The annual winter census of eastern monarch butterflies at their overwintering sites in the oyamel fir forests of central Mexico counted 2.2 hectares of forest coverage occupied by monarch colonies in the 2025 to 2026 season, down 22 percent from the 2.84 hectares recorded the prior year, the World Wildlife Fund Mexico reported in February 2026. The figure places the population well below the 6 hectare threshold that scientists consider the minimum viable size for a self-sustaining population capable of weathering a single catastrophic weather event.
How the Census Works | Hectares as a Population Proxy
Because counting individual monarchs at the overwintering sites is impractical, researchers measure the total area of forest covered by butterfly clusters, where monarchs pile so densely on oyamel fir branches that trunks and limbs are entirely obscured. Each hectare of occupied forest is estimated to contain between 10 million and 50 million individual butterflies. The 2.2 hectare figure for 2025 to 2026 translates to a rough population estimate of 22 million to 110 million butterflies. At the population's recorded peak in 1996, the census counted 21 hectares, representing an estimated 1 billion or more butterflies.
The western monarch population, which overwinters along the California coast rather than in Mexico, is counted separately by the Xerces Society's Thanksgiving count. The 2025 western count recorded approximately 233,000 butterflies, a modest recovery from the population's recorded low of 1,914 in 2020, but still more than 99 percent below historical estimates of 10 million western monarchs in the mid-20th century.
Drivers of Decline | Milkweed, Deforestation, and Climate
Researchers attribute the ongoing decline of the eastern monarch to three interlocking factors. The first is milkweed loss. Monarchs lay their eggs exclusively on milkweed plants, and caterpillars feed only on milkweed leaves. The United States has lost an estimated 1.3 billion stems of milkweed since 1999, driven largely by the adoption of herbicide-tolerant genetically modified corn and soybean crops, which have allowed widespread application of glyphosate to formerly milkweed-rich agricultural fields in the Midwest corn belt.
The second driver is illegal and legal logging at the Mexican overwintering sites, where the oyamel fir forest ecosystem exists only in a narrow elevation band between 2,400 and 3,600 meters in the states of Michoacan and Mexico. Despite protections, the World Wildlife Fund Mexico estimates that between 100 and 200 hectares of overwintering habitat have been lost to degradation and illegal logging since 2000. The third driver is climate disruption to the migratory phenology, the timing of the butterflies' departure from Texas in spring, their northward movement tracking milkweed green-up, and their return migration in fall.
U.S. Fish and Wildlife Service Listing Decision
The U.S. Fish and Wildlife Service proposed listing the monarch butterfly as threatened under the Endangered Species Act in December 2020. As of June 2026, a final listing decision remains pending, delayed by litigation, interagency review, and a backlog of listing candidates. Conservation organizations including the Center for Biological Diversity and the Xerces Society have sued to compel a final determination, arguing that each additional year without federal protection allows further habitat destruction to proceed without the mitigation requirements that an ESA listing would trigger.
A threatened listing would not ban monarch butterfly observation or incidental harm from agricultural operations, but it would require federal agencies to consult with the Fish and Wildlife Service before approving projects that could harm monarch habitat, and it would authorize funding for habitat restoration and milkweed corridor programs through the Section 6 grant mechanism.
What Is Being Done | Milkweed Corridors and Citizen Science
Restoration efforts are underway at multiple scales. The National Wildlife Federation's Garden for Wildlife program has enrolled more than 300,000 certified wildlife habitats featuring milkweed plants, and the USDA Natural Resources Conservation Service offers cost-share payments to farmers who plant pollinator habitat buffers along field edges. The HoneyNewspaper environment desk has covered pollinator corridor initiatives connecting the Texas migration bottleneck to overwintering habitat in Mexico.
Citizen scientists contribute population data through Journey North, a project that tracks monarch sightings across North America and has documented northward migration phenology since 1994. The data has become a primary source for researchers modeling how climate change is altering the timing of the migration relative to milkweed availability, a mismatch that reduces larval survival rates.
HoneyNewspaper animal welfare and wildlife coverage includes ongoing reporting on the Endangered Species Act listing backlog and community-level pollinator habitat initiatives. Corporate accountability reporting covers the role of agricultural chemical manufacturers in milkweed decline and herbicide-tolerant crop adoption.