Are Monarch Butterflies Poisonous?

Written by Krishna Maxwell
Updated: September 30, 2022
© iStock.com/Christophe Merceron
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Key Points

  • Butterflies can’t bite or harm humans. That said, they are toxic to anyone who eats them though they would have to eat a number of them.
  • Birds or other animals who eat monarch butterflies become ill. This discourages them from eating any more of them, which is the only for of defense the butterflies have.
  • The butterflies don’t produce this poison themselves. Instead, their toxic bodies come from the diet of milkweed they consume in the larvae stage.

Monarch butterflies are known for their beautiful and bold colors, decorating their environment with orange and red hues that are enjoyable to watch float around. Despite their beauty, they have a very effective way to protect themselves from predators — poison. So, are monarch butterflies poisonous? These insects are highly poisonous and dangerous, helping them to protect the species with a special toxin. However, they don’t produce this poison themselves. Instead, their toxic bodies come from the diet they consume in the larvae stage. Read on to learn more about the poisonous monarch butterfly.

What Makes Them Poisonous?

The simple answer to this question comes down to a major plant in the diet of the monarch butterfly caterpillar — milkweed. Milkweed is primarily known for its role in the life cycle of a monarch butterfly, found primarily in the Americas. Though milkweed is an essential part of the monarch butterfly’s diet, every single part of the plant has cardiac glycosides.

Exposure to glycosides can lead to uncomfortable side effects for most creatures. The most common reactions to milkweed include nausea, diarrhea, weakness, confusion, seizures, changes in heart rhythm, respiratory paralysis, and, in extreme cases, death. If you touch your eye or skin with milkweed, they can become incredibly irritated. Despite these effects, the monarch butterfly has no bad reaction to it.

As the toxins of the milkweed leaves are processed through the digestive system of the monarch butterfly as a caterpillar, they accumulate. The toxins survive through the change from a caterpillar to a butterfly, making it as dangerous to predators as milkweed already is.

Monarch butterflies are toxic because of the toxins inside the milkweed they consume.

©Kate Besler/Shutterstock.com

What Toxins Do They Use?

The toxins inside milkweed are called cardenolides or cardiac glycosides, which are the reason that monarch butterflies are toxic. Milkweed is the sole source of nutrients for monarch butterflies since they have gradually evolved in such a way that they are immune to toxicity. Their bodies produce a protein that the cardenolide toxins don’t interfere with. Though all animals have this sodium pump, eating milkweed would cause them to go into cardiac arrest.

According to scientific studies, scientists believe that there is a mutation in the DNA of monarch butterflies that prevents the cardenolides from binding to it. Without binding to a particular amino acid, monarch butterflies are not at risk when exposed to the toxicity of milkweed. They end up storing it in their body, allowing it to become an integral part of their wings and the rest of their body after they make the transition from caterpillar to butterfly.

When the caterpillar stores the toxins, they use them to their advantage against predators. The toxins cannot be made by the larvae themselves since the caterpillar is not technically poisonous. If it consumed any other plant for its main diet, it wouldn’t end up being dangerous to predators. It is only with the accumulation of these glycosides that the monarch butterfly ends up with any defense against predators. Unfortunately, that doesn’t protect them entirely.

The result of these toxins is primarily an unpleasant taste, but it can also wreak havoc on the digestive system of the animals that eat them. Though it is not deadly to all predators, it is enough that most animals will even stay away from lookalike butterflies out of fear that they will taste the same.

Who Should Avoid Them?

The entire point of the monarch butterfly’s poison is to deter predators, like birds and small mammals. Any animal that typically eats insects as a regular part of their diet knows to look out for the warning colors, which are an indication that the insect has poison. Predators quickly become sick after eating the monarch butterfly, which is why many predators go after the eggs or the caterpillars instead.

Though it is not deadly for humans to eat these butterflies, the toxins will also cause digestive discomfort for them. Ultimately, as beautiful as this creature may be, their bold colors are a sign for all predators to stay away.

Surprisingly, the bad taste of the toxins isn’t enough to deter all predators from eating the monarch butterfly. Some of the natural enemies of this insect include birds and wasps, though the parasites that accumulate in the monarch’s body also put them at risk.

Milkweed is the only plant that monarchs can raise their young on and it is becoming more and more rare. If you would like to help ensure the survival of the monarch butterfly, the best thing that you can do is to plant milkweed or avoid cutting it down when. While milkweed is toxic, it is not dangerous to be around. The sap can cause eye irritation, but it is a thick, milky-white fluid that is very easy to see and avoid. There are no other animals who will eat milkweed as it has an extremely unpleasant taste. It is not a concern to have in yards or pastures all animals other than monarch butterflies avoid it.

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Monarch Butterfly
Monarch Butterfly
© iStock.com/Christophe Merceron

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About the Author

Krishna is a lifelong animal owner and advocate. She owns and operates a small farm in upstate New York which she shares with three dogs, four donkeys, one mule, and a cat. She holds a Bachelors in Agricultural Technology and has extensive experience in animal health and welfare. When not working with her own animals and tending her farm, Krishna is helping other animal owners with behavior or management issues and teaching neighboring farmers about Regenerative Agriculture practices.

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