Boulder’s Mighty Migrator

This is the tale of the Monarch butterfly (Danaus plexippus) whose existence depends on a 1800 mi / 2900 km migration from Boulder to a specific grove of oyamel trees in Angangueo, Mexico. These mighty migrators employ a complex but fragile lifecycle which permits them to overwinter where they can survive the cold while still getting access to our essential northern milkweed plants in the warmer months. This is the story of a unique and environmentally vulnerable lifecycle.

Monarch Butterfly

Boulder Monarch Butterfly (Danaus plexippus) Fueling Up on Verbena Nectar August 31, 2017

The tale begins at underside of the leaves of the Milkweed Plant (Asclepias) when the Monarch butterfly lays her eggs.

Milkweed Plant

Milkweed Plant with Seedpods

Milkweed With Monarch Eggs

Boulder Milkweed Leaf with Monarch Eggs

As with all butterflies, the eggs hatch into baby caterpillars, also called the larvae, in about 4 days. The caterpillar eats the milkweed leaves which fuel rapid growth. When Monarch larvae ingest milkweed, they also ingest the plants’ toxins, called cardiac glycosides. They sequester these compounds in their wings and exoskeletons, making the larvae and adults toxic to many potential predators. Vertebrate predators may avoid Monarchs because they learn that the larvae and adults taste bad and/or make them vomit. The bright coloration of the larvae and mature butterflies is a warning to predators.

After several molts and about two weeks the caterpillar will be fully-grown and will find a place to attach itself so that it can enter it’s protective pupa and start the 10 day process of metamorphosis to transition from caterpillar to butterfly.


Caterpillar Chrysalis (

The Caterpillar Enters Metamorphosis (photo from

Monarch Emerges (

The Butterfly Emerges (photo from

The monarch butterfly will emerge from the pupa and fly away, feeding on flower nectar and mating during the two to six weeks of its lifespan. This first generation monarch butterfly will then die after laying eggs for generation number two. This cycle will repeat for 4 generations until fall when an ancient call to migrate is detected.

The forth generation of monarch butterflies does not die after two to six weeks. It will not mate and it produces the larger wings necessary to fly and glide for its trip to Mexico. This generation of monarch butterflies migrates to warmer climates and will live for six to eight months until it is time to start the whole process over again. By the end of October, the population of monarchs east of the Rocky Mountains migrates to the sanctuaries of the Mexican Mariposa Monarca Biosphere Reserve within the in oyamel fir tree forests in the Mexican states of Michoacán and México. How they navigate is still a mystery but they must have evolved a map and navigational system to guide them.



Monarchs Flocking on Oyamel Fir Tree (


The insects will rest in a dormant stage called diapause for 3-4 months until they are cued to eat, mate and to start the multigenerational flight north to lay eggs on milkweeds and start the process over again.

This delicate lifecycle was forged by natural selection through the ages but it represents prehistoric conditions that may no longer exist. Climate change and habitat destruction have landed the Monarch on the threatened list. The Mexican government has established a biosphere reserve to protect the fir tree forests but there is still illegal tree cutting. Milkweed is poisonous to livestock so farmers and homeowners spray herbicides on milkweed plants in the U.S., and climate change is messing up the temperature cues and flyway conditions which drive the great migration.

In an attempt to counter two decades of destruction, the U.S. Fish and Wildlife Service launched a partnership with two private conservation groups, the National Wildlife Federation and the National Fish and Wildlife Foundation to  grow milkweed in the hopes of saving the monarchs.

Here in Boulder we can help by planting and helping milkweed to propagate. I went hiking with a naturalist in August 2016 and when he spotted a patch of milkweed we all blew on the open pods to help spread their seeds. You can also help by adopting a monarch.


Milkweed Blowoff-1.jpg

Milkweed Blow-off

It appears that climate change tends to leave Monarchs stuck in Northern climates. Our climate is changing faster than these fragile creatures can evolve. Monarch butterflies are thriving in 2019 but it could be the last.

This is a tale about the preservation of a unique butterfly but it may also be a warning about our fragile survival as well.



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2 Responses to “Boulder’s Mighty Migrator”

  1. Susan Litt Says:

    nicely done as always


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