Few insects are tracked as closely as the monarch butterfly. Every winter, volunteers in California count clustered butterflies along the coast, and scientists in Mexico measure how many acres of forest are draped in orange wings. The numbers they report often swing dramatically from one year to the next, which can make headlines confusing: one winter brings news of a rebound, the next of a near collapse. This article explains why monarch numbers change so much, how they are measured, and what the long-term trend actually shows.
Two Populations, Two Counting Methods
North America has two main migratory monarch populations. The eastern population, by far the larger, breeds across the eastern United States and southern Canada and spends the winter in high-elevation oyamel fir forests in central Mexico. The western population breeds west of the Rocky Mountains and overwinters mainly in groves along the California coast.
Because the butterflies pack together so densely in Mexico, scientists there do not count individuals. Instead, they measure the total forest area occupied by colonies. According to the Monarch Joint Venture, each hectare (about 2.47 acres) holds an estimated 20 to 30 million monarchs, with a median estimate of about 21 million. In California, the Xerces Society coordinates the Western Monarch Count, in which hundreds of volunteers visit overwintering sites around Thanksgiving and estimate the number of butterflies directly.
What the Recent Numbers Show
The western population illustrates how volatile counts can be. Researchers estimate that more than 4.5 million monarchs overwintered on the California coast in the 1980s. The 2020 count found only 1,914. Three years later, the 2023 count tallied 233,394. Then the number dropped again to 9,119 in the 2024 count. The 2025 count, released in early 2026, recorded 12,260 butterflies at 249 sites, the third-lowest total since the count began in 1997.
The eastern population also fluctuates. In the 2024-25 winter, monarchs occupied about 4.42 acres (roughly 1.8 hectares) of forest in Mexico. The following winter, figures released by WWF-Mexico and partners in early 2026 showed about 7.24 acres (roughly 2.9 hectares), an increase of 64 percent. For context, the occupied area exceeded 18 hectares in the mid-1990s, and the Monarch Joint Venture notes that at least 6 hectares is considered necessary for a resilient population.
Reason 1: Many Generations Compound Small Changes
A monarch’s year involves several generations. The butterflies that overwinter are a special long-lived generation that can survive for months. In spring they fly north, breed and die, and their offspring continue the journey; it typically takes several generations to recolonize the breeding range before the final generation of summer flies south again. Because each generation’s success multiplies the next, a modest improvement or setback at every stage can produce a very large difference by autumn. A good spring followed by a good summer can multiply numbers; a poor start can leave too few butterflies to rebuild even if later conditions are favorable.
Reason 2: Weather and Climate
Monarchs are highly sensitive to temperature and rainfall. Hot, dry conditions in spring and summer can reduce milkweed quality, dry out nectar flowers and shorten adult lifespans. Drought in Texas and the southern Plains, a key spring breeding and fall migration corridor for eastern monarchs, has been linked to poor years. Severe storms at overwintering sites can kill large numbers of butterflies directly; in Mexico, unusually cold, wet weather has caused major die-offs in the past. In California, winter storms and the condition of groves influence where butterflies cluster and how many survive. The Monarch Joint Venture lists temperature and rainfall patterns, along with habitat quality and availability, among the factors affecting overwintering numbers.
Reason 3: Milkweed and Nectar
Monarch caterpillars can eat only milkweed, so the amount and location of milkweed across the breeding range strongly limit how many butterflies can be produced. Large losses of milkweed in agricultural regions over recent decades, along with conversion of grasslands and roadside habitat, have reduced breeding capacity. Adults also need abundant nectar flowers, especially during fall migration when they must build up fat reserves to survive the winter. Years with good bloom along migration routes tend to support healthier overwintering populations.
Reason 4: Overwintering Habitat
Both populations depend on very specific winter sites. In Mexico, the fir forests provide a microclimate that keeps butterflies cool enough to conserve energy but protected from freezing. Illegal logging, forest degradation and climate shifts threaten that microclimate. In California, monarchs rely on coastal groves of eucalyptus, Monterey pine, Monterey cypress and other trees that offer shelter from wind and the right humidity. Development, tree loss and aging groves reduce the number of suitable sites.
Reason 5: Pesticides, Disease and Counting Itself
Insecticides, including systemic products that can be present in milkweed and nectar, and herbicides that remove milkweed both affect monarchs. A protozoan parasite called Ophryocystis elektroscirrha, or OE, can weaken infected butterflies, and its prevalence may be higher where monarchs breed year-round. Finally, part of the year-to-year variation reflects measurement. Counts take place on specific days at known sites, and butterflies may shift to new or unmonitored locations or spread out across inland areas in mild winters, which can lower recorded numbers even when the population has not collapsed as much as the total suggests.
The Long-Term Trend
Short-term rebounds should not hide the bigger picture. The U.S. Fish and Wildlife Service estimates that the western population has declined by more than 95 percent since the 1980s and the eastern population by roughly 80 percent. In December 2024, the agency proposed listing the monarch as a threatened species under the Endangered Species Act. The final decision was not issued by the original deadline of December 2025; in 2026, a federal court consent decree set June 6, 2030, as the deadline for a final listing decision.
How People Can Help
Because monarchs respond quickly to conditions, local actions matter. Planting native milkweed suited to your region, adding a variety of native nectar plants that bloom from spring through fall, reducing pesticide use, and joining community science programs such as the Western Monarch Count or Monarch Watch tagging all contribute data and habitat. Consult your regional native plant society or extension service to choose species appropriate to your area.
The Bottom Line
Monarch numbers change dramatically from year to year because weather, food plants and habitat effects compound across multiple generations and because counts capture just one moment at specific sites. These swings sit on top of a long-term decline, which is why scientists focus on multi-year trends rather than any single winter’s headline.
Sources
- Monarch Joint Venture: Western Monarch Count marks another low year
- Monarch Joint Venture: Population Trends
- AccuWeather: Monarch population rises in Mexico, but California numbers fall near record lows
- DTN: Monarch Listing Decision Due by 2030
Featured image: Thomas Bresson / Wikimedia Commons, CC BY 2.0
