Monarch Butterfly

The monarch butterfly, called scientifically Danaus plexippus, or “sleepy transformation” in reference to their ability to metamorphosize from a striped yellow, black and white caterpillar to an orange and black butterfly. Monarch butterflies were originally found in both North and South America, but they are now no longer found in South America. North American monarchs can be found in two groups: the western monarchs and the eastern monarchs. Smaller populations can be found in Hawaii, Portugal, Spain, Australia, and New Zealand.

Females lay their eggs individually on a milkweed plant, stuck on with homemade glue. Over 2-5 weeks, they will lay 300-500 eggs. The eggs hatch into caterpillars after a few days, and then focus on growing. They only eat milkweed, which is toxic, but monarchs have evolved to tolerate it and in result are poisonous to eat. Their black and orange distinctive coloring warns predators of this, which some other species of butterflies have used to their advantage by mimicking the pattern. In two weeks they spin chrysalis around themselves, entering the pupa stage, and a week or two later emerge as an adult monarch butterfly. Adults feed on nectar from many native plants including milkweed. They will only live a few weeks, except for the migratory generation born in the fall that will live up to 8 months.

Monarch butterflies are famous for their migration, but not all monarchs migrate. Monarchs born in the spring and early summer don’t migrate, instead immediately reproducing. If they are born in the late summer or fall, they will migrate to escape the coming winter. Western monarchs lay their eggs in the warmer months west of the Rocky Mountains and before the winter migrate to southern California, while eastern monarchs breed in the Great Plains and Canada in the warmer months and in Central Mexico overwinter. Some may fly 3,000 miles.

Once the eastern monarchs reach Mexico they wait out the winter in oyamel fir trees. After winter they begin flying north, but they will never get back to Canada. Instead they stop along the route and lay eggs, and their children will continue the journey. It may take 4-5 generations to actually get to Canada. Western monarchs, on the other hand, head to the coast of California to wait for winter to end. In spring they head back to various other western states. Monarchs make this long journey by using the sun as well as their own magnetic compass to navigate. They also have genetically efficient muscles that help them make such a long flight. Another fascinating feature of this butterfly is how they communicate. They talk using scents and by releasing chemicals, and their colors convey their poisonousness to predators.

Western monarchs have declined around 99% since the 1980s; eastern monarchs by 80%. The main reason for the decline of Monarch populations is loss of milkweed. Milkweed is both the only plant monarch caterpillars will eat and, because of this, the only place monarchs will lay eggs. Milkweed has been purposefully removed from agricultural fields, where it used to be commonly found, and has also accidentally been removed because of increased pesticides and mowing. Climate change affects monarchs because they are sensitive to temperature and weather changes, which might affect biological processes like knowing when to reproduce and migrate. Extreme weather events caused by climate change also affects the availability of milkweed and their habitats in the winter. Temperature changes may also directly kill monarchs before they reproduce. Urban development also fragments and damages their habitat, and insecticides kill the monarchs while pesticides kill their food.


Image courtesy of The National Wildlife Federation

Loss of habitat and climate change also affect many other species, so by helping the monarch butterfly we also help other species including milkweed species.

If you live in Canada, the United States, or Mexico, the main way you can help is by planting native species, especially milkweed. Monarchs coevolved with native species, so both live best with the other. Unfortunately, to try and attract monarchs, some people have planted tropical milkweed, or Asclepias curassavica, an ornamental milkweed found in nurseries that is native more in South America. It is easy to grow and attracts monarchs so they can lay their eggs, but unfortunately, tropical milkweed when planted in non-native areas may sometimes encourage monarchs not to migrate and lay eggs through the winter. This exposes them to diseases and complications they usually avoid by migrating. Please make sure the milkweed you are planting is native to your area. You can also participate in the Monarch Watch Tagging Program by announcing any tagged monarchs you find and tagging your own, which helps scientists learn more about monarch migration. Their website is linked below.

true-wildlife-ishikawa's frog-3-1.jpg

Image courtesy of Melissa Scott

Image courtesy of Edward Parker, World Wildlife Fund

Image courtesy of Ingo Arndt

Image courtesy of Joel Sartore

Further Resources

Works Cited

Monarch butterfly. (2020, April 01). Retrieved October 24, 2020, from

Monarch Butterfly. (n.d.). Retrieved October 24, 2020, from

Monarch Butterfly. (n.d.). Retrieved October 24, 2020, from

Image courtesy of Morgan Heim of Day's Edge Productions