Are Monarch butterflies toxic?

Narveen Jandu
23 January 2012

Have you ever heard the saying: “You are what you eat”? This saying is especially true for Monarch butterflies.

Monarch butterflies have a fascinating life-cycle and undergo four stages of transformation. During these stages, their diet includes milkweed. Let’s explore the life of a Monarch butterfly and how their diet contributes to their toxicity.

Fast Fact: The scientific name for the Monarch Butterfly is Danaus plexippus.

The life of a butterfly begins as an egg. Eggs are laid by adult Monarch butterflies on the underside of milkweed plants and hatch a few days later as worm-like larvae, called caterpillars. After a couple of weeks, the caterpillar enters the pupa stage, also known as the chrysalis stage. This is the life stage where the caterpillar hangs upside down and spins a silk encasing around itself. After another couple of weeks, the beautiful orange wings of the Monarch butterfly emerge from this encasing. Its life cycle is complete!

As it turns out, milkweed is the only thing Monarch caterpillars can eat. Milkweed is a tall wildflower with a thick-hairy stem. It can grow to a height of three to four feet tall. The leaves of milkweed are big and broad – as big as the palm of your hand. The flowers are often whitish-pink to purple and are found at the top of the plant in a globe-like cluster.

Fast Fact: The best way to attract Monarch butterflies to your backyard is to grow milkweed.

A characteristic feature of this plant is the thick milky sap that runs throughout the stem and leaves. This milky sap contains chemical compounds, including alkaloids and cardenolides (or cardiac glycosides). When caterpillars feed on milkweed leaves, the cardenolides enter their bodies and are stored there until the adult butterfly emerges. The adult Monarchs have cardenolides in their wings and bodies.

Cardenolides are not harmful to Monarchs, but they can cause birds, for example, to vomit when ingested. If birds try to eat the caterpillars or the butterflies, they stop feeding because of the bad taste of the cardenolides.

Although Monarch butterflies are toxic because of their milkweed diet, this is a form of protection from predators.

Learn More!

Article first published March 15, 2011.

Narveen Jandu

Narveen is currently a Lecturer in Cell Biology and a Curriculum Fellow in Cancer Biology at Harvard Medical School. Prior to moving to Boston, she was a post-doctoral research fellow at Stanford University, where she was studying the interactive effects of gut microbes on immune cell homing and trafficking. Narveen received her PhD from the University of Toronto - her thesis was on the pathogenic effects of Escherichia coli O157:H7 on an innate immune signalling cascade of intestinal epithelial cells. Prior to her PhD training, she completed a Master's degree at McMaster University and she completed her undergraduate degree at Wilfrid Laurier University.

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