The question of whether the Monarch butterfly (Danaus plexippus) is rare is complicated. While it is one of North America’s most recognizable insects, its populations have experienced severe, decades-long declines, making its future stability uncertain. This migratory species is famous for its multi-generational journey across the continent and is now considered critically vulnerable. The current situation is not one of inherent rarity but rather catastrophic population loss that has pushed this formerly common butterfly toward the brink of collapse.
Understanding the Eastern and Western Populations
The Monarch population in North America is divided into two migratory groups separated by the Rocky Mountains. These groups follow different routes and have faced varying degrees of decline. The Eastern population breeds across the central and eastern US and Canada. This group undertakes a journey of up to 3,000 miles to overwinter in the Oyamel fir forests of central Mexico.
The Western population breeds west of the Rockies, primarily in California, and overwinters along the Pacific coast. While the Eastern population is significantly larger, the Western group has experienced a far more drastic reduction. The Eastern population has declined by an estimated 80 to 90 percent since the mid-1990s. The Western population has plummeted by over 95 percent since the 1980s.
The Western population has seen annual overwintering counts drop from millions to alarmingly low numbers, with some years recording fewer than 2,000 individuals. This disparity underscores why public alarm regarding Monarch rarity often focuses on the Western migratory phenomenon. Although the populations are genetically similar, their conservation outlooks are not identical.
Key Environmental Pressures Causing Decline
The primary driver of the Monarch’s decline is the widespread loss of its host plant, milkweed (Asclepias species), in its breeding grounds. Milkweed is the only plant Monarch caterpillars can eat, and its removal has directly reduced the butterfly’s reproductive success. This loss is linked to modern agricultural practices, particularly the increased use of herbicides like glyphosate on genetically modified crops. Spraying has eliminated milkweed from millions of acres of farmland across agricultural regions.
Habitat destruction at overwintering sites also contributes to the population drop. The Eastern population’s sanctuary in Mexico’s Monarch Butterfly Biosphere Reserve is threatened by illegal logging of Oyamel fir trees. These trees provide a canopy that regulates the microclimate, shielding the clustered butterflies from freezing rain and extreme cold.
The Western population faces similar pressures, with coastal overwintering sites in California continually being lost to urban development and habitat degradation. Climate change introduces instability that disrupts the Monarch’s multi-generational life cycle. Increased extreme weather events, such as droughts and unseasonal freezes, directly kill butterflies and disrupt migration timing and reproduction. Changes in temperature can also alter the blooming schedule of nectar plants, leaving adults without the fuel needed for migratory flights.
Current Conservation Status and Monitoring Methods
Scientists measure the status of the two migratory populations at their wintering sites. The Eastern population is monitored by calculating the total area, measured in hectares, that the clustered butterflies occupy in the Mexican Oyamel forests. The Western population’s status is determined by conducting a direct count of individual butterflies at overwintering groves along the California coast.
In 2022, the International Union for Conservation of Nature (IUCN) listed the migratory Monarch butterfly as “Endangered” on its Red List of Threatened Species. This international designation recognizes the severe threat but does not provide legal protection within the United States. In the U.S., the migratory Monarch has been petitioned for listing under the Endangered Species Act (ESA).
The U.S. Fish and Wildlife Service determined the listing was “warranted but precluded.” This status indicates that while the species meets the criteria for protection, other higher-priority species must be addressed first. The Monarch currently lacks ESA protection within the U.S. However, the Service has since proposed to list the species as “Threatened,” signaling a strong likelihood of future federal protection.
Actionable Steps for Citizen Conservation
Individuals can play a direct role in Monarch conservation by providing essential habitat in their communities. Planting native milkweed species is crucial, as these are the only plants Monarch caterpillars can consume. It is important to source milkweed seeds or plants native to the local region, ensuring they are best suited for the climate and Monarch life cycle.
Homeowners should also plant native nectar-producing flowers that bloom throughout the seasons to provide energy for adult Monarchs. Tropical milkweed (Asclepias curassavica) must be managed carefully, as its year-round growth encourages a debilitating parasite (Ophryocystis elektroscirrha). If planted, it must be cut back in the fall and winter to prevent parasite accumulation.
Eliminate or severely limit the use of insecticides and herbicides in gardens and yards. Pesticides, especially neonicotinoids, are highly toxic to caterpillars and adult butterflies. Creating a safe, chemical-free zone with native milkweed for larvae and nectar sources for adults is the most effective way to support the Monarch’s recovery.