Monarch butterfly conservation is one of the most visible insect conservation stories in the world, partly because the monarch’s biology makes its population changes legible in ways that most insects’ do not. The annual migration covers up to 4,800 kilometres. It begins from overwintering sites in central Mexico to summer breeding grounds across North America. It is the longest regular insect migration on Earth. When the population collapses, the forests of oyamel firs at Cerro Pelado and Sierra Chincua fill with fewer butterflies. When it recovers, the numbers are visible, countable, and reported.
TL;DR: Monarch butterfly populations declined by approximately 80% between the 1990s and 2010s. The primary causes are milkweed loss from herbicide use, deforestation of Mexican overwintering sites, and climate disruption of migration timing. Population counts have shown partial recovery in some years, but the species remains under threat. Conservation actions include milkweed planting, habitat corridors, and protected area management.
| Fact | Detail |
|---|---|
| Scientific name | Danaus plexippus |
| Migration distance | Up to 4,800 km (3,000 miles) |
| Overwintering sites | Oyamel fir forests, Michoacan and Estado de Mexico, Mexico |
| Population peak estimate | ~1 billion individuals in 1990s |
| Population low estimate | ~33 million individuals (2013–2014 season) |
| Host plant | Milkweed (Asclepias species) — larvae feed exclusively on milkweed |
| IUCN status | Endangered (migratory population; assessed 2022) |
The Migration
The eastern monarch population overwinters in a remarkably specific area: twelve or so mountain sites in central Mexico, at elevations between 2,400 and 3,600 metres, in oyamel fir forest. The butterflies cluster in such density that branches bend. The area required to sustain the entire eastern population is roughly 10 hectares in a peak year — an absurdly small footprint for a population counted in hundreds of millions at its height.
In spring, the overwintering generation begins moving north. They mate, lay eggs on milkweed, and die — the next generation continues north. It takes three to four generations to complete the northward migration. In late summer, a final “super generation” emerges that will live eight times longer than the summer generations, make the entire southward journey to Mexico alone, and overwinter there before the cycle begins again. No individual completes a round trip. The navigation information is somehow encoded across generations.
Why the Population Declined
Three factors drove the decline from estimated peak populations of around one billion individuals in the 1990s to a low of approximately 33 million in the 2013–2014 season.
Milkweed loss. Monarch larvae feed exclusively on milkweed (Asclepias species). The widespread adoption of herbicide-resistant crops in the US Midwest from the late 1990s onward allowed the elimination of milkweed from agricultural fields that had previously supported it. Common milkweed (Asclepias syriaca) had grown in corn and soybean fields for generations. Glyphosate-resistant varieties allowed complete weed elimination. Milkweed acreage in the Midwest collapsed.
Deforestation of overwintering sites. Illegal logging in the Mexican biosphere reserve reduced the oyamel fir forest that the butterflies depend on. Legal and illegal agriculture encroached on the reserve boundaries. The forest both shelters the butterflies from frost and regulates the humidity conditions they require to survive winter without depleting their fat reserves.
Climate disruption. The monarch’s migration is timed by day length and temperature. Climate change has altered the flowering timing of milkweed and nectar sources, and increased the frequency of extreme weather events that can kill large numbers of butterflies. Drought conditions reduce milkweed quality and availability. Warmer winters at overwintering sites disrupt the metabolic dormancy the butterflies require.
Monarch Butterfly Conservation Responses
Conservation responses have developed at multiple scales.
Milkweed restoration. Planting native milkweed species along migration corridors is the most direct individual action available for monarch butterfly conservation. The Monarch Waystation programme, run by Monarch Watch at the University of Kansas, has certified over 35,000 habitat gardens. Native milkweed species, such as Asclepias tuberosa, Asclepias incarnata, and Asclepias syriaca depending on the region, are more valuable. They are more beneficial than the commonly available tropical milkweed (Asclepias curassavica). Tropical milkweed can disrupt migration timing if left standing year-round in warmer climates.
Mexican reserve protection. The Monarch Butterfly Biosphere Reserve in Mexico was established in 1980 and expanded in 2000. WWF-Mexico and local ejido communities have developed programmes. These provide economic alternatives to logging. The alternatives include ecotourism, sustainable forestry certification, and payment for ecosystem services. Illegal logging within the reserve has decreased significantly since peak years in the early 2000s. However, it has not stopped entirely.
US and Canadian policy. In 2014 the US Fish and Wildlife Service was petitioned to list the monarch under the Endangered Species Act. The listing decision has been repeatedly delayed. The IUCN listed the migratory monarch population as Endangered in 2022 — an assessment covering the migratory phenomenon itself, which is considered a distinct conservation unit regardless of the overall species status. Canada has listed the monarch as a species of Special Concern under the Species at Risk Act.
Current Population Status
The World Wildlife Fund and partners conduct population counts annually at the Mexican overwintering sites. They measure the area of forest occupied by butterflies. The 2022–2023 season showed recovery to approximately 2.84 hectares, up from the historic low of 0.67 hectares in 2013–2014. The long-term trend remains below the informal target of 6 hectares considered necessary for population viability.
Western monarchs — a distinct population that overwinters on the California coast — crashed to around 2,000 individuals in 2020. They then recovered to over 200,000 in 2021. This fluctuation is not fully explained and illustrates the difficulty of interpreting short-term population data.
Drawing Danaus plexippus
The Fiurdelin Danaus plexippus illustration focuses on what makes the monarch immediately identifiable: the deep orange ground colour, the black venation, the white spots in the border, and the characteristic wing shape with its relatively straight forewing edge. These are the diagnostic features — the ones a field guide would prioritise — and they are also the features that make the monarch one of the most recognisable insects in North America.
The colouration is aposematic — a warning to predators that the butterfly is toxic. Monarchs accumulate cardenolides from the milkweed they eat as larvae. Birds that eat them vomit. The viceroy butterfly mimics the monarch’s colouration closely enough that predators avoid it too, despite the viceroy not being toxic. Drawing that warning colouration accurately means getting the orange right — not too warm, not too yellow, the specific saturated orange-red that the monarch’s wing actually is.
The Fiurdelin Danaus plexippus illustration is available as a print. Browse the full Fiurdelin collection.
