How milkweed plants defend against monarch butterflies

Anurag looking at a monarch caterpillar on a milkweed plant.

The activities are as follows:

For millions of years, monarch butterflies have been antagonizing milkweed plants. Although adult monarchs drink nectar from flowers, their caterpillars only eat milkweed leaves, which harms the plants. This is an ecological interaction called herbivory. The only food for monarchs is milkweed leaves, meaning they have evolved to be highly specialized, picky eaters. But their food is not a passive victim. Like most other plants, milkweeds fight back with defenses against herbivory.

Monarch butterflies lay their eggs on the underside of milkweed leaves. After eggs hatch, caterpillars start to feed and quickly meet the plant’s first defense. Milkweed leaves are covered in thousands of tiny hairs, called trichomes, that the caterpillar needs to shave off before they can take a bite. The next challenge happens when the caterpillar takes a bite of the leaf. They get a mouthful of latex, which is sticky like Elmer’s glue. The caterpillars have to be very careful in how they feed. They cut the veins in the leaf to drain out the latex before continuing to feed on the leaf. Even after monarch caterpillars make it past the trichomes and latex, there’s another defense they need to overcome. Milkweed leaves have chemical toxins called cardiac glycosides, which are poisonous to most animals. As they feed, monarchs eat some of this poison.

Anurag is a scientist who has long been fascinated by plants and their defenses. He thinks this comes from the fact that his mother was such an avid gardener. She would grow food, such as peppers, squashes, and tomatoes. He looks back and has memories that are associated with garden plants and their defenses. For example, he remembers eating a bitter cucumber as a kid and spitting it out. He also can still recall the bitter aroma on his hand after brushing against the sticky tomato leaves. And plants that are tough and stringy, like kale, are not as tasty to eat. These traits are examples of plant defenses in action, making them harder or less enjoyable to eat, reducing herbivory.

Anurag collecting data on milkweed plants.

Anurag first started studying milkweeds 20 years ago, based on a recommendation from a friend. His friend told him of the bitter, sticky, and furry leaves that were treasured by the monarch butterfly caterpillars. This led him to study the paradox of coevolution. The milkweed and monarch have such a tight relationship that over time, milkweeds have evolved multiple ways to defend themselves against their herbivores. In response, monarchs have evolved to overcome those defenses because they need to eat the milkweed. This arms race may continue to shift back and forth over the course of evolutionary time.

This back-and-forth battle between caterpillar and plant intrigued Anurag. He wanted to know whether milkweed’s defensive traits are still effective against monarchs, or have monarchs evolved in ways that make them unaffected by the defenses? Because each defense trait might be at a different phase in the coevolution process, perhaps some would be effective defenses to herbivory, but others would not be effective. He predicted that monarchs would be harmed by all three milkweed defense traits (trichomes, latex, and cardiac glycosides), but that some would cause more harm than others.

To test his ideas, Anurag and his collaborators grew monarch caterpillars on 24 different North American milkweed species. They put a single newly hatched caterpillar on each plant and had five replicate plants per milkweed species. They recorded each caterpillar’s growth over the course of 5 days to see how healthy it was. They also measured the amount of trichomes, latex, and cardiac glycosides in each plant to determine their level of defense. Once they had their data, they looked for a relationship between caterpillar growth and plant defense traits to determine which made the best plant defenses. The better the defense, the less caterpillars would grow.

Featured scientist: Anurag Agrawal (He/Him/His) from Cornell University

Flesch–Kincaid Reading Grade Level = 8.5

Additional teacher resources related to this Data Nugget include:

  • Anurag has other examples, data, and related stories in his book: Monarchs and Milkweed, which is written for budding scientists and interested naturalists:
  • Students can learn more about Anurag, his research, and his lab at his website: which includes blog posts about monarch conservation, the community of insects on milkweed plants, videos of talks and presentations, and other things related to his research and teaching at Cornell University.
  • A scientific article based on this research: Agrawal, A. A., Fishbein, M., Jetter, R., Salminen, J. P., Goldstein, J. B., Freitag, A. E., & Sparks, J. P. (2009). Phylogenetic ecology of leaf surface traits in the milkweeds (Asclepias spp.): chemistry, ecophysiology, and insect behavior. New Phytologist183(3), 848-867.
  • Learn more about Anurag and his research in this YouTube video!


  1. Zachary Smith says

    What about the fertilizer effect of the caterpillar poop for the plant? Also, when the caterpillars eat most the leaves, the aphid infestation is removed, allowing for new healthy leaf growth.
    Has anyone studied these effects?
    I raise monarchs and milkweed and would like to know.
    Thank you

  2. Robert Schaffrath says

    Looking at the relationship between just milkweed and monarch caterpillars, it is definitely a one-way relationship that does not benefit the milkweed plant. I gathered up some milkweed seeds three years ago and this third year was the first time that three of the surviving plants were large enough to attract female butterflies and support caterpillars (who managed to strip most of the leaves from two of the plants). I did wind up with five butterflies though and was able to watch two of them emerge from their chrysalis state. The plants are still not mature enough to flower though and will take at least another year or two grow larger.

    However, if you throw humans into the equation, then there is IMHO a benefit to the plant. This would be similar to the relationship humans have with corn. Corn is no longer able to reproduce on its own due to millennia of breeding by humans. Without human intervention, it would most likely die off according to a paper published several years ago. Instead, a symbiotic relationship has developed between humans and corn where humans grow it and in return provides nourishment to humans. Likewise, humans like monarch butterflies. In order to attract butterflies and grow caterpillars, humans plant milkweed just like I did. One finds milkweed planted by people in many cultivated gardens and parks for the sole purpose of attracting and growing butterflies. If humans had no interest in butterflies, no one would be planting milkweed and there would be much less of it than there is. Milkweed reproduction is supported by humans who want to raise butterflies. So the milkweed plant, in a roundabout way, does benefit from being a food source for monarch caterpillars that in turn encourages humans to grow it.

    • Milkweed’s relationship to man precedes the more recent “butterfly garden” trend. During WWII, children were dispatched to roadsides and meadows to gather milkweed pods in gunnysacks. The seed filaments were separated at industrial looms to make parachutes for armed services. Whether this ultimately helped milkweed populations, I don;t know. I suspect it had the opposite effect in the regions where the practice was common.

  3. Hi Alice, the milkweed plant does not appear to benefit in any way from the monarch caterpillars that eat them. As an adult butterfly, monarchs visit flowers, but they are generally poor pollinators of milkweed (large bodied bees and wasps do the bulk of pollination). So, yes, the benefit seems one-way, both as food and chemical protectants for the monarch! Thanks for your question.

  4. Does the milkweed plant benefit in any way from being the monarch’s host plant? or is all the benefit one-way, namely, the protective toxic effect the Monarchs acquire thru consuming the milkweed?

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