Monarch Butterfly Winter Count Drops 59 Percent in Mexico

Eastern Migration Shrinks to Second-Lowest Area Recorded

The annual survey of eastern monarch butterflies overwintering in the oyamel fir forests of central Mexico revealed a steep contraction in population coverage during the 2023-2024 winter season. Field monitoring conducted by environmental authorities and conservation groups documented that monarchs occupied just 0.9 hectares of forest canopy across the biosphere reserve. This figure represents a 59 percent decrease from the 2.21 hectares recorded during the previous winter monitoring period, marking the second-lowest spatial coverage recorded since systematic tracking began in 1993.

Biologists attribute this marked reduction to severe heat and drought across key North American breeding territories during the spring and summer migration corridors. High summer temperatures coupled with prolonged dry spells dramatically reduced the density and quality of milkweed, the exclusive host plant required by monarch caterpillars. As milkweed withered prematurely, successive generations of monarchs encountered reduced breeding success, resulting in fewer migratory adults embarking on the thousands-of-miles journey southward to the trans-volcanic mountain ranges of Michoacan and the State of Mexico.

The spatial measurement of occupied canopy serves as the primary scientific metric for evaluating the health of the eastern monarch population, because counting individual butterflies packed densely into tree branches remains logistically infeasible. Historical data indicates that the population covered more than 18 hectares at its peak in the mid-1990s. Conservation biologists emphasize that a sustained minimum threshold of 6 hectares is widely regarded as necessary to shield the migratory phenomenon from catastrophic localized events, such as winter freeze storms or disease outbreaks.

Western Monarch Numbers Reflect Persistent Historical Loss

On the Pacific coast, the western monarch butterfly population continues to struggle well below historical benchmarks, according to survey findings released by the Xerces Society for Invertebrate Conservation. During the annual Western Monarch Thanksgiving Count, community science volunteers tallied more than 233,000 butterflies overwintering across coastal California groves. While this total reflects a partial stabilization compared to the disastrous lows of 2020 and 2021 when fewer than 2,000 butterflies were observed, it remains drastically lower than the millions that populated coastal valleys in the 1980s.

Western monarchs complete a shorter migration cycle than their eastern counterparts, migrating inland to breeding areas across California, Nevada, Oregon, Washington, and Idaho before returning to eucalyptus, pine, and cypress groves along the California coastline. Coastal habitat degradation, urban development, and extreme winter storm events have severely thinned the protective tree canopy at key overwintering grounds. The loss of windbreaks leaves resting clusters exposed to freezing winds and heavy rains, compounding winter mortality.

The Xerces Society and state wildlife officials warn that without robust protection of critical overwintering sites, the western migration remains acutely vulnerable to climate shocks. Environmental stress in agricultural breeding zones, including atmospheric heat waves and widespread pesticide exposure, continues to suppress spring reproductive output, preventing the population from mounting a sustained recovery toward multi-million baseline counts.

Federal Status and the Endangered Species Act Evaluation

In response to long-term population declines across both flyways, the U.S. Fish and Wildlife Service continues evaluating the monarch butterfly for potential listing under the federal Endangered Species Act. The agency previously determined in December 2020 that listing the monarch as threatened or endangered was warranted but precluded by higher-priority listing actions for other species. As a result, the monarch was designated a official candidate species, requiring annual status reviews while federal biologists updated population risk models.

The Endangered Species Act evaluation focuses on the cumulative impacts of three primary threat drivers: the loss of native milkweed host plants across agricultural landscapes, climate change acceleration leading to erratic weather patterns, and widespread exposure to insecticides and herbicides. Federal review procedures involve analyzing sophisticated demographic models that project extinction probabilities under varying land-management and climate scenarios across North America.

A formal listing under the Endangered Species Act would establish federal protection standards, mandate critical habitat designations, and enforce interagency consultation for federally funded or permitted projects that affect monarch corridors. It would also accelerate federal grant allocation for state-level habitat restoration, offering legal mechanisms to preserve essential breeding and overwintering grounds.

Environmental Drivers of Milkweed Deficit and Habitat Loss

The fundamental engine of the monarch decline remains the widespread loss of milkweed species, particularly across the agricultural heartland of the U.S. Midwest. Over the past three decades, the widespread adoption of herbicide-tolerant crop varieties accompanied by increased application of broad-spectrum herbicides virtually eliminated milkweed from corn and soybean fields. Historically, farm fields provided millions of acres of milkweed interspersed among crops, serving as the core engine for summer reproductive generations.

In addition to herbicide-driven weed control, industrial land transformation, grassland conversion, and roadside mowing schedules have steadily reduced marginal breeding habitat along migration highways. Neonicotinoid insecticides used widely in seed treatments and crop protection pose a parallel chemical threat. Research demonstrates that sub-lethal exposure to neonicotinoids can impair monarch caterpillar growth, reduce butterfly navigational capabilities, and lower adult longevity.

Superimposed upon chemical and land-use stressors, climate change introduces unpredictable phenological mismatches. Warmer spring temperatures often trigger early monarch migration before milkweed plants have fully emerged in northern states, or induce excessive heat during summer months that degrades floral nectar resources. These intersecting environmental pressures leave monarch generations with diminished reserves as they prepare for autumn migration.

Cross-Border Conservation Benchmarks and Population Synthesis

By synthesizing monitoring data from both North American migratory routes, a comprehensive picture of population health emerges across international boundaries. In the eastern flyway, spatial area measurements from Mexican forest reserves serve as the standard metric, whereas western flyway monitoring relies on direct volunteer counts in California coastal groves. The following consolidated synthesis highlights the contrasting monitoring methodologies, current levels, and historic baselines across both populations.

In the eastern flyway, the 2023-2024 occupied forest area measured 0.9 hectares, representing a 59 percent drop from 2.21 hectares in 2022-2023 and sitting far below the 6.0 hectare conservation target recommended by scientists. Peak historic coverage reached 18.19 hectares in the winter of 1996-1997. In the western flyway, the 2023 Thanksgiving count recorded 233,000 butterflies, representing a substantial drop from historic 1980s benchmarks exceeding 4.4 million butterflies across coastal wintering groves. This comparative consolidation combines spatial monitoring data from the World Wildlife Fund Mexico and CONANP as reported by Associated Press with volunteer tally data compiled by the Xerces Society as reported by Reuters, anchored against U.S. Fish and Wildlife Service regulatory benchmarks.

This cross-border synthesis underlines that despite differing survey techniques, both populations face parallel environmental headwinds. The spatial reduction in Mexico mirrors the persistent numerical suppression in California, demonstrating that localized conservation efforts must be coupled with continent-wide landscape restoration to reverse multi-decadal declines.

Restoration Strategies and Immediate Action Priorities

To counter habitat fragmentation, state agencies, non-profit organizations, and agricultural groups have launched large-scale habitat restoration initiatives across North America. Federal conservation programs incentivize farmers and ranchers to establish pollinator meadows, plant native milkweed, and preserve native prairies through working-lands conservation easements. Linear corridors along interstate highways, utility rights-of-way, and urban park systems are increasingly managed to support milkweed and nectar-producing native wildflowers.

Urban and suburban communities are also playing an expanded role in monarch conservation through residential planting campaigns. Organizations encourage homeowners and municipal planners to plant locally native milkweed species—such as common milkweed, swamp milkweed, and butterfly weed—while avoiding non-native tropical milkweed. Tropical milkweed can linger late into autumn in southern regions, disrupting natural migratory cues and harboring the protozoan parasite Ophryocystis elektroscirrha, which severely weakens monarch adults.

International collaboration between Canada, the United States, and Mexico remains essential for maintaining the migratory flyway network. Joint initiatives focus on protecting winter forest sanctuaries from illegal logging, expanding spring nectar corridors through northern Mexico, and coordinating scientific monitoring. Rebuilding resilient monarch populations requires sustained investment across every geographic link in their annual cross-continental journey.

What happens next

The U.S. Fish and Wildlife Service will proceed with its regulatory process to determine whether to formally propose listing the monarch under the Endangered Species Act. Meanwhile, conservation groups and state wildlife departments will conduct upcoming overwintering counts in California and Mexico to assess population recovery following summer breeding conditions.