Monarch butterflies (Danaus plexippus) are perhaps the most well-known and beloved butterflies in North America. Seen from coast to coast, their arrival in northern states and Canadian provinces is viewed by many as a welcome sign of the change in seasons from spring to summer. However, the monarchs in different parts of North America might not be quite the same! Read on to discover what makes eastern monarchs different from western ones, and how we can help them.

What are eastern monarchs?

Monarch butterflies in North America) are divided into different populations based on where they live (and migrate to).  The eastern population (“eastern monarchs” breeds east of the Rocky Mountains, and overwinters in central Mexico. The western population (“western monarchs”) breeds west of the Rockies, and overwinters in forest groves on the California coast. There is also a southern Florida population, which is much smaller in numbers than the other two. In another major difference, this southern Florida monarch population does not seem to migrate, and instead breeds year-round in the same area.

Are eastern monarchs different from other monarch butterflies?

All three populations of monarch butterflies — eastern, western, and Florida — are all part of the same species: Danaus plexippus. The populations are still closely related, and can breed with each other when they end up in the same place, but generally live in separate parts of North America.

The eastern monarch population is the largest of all monarch populations, both in numbers and geographic range. There are approximately 100 times more eastern monarchs than western ones. Eastern monarchs  live across a geographic area accounting for about 70% of the total North American monarch range. The eastern monarch range includes Mexico, eastern U.S., and southern Canada, while the western monarch range includes the western U.S. and southwestern Canada.
 

Why are there different monarch populations?

While monarchs are impressive fliers, there are some trips that are tough even for them. The Rocky Mountains, along with the deserts of the American Southwest, form a natural barrier that would take too much energy for monarchs to cross. So, monarchs on the west of the Rockies are blocked from traveling east, and vice versa, leading to separate populations.

How far do eastern monarchs fly during their epic migration?

Both eastern and western monarchs migrate south each year to avoid the harsh winter cold. In autumn, most eastern monarchs go to overwintering sites in Michoacan, Mexico. This is an epic journey, with some individuals flying up to 3,000 miles! (Western monarchs travel a much shorter distance of 300-1,000 miles to their overwintering sites.)

Eastern monarchs begin arriving in Mexico in late October. They overwinter in high elevation forests, gathered in dense clusters on oyamel fir trees. The oyamel forest provides the right conditions: cool but not frigid. The forest also provides protection from rain, snow, hail, and wind. This is important because extreme weather can have a devastating impact when large numbers of monarchs are concentrated in one location.
 

Are eastern monarchs threatened or endangered?

In December 2024, the U.S. Fish and Wildlife Service proposed that monarch butterflies be listed as Threatened and granted protection under the Endangered Species Act (ESA). These protections would be for the entire species, including both eastern and western populations.

Both eastern and western monarch populations have declined severely over the past two decades. Long-term monitoring of monarchs at their overwintering sites show that eastern monarchs have dropped by more than 80% since the 1990s, and western monarchs by over 95% since the 1980s.

The Fish and Wildlife Service estimated that, if nothing changed, the eastern monarch population had a less than 10% chance of extinction within 10 years, but a 56-74% chance within 60 years. (Western monarchs fared worse, with a 99% chance of extinction within 60 years). Endangered Species Act protections are needed to help monarchs recover and avoid extinction. 
 

What are the threats to eastern monarch butterflies?

Eastern monarchs are facing similar problems to other monarch butterflies: large-scale habitat loss, lack of native milkweed and  nectar plants, pesticides, and extreme weather caused by climate change. 

Habitat loss is caused by all sorts of land development that reduce the size of or replace natural areas, such as expanding cities & suburbs or converting fields into farmlands or industrial spaces. This gives monarchs fewer milkweed plants to lay eggs on, fewer native wildflowers to feed on as adults, and fewer trees to roost in at night during their migration. 

At overwintering sites in Mexico, logging and deforestation has damaged the forests of oyamel fir trees that eastern monarchs need to survive the winter. These forests protect overwintering monarchs from extreme weather such as the winds, rain, and low temperatures of winter storms. But as oyamel fir forests are cut down, this concentrates the monarchs in smaller areas, which makes them more vulnerable to storms and other disturbances. 

Monarchs, and all butterfly species for that matter, also need protection from pesticides. Insecticides can hurt or kill monarch caterpillars and adults, from being sprayed directly or potentially just from landing on a plant that was sprayed weeks ago. In addition, herbicides are often used to kill the milkweed and other native flowering plants that monarchs rely on. Many farms across North America use herbicide-resistant crops as a way to easily spray more herbicides, leading to fewer and fewer patches of milkweed and other native plants where there used to be plenty. 

Migratory monarchs are also impacted by the physical dangers they encounter on their migratory journeys. Monarch “flyways” — the routes they take to their overwintering sites—  cross several large highways and monarchs are susceptible to car strikes, particularly in areas where they are converging in large numbers.

Why are milkweed and other native plants so important for monarchs?

Milkweeds serve as host plants for monarch caterpillars. Nectar sources provide nectar that appeals to monarch adults. Many milkweed species serve double duty by also producing flowers with nectar that is sought after by monarch adults. During fall migration, abundant nectar sources are important to fuel their long flights.

Four easy ways you can help eastern monarchs

Wherever you are, there are many things you can do right now to make monarchs’ lives easier:

• Plant milkweed and monarch nectar plants that are native to your area! You don’t need to convert your whole lawn (although that’d be great!); even just turning 10% of the space into a native plant garden can make a huge difference.  (But avoid tropical milkweed; it's non-native, and bad for monarchs).
• Protect monarchs from pesticides! Avoid using pesticides whenever possible, and follow our advice for buying pesticide-free plants.
• Join a community science project! Xerces and other groups still have a lot of questions about monarchs that will inform our conservation work. See a monarch getting nectar from a flower? Take a photo and send it to our Monarch Nectar Plant Database! You will be helping us make our Monarch Plant Guides for each region. The International Monarch Monitoring Blitz is another great project to try out.
• Help other people learn about monarchs and how they can get involved! Talk to your friends, neighbors, and online communities. Every little bit of awareness and habitat helps, and for monarchs to really recover, they will need a lot of us to pitch in.