year to year, and there have been multiple instances of sizeable population recoveries. For instance, the 2024/2025 overwintering population nearly doubled over the previous year, although it is still well below the 30 year median. Population recoveries provide hope that with ample habitat availability and favorable weather conditions, monarchs have the ability to bounce back. Because weather conditions can be variable and unpredictable, maintaining quality habitat for monarchs is imperative for their resiliency as a species. Ensuring that habitat is broadly geographically distributed should further protect the viability of eastern monarchs from localized catastrophic events.
In response to the declining monarch populations, the United States Fish & Wildlife Service completed a Special Status Assessment for monarchs to assess the need for federal protections under the Endangered Species Act. Through the SSA, the Fish & Wildlife Service determined that Eastern Monarchs are at a heightened risk of extinction within the next century, and recommended that the species be listed as federally “threatened”. This threatened designation could enhance ongoing voluntary efforts toward monarch conservation through several avenues, including increasing monarch habitat.
The threatened listing would allow increased funds to be devoted to monarch conservation, further promoting voluntary efforts to protect and restore prairies habitat. The widespread geographic distribution of monarchs offers a unique opportunity for people across most of the US to be involved in the conservation of a threatened species; in fact, the FWS makes it clear that active participation in conservation efforts by landowners is necessary for the future of monarchs.
