Monarch butterflies are endangered primarily because of massive habitat loss, pesticide use, and climate change. The migratory monarch was officially classified as Endangered on the IUCN Red List in 2022, after its population shrank by an estimated 22% to 72% over the prior decade. The western population, which overwinters in California, has been hit hardest, declining by roughly 99.9% from as many as 10 million butterflies in the 1980s to just 1,914 in 2021. The larger eastern population, which overwinters in Mexico, dropped by 84% between 1996 and 2014.
Milkweed Loss From Herbicide-Tolerant Crops
The single biggest driver of the monarch’s decline is the disappearance of milkweed, the only plant monarch caterpillars eat. Milkweed once grew abundantly between rows of corn and soybeans across the Midwest, but the widespread adoption of genetically modified, herbicide-tolerant crops changed that. Farmers began spraying entire fields with glyphosate, which killed the milkweed along with every other non-crop plant. Between 1999 and 2010, milkweed across the Midwest landscape declined by an estimated 58%, and monarch production in the region fell by 81% over the same period.
This matters because the Midwest is the monarchs’ most productive breeding ground. Researchers have confirmed that monarch production in the Midwest each year is directly correlated with the size of the overwintering population in Mexico the following winter. Fewer milkweed stems means fewer caterpillars, which means fewer adults completing the fall migration. A U.S. Geological Survey study calculated that it takes about 28.5 milkweed stems to produce a single monarch that successfully reaches Mexico. To sustain a healthy population of around 127 million overwintering butterflies, roughly 3.62 billion milkweed stems are needed across the U.S. Only about 1.34 billion remain, leaving a gap of 1.8 billion stems that need to be restored.
Pesticides That Kill Caterpillars on Contact
Even milkweed that survives near agricultural fields can be contaminated. Neonicotinoids, a widely used class of insecticides applied as seed coatings on corn and other crops, leach into surrounding soil and get absorbed by nearby plants, including milkweed. Field experiments have shown that milkweed growing directly alongside corn treated with the neonicotinoid clothianidin contained concentrations of 7 to 21 parts per billion in its leaves. That range overlaps with the lethal dose for monarch larvae, which is around 15.6 parts per billion. Caterpillars feeding on this contaminated milkweed had lower survival rates compared to those on milkweed near untreated fields. At lower, non-lethal exposures, larvae still showed reduced growth rates and shorter body lengths.
Destruction of Overwintering Forests
Every fall, eastern monarchs funnel into a handful of mountain sites in central Mexico, clustering on oyamel fir trees at high elevations where the cool, humid conditions keep them alive through winter. These forests are irreplaceable. Monarchs cannot survive the winter without the specific microclimate created by dense tree canopy, which buffers them against freezing temperatures and keeps humidity high enough to prevent dehydration.
Between 2001 and 2012, over 2,100 hectares of forest in the core zones of the Monarch Butterfly Biosphere Reserve were damaged, with roughly 1,250 hectares completely cleared and another 925 hectares degraded. Illegal logging was responsible for the vast majority of this loss: about 1,500 hectares were destroyed by large-scale operations and another 554 hectares by small-scale, subsistence-level cutting. Michoacán state bore 88% of the damage. Enforcement efforts managed to halt large-scale illegal logging by 2012, but small-scale cutting for local firewood and timber remains a serious and growing problem.
The most recent overwintering survey, from the 2024–2025 season, found monarchs occupying 1.79 hectares of forest in Mexico. That was a 99% increase from the previous year’s 0.9 hectares, but these numbers fluctuate wildly from year to year and remain far below historical levels.
Climate Change and Extreme Weather
Rising temperatures are compounding every other threat monarchs face. Heat and drought stress milkweed plants, reducing both their abundance and nutritional quality for developing caterpillars. Larvae that feed on heat-degraded milkweed develop more poorly. At the same time, prolonged exposure to temperatures above 36°C (97°F) directly increases caterpillar mortality, slows development through the pupal stage, and produces smaller, weaker adults.
Climate shifts are also disrupting the timing of migration itself. Warmer springs in the southern U.S. and Mexico have triggered earlier breeding, sometimes before milkweed is available at the right growth stage. This mismatch between when butterflies are ready to lay eggs and when their host plants are ready to support larvae can reduce reproductive success across an entire generation. On the other end, severe weather events during migration, including late-season freezes at overwintering sites and catastrophic storms, have killed millions of monarchs in single events. Drought also increases the frequency and intensity of wildfires in both the Mexican mountain forests and California coastal habitats where monarchs spend the winter.
Western Monarchs Face Steeper Odds
The western population, which breeds west of the Rocky Mountains and overwinters along the California coast, has been in far more precarious shape than its eastern counterpart. From a baseline of roughly 10 million butterflies in the 1980s, the population collapsed to under 2,000 by 2021. The most recent count, from late 2025, recorded approximately 12,260 monarchs across 249 sites. That’s a meaningful recovery from the worst lows, but still a fraction of what the population once was. Western monarchs face the same core threats (habitat loss, pesticides, climate change) compressed into a smaller geographic range, making them more vulnerable to any single bad year.
Where Legal Protections Stand
Despite its Endangered status on the IUCN Red List, the monarch butterfly is not yet federally protected in the United States. In December 2024, the U.S. Fish and Wildlife Service proposed listing the monarch as threatened under the Endangered Species Act, accompanied by a proposed critical habitat designation at overwintering grounds in coastal California. The public comment period, originally set to close in March 2025, was extended to May 2025. Protections would not take effect until a final rule is issued.
Conservation groups have set a target of restoring the eastern overwintering population to a level that consistently occupies about 15 acres (roughly 6 hectares) of forest in Mexico, which would represent a stable and resilient population. Reaching that goal will require restoring approximately 1.8 billion milkweed stems across the Midwest and Great Plains, a task complicated by the fact that location matters. Milkweed needs to be reestablished in the specific agricultural landscapes and roadsides where monarchs actually breed, not just anywhere it will grow.