By Lisa Mason, CSU Extension Horticulture Specialist, Entomologist

My Heart is a Flutter: Experiencing the Monarch Butterfly Migration in Mexico

Great news – the monarch numbers are up!

Monarchs in Mexico

Monarch butterflies are an iconic species in North America. These bright orange and black butterflies are famous for their 3,000+ mile migration. How do these small creatures navigate such an extensive journey every year? This question has fascinated scientists, and while we have theories on how they migrate, we still do not know the answer.

In January, I had the privilege of traveling to Mexico to visit the overwintering population of monarch butterflies. We flew to Mexico City and drove over three hours to the El Rosario Monarch Butterfly Sanctuary in the Michoacán state. At the reserve, we rode on horseback up the steep mountain and then hiked to the butterflies. The butterflies live between 10,000 – 11,000 feet in elevation. The temperatures were in the 50s Fahrenheit.

When we arrived, we had to stay completely silent to not disturb the butterflies. We could HEAR the butterflies flying! The number of butterflies sounded like a light rain or breeze through the forest. We had to watch every step because there were butterflies on the ground. They can only fly when the temperatures are 55°F or warmer. It can take a butterfly a few days to crawl from the ground back up to a tree where it will be safer from predators.

Many bird species prey on other insects instead of monarch butterflies because their bodies contain toxins from feeding on milkweed plants as caterpillars. However, we came across dead butterflies that were missing their abdomens. Turns out, two species of birds readily prey on monarch butterflies: Black-backed orioles and black-headed grosbeaks. These savvy birds have strategies to avoid the toxins. For instance, the grosbeak only eats the abdomen of male butterflies because they carry fewer toxins than female butterflies.

The predominant tree species in the forest is the Oyamel fir or sacred fir (Abies religiosa). These forests provide the perfect microclimate for the butterflies to overwinter. The dry forests reminded me of Colorado’s forests in the summer, with one key distinction: The diversity and abundance of wildflowers in the forests far exceed wildflowers found in Colorado’s forests. For example, I was amazed by the diversity of Salvia (Salvia spp.) plants. Salvia plants in Colorado are cultivated for consumers. They are a popular choice at garden centers. These beautiful perennials are great pollinator-friendly additions to waterwise landscapes. iNaturalist has over 250 species of Salvia documented in Mexico. The biodiversity of wildflowers was incredible to observe.

The butterfly sanctuary is managed by the indigenous communities. They deeply care about the monarch butterflies and the land where they overwinter. The butterflies arrive in Mexico around Dia de los Muertos, the Day of the Dead (November 1) when souls of the ancestors come to visit the living. They believe the monarch butterflies are souls of past loved ones visiting.

Migration

Adult butterflies only live two to five weeks. The only exception is the overwintering generation of monarchs that can live up to nine months in Mexico. Once spring arrives, this overwintering generation will migrate north to Texas and surrounding areas. The females will lay eggs for the next generation. Once the next generation becomes adult butterflies, they continue to migrate north. After a few weeks, they will lay eggs for another generation further north. Typically, monarchs will have two to three generations throughout the summer season. Once fall arrives, the fourth generation, also known as the overwintering generation, will begin to migrate south back to Mexico. How does each generation of monarch know how to navigate migration? For other migratory species like Swainson’s hawks, they follow their parents and large groups of hawks to the overwintering grounds in Argentina. Scientists are still researching how monarchs are able to migrate to the same location every year. Recent research suggests they use a combination of the sun’s position in the sky, landmarks like mountains, and an internal magnetic compass. Genetics may also play a role in the ability to navigate.

Monarchs have two migratory pathways in North America. The eastern monarch population migrates from Mexico up north through the Midwest and eastern US. The western monarch population migrates from the Pacific Coast of California to the states west of the Rocky Mountains. While Colorado is not one of the main migratory corridors, you can still see monarchs throughout our state.

Monarchs in Colorado

Identification

While the bright orange butterflies can be hard to miss, Colorado has a variety of orange-colored butterflies. You can identify a monarch butterfly by the black veins on the wings in addition to the bright orange color. They also have white spots on the edges of the wings. The wingspan usually ranges from 3-4 inches long. You can identify if the butterfly is a male or female by looking at the hind wing. If the butterfly is a male, it will have one black spot on each hind wing along one of the center veins. If the butterfly is female, she will not have a spot.

Monarch caterpillars have contrasting black, yellow, and white stripes on their bodies. Caterpillar size varies depending on what instar or stage of growth the caterpillar is in.

Life Cycle

All butterflies, including monarchs, go through a lifecycle called metamorphosis that includes an egg, caterpillar, a pupa called a chrysalis, and an adult butterfly. When monarchs are caterpillars, their job is to feed as much as possible. They feed exclusively on milkweed plants (Asclepias spp.). Once they have fully grown, they will find a safe space to form a chrysalis. The chrysalis is a protective covering for the caterpillar while it transforms into a butterfly. It begins as a pale green color, then gradually turns black and orange as the butterfly gets ready to emerge. After emergence, the butterfly will soon search for a mate and the female will begin laying eggs on new milkweed plants.

Butterfly Mimicry

Other species that look similar to the monarch butterfly include the queen butterfly (Danaus gilippus) that hosts on milkweed and dogbane plants in the Apocynaceae family, and the viceroy butterfly (Limenitis archippus) that hosts on plants in the Salicaseae family including willows, cottonwoods, and poplars.

These three butterflies look similar for one reason: mimicry! The contrasting orange and black colors serve as a warning to predators that the insect may be distasteful and potentially toxic. Milkweed is full of a compound called cardenolides. When a caterpillar feeds on milkweed plants, the cardenolides stay in the body of the monarch which makes it distasteful and toxic to predators like birds. The predators learn to stay away from insects with the bright coloring.

For a long time, scientists suspected viceroy butterflies mimicked monarchs in a form of Batesian mimicry, meaning that the viceroy appeared toxic and distasteful to predators based on coloring and wing shape, but they were not actually toxic or distasteful. Further research indicates that the monarch, viceroy and queen butterflies may exhibit Müllerian mimicry, meaning all three can be distasteful or toxic to predators. More research is needed to fully understand this mimicry relationship between the butterflies because variations in the butterfly’s colors, wing shape, distastefulness, and toxicity vary among different regions and caterpillar host plants.

(left) A viceroy butterfly can be differentiated from a monarch by the black, circular line through the hind wing that is perpendicular to the other black veins. (right) A queen butterfly has more white spots on the hind wings than a monarch. Colors can be variable but often they are a darker orange color than monarch butterflies. Photos: Lisa Mason

Supporting Monarchs and Other Butterflies

Monarch butterflies continue to decline due to host plant habitat loss and loss of overwintering areas, climate change, and overuse and misuse of pesticides. In December 2024, the U.S. Fish and Wildlife Service submitted a proposal to list the monarch butterfly on the Endangered Species Act.

You can support monarchs and other butterflies by providing food, habitat, water and space in your landscape. Each species of butterfly has a different caterpillar host plant; for instance, monarchs rely on milkweed plants for caterpillar food. Adult butterflies will visit a variety of other flowers for nectar. Plant flowers that have different bloom times in order to have flowers all season. Butterflies need sunny areas and places to shelter from wind and weather. Planting a variety of trees and shrubs can help provide sheltering areas. Be mindful of pesticide use because they can harm caterpillars and butterflies.

Click here for more information on attracting butterflies to your landscape.

For more information on monarch butterflies, visit MonarchJointVenture.org.

To learn more about butterflies in Colorado, read this CSU Co-Horts blog post called The Fascinating Lives of Butterflies.