Friday, April 15, 2011

Toxic Beauty


One never fails to notice the beauty and innocence of a monarch butterfly. It does not try to hurt anything or is associated in any negative way, just something pretty to look at on a beautiful spring day…right? It will come as a shock to you when I describe how it is filled with toxic substances that will harm animals that try to eat it!

The monarch butterfly (Danaus plexippus) in the larval stage feeds on species of milkweed (Asplepias) [2]. It is the host plant for almost the entire monarch’s lifecycle. Milkweed is a type of sprouted plant that contains “milk” which is white latex. The “milk” is acidic and poisonous to many animals. The monarch from larval stage to caterpillar does not eat anything but the milkweed milk, meaning that it absorbs these compounds into its body and will store them all throughout its lifetime. This will cause harm to any animal trying to eat the not so innocent butterfly. They are found all throughout the world beginning in June till about late summer or early fall. The monarch butterfly is one of the fewbutterflies that migrate south during the cold weather [5]. The interesting ability to use the milkweed to synthesize the bitter substance within has allowed the monarch butterfly to be one of the most recognized and successful animal species.

Description of the Relationship

The monarch is fully dependent on the milkweed, where one finds a monarch butterfly there will most certainly be milkweed around. This relationship with milkweed is unique, not every butterfly can synthesize and take in the milkweed poison. This is because milkweeds contain alkaloids called cardiac glycosides, which make the plant taste bad and the monarch poisonous [4]. Milkweed belongs to the genus Asclepias, there are many species within Asclepias that monarch’s use. The common milkweed is Asclepias syriaca. The monarch butterfly belongs to Danaus plexippus. A female monarch butterfly will lay its eggs on the underside of the leaf of a sprouted milkweed plant. The eggs are about the size of a typed period on this page! About three to ten days later a tiny caterpillar with yellow and black stripes will hatch. It will eat the egg casing and then eat large amounts of milkweed very quickly. As the caterpillar grows from eating, it has to shed its skin four times to allow for that rapid growth to occur. At the final stage the larva is about 5 cm long; this growth rate is huge! Imagine a human baby growing that fast…it would be bigger than a school bus in the same amount of time [8]!

After it eats for about two weeks non-stop it looks for a place to pupate. It hangs down in a “J” under a branch or ledge. After about one to two days inside, the caterpillar will change into a green chrysalis with gold spots. Then in about one to three weeks the chrysalis will become transparent with the case thinning out. The monarch butterfly will emerge out. The monarch butterfly will have strong wings because they migrate, so in the beginning as a butterfly, large amounts of liquid is pumped from the body to the wings. The male butterflies are distinct from the females in that they have a black spot in the third vein of each hind wing. This spot produces perfume that attracts a female. Now as an adult butterfly, they can drink any type of nectar not just specifically milkweed, whereas in the larval stage they can only feed on milkweed. The butterfly in the larval stage took in enough milkweed poison to last for its entire life. Therefore, birds and other predators have learned to stay away from monarch butterflies [2,8].

Looking from the perspective of the milkweed, the monarch help the plant pollinate its seeds. This serves as a benefit to the plant, therefore making this a mutualistic relationship. The monarch benefits with food and shelter while the plant receives pollination. However, pollination can occur with other types of butterflies, bees, ants, and moths, not just specifically monarch butterflies [4]. But monarch butterflies cannot feed at the larval stage on anything but the milkweed. The monarch, therefore, is in an obligated relationship with the milkweed.

The need for continual change for survival and reproductive fitness as the Red Queen Hypothesis states has allowed an interesting co-evolutionary relationship to develop between the monarch butterfly (Danaus plexippus) and viceroy butterfly (Limenitis archippus). The evolutionary arms race has created a mimicking version of the monarch to reap the benefits of the appearance. Birds or any other predator preying on butterfly know to stay away from the monarch. The vibrant appearance gives the predator a hint to stay away. This type of mimicry is called batesian mimicry. The harmless species (viceroy butterfly) has evolved to imitate the warning signals of a harmful species (monarch butterfly) [3]. Take a look and see if you can tell the difference.

Can you see the difference?

(On the left is a viceroy butterfly, the right is a monarch butterfly)

Cost/Benefits Analysis

Monarch butterfly’s entire existence depends on the milkweed. If one day there were to be no milkweed the caterpillar would have no food and would not survive. This dependency can constitute as a cost to the monarch, this is one of the reasons why the monarch population is dwindling. The use of transgenic crops is killing the milkweeds and is not allowing the monarch caterpillar to have a place for food and protection [2]. Unless some evolutionary adaption occurs where the monarch larvae have another source of food, these butterflies may not be around anymore.

There are also some scenarios where there is a cost on the milkweed due to the caterpillar. The caterpillar is harming the plant when feeding at large amounts on the leaves. However, this does not reduce the ability of reproductive success of the plant and the plant has adapted the ability to repair itself. Although some argue that the repairing still sets a cost on the plant where energy is lost. The other side of the situation is the monarchs actually help the plant pollinate [1]. Both the milkweed and butterfly are benefiting from each other. Whichever the case the relationship will only continue to exist as long as the milkweed plants are around.

So beware to predators preying on the beautiful and “naive” monarch butterfly! Consuming will cause harm!


[1] Agrawal, Anurag. "Researcher Shows Evolution of Milkweed Defense System." - Science News, Technology, Physics, Nanotechnology, Space Science, Earth Science, Medicine. Web. 13 Apr. 2011.

[2] Ames, Doris. "Milkweed and Monarchs." Native Orchid Conservation Inc. Web. 13 Apr. 2011

[3] "Co-evolution and the Evolutionary Arms-race." WVU Parkersburg. Web. 13 Apr. 2011

[4] Froehlich, Shirley. "Milkweeds & Monarchs." Prairieoriginals. Web. 13 Apr. 2011

[5] "Monarch Butterfly and Milkweed." Monarch Butterfly and Milkweed. 12 Oct. 2001. Web. 13 Apr. 2011.

[6] "Monarch." The World of the Butterfly | Why Butterfly. Web. 13 Apr. 2011.

[7] Pratt, Jim. "Life Cycle of the Monarch Butterfly." Zimbio. 30 June 2010. Web. 13 Apr. 2011.

[8] "The Monarch Butterfly." Kidzone - Fun Facts for Kids! Web. 15 Apr. 2011


  1. You mentioned that the butterfly lays its eggs on a milkweed plant. Does the butterfly have any modification or anything that allows it to detect that the plant its laying it eggs on is a milkweed plant. Also, is there not any other way that the butterfly could survive without the wilkweed plant. By this I mean can it not eat anything else to live?

  2. Prerana: The monarch uses a combination of both visual and chemical cues to find milkweed. Once they land on a plant, they use sensory organs that are on their feet and heads to tell them if it is a milkweed. They also use these sense organs to determine the quality of the milkweed.
    No, they cannot survive without milkweed plant, in the larval stage this plant is necessary for development but once they have become a butterfly they can feed off different plants. There are many species of milkweed and monarchs can eat most of them. They also eat a plant that is not milkweed, however, it is in the same family. It is called Cynanchum laeve, or sand vine.

  3. The Red Queen Hypothesis does help create new evolutionary species. You mentioned that there is a harmful bird just like the viceroy butterfly. How does the monarch butterfly harm the plant or other species?

  4. This comment has been removed by the author.

  5. The viceroy butterfly is not harmful when eaten and it is a butterfly not a bird. The viceroy butterfly does not eat milkweed as a larva so does not have the toxins in the body. It looks like the monarch but does not actually harm birds that eat it, but because it looks like the monarch the birds will stay away from eating the viceroy butterfly. The birds have learned to recognize the colors of the monarch because they know the monarch tastes bad.
    Does this answer your question?

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