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Monarch caterpillars turn milkweed toxin into a survival tool

Monarch butterflies are known for their spectacular migrations, but their journey begins on a surprisingly toxic menu. The monarch caterpillars munch on the leaves of milkweed, a plant notorious for its milky, white latex – a sticky sap packed with toxins.

But how do these tiny creatures avoid getting poisoned? The answer lies in their metamorphosis of taste and a clever trick they develop as they grow.

Journey of young monarch caterpillars

When a tiny monarch caterpillar emerges from its egg, milkweed is its only food source. However, the milky sap, or latex, is a deterrent for most animals. It’s like a built-in security system for the plant.

Young caterpillars inherit a clever way to avoid the toxic substance. They act like picky eaters, carefully nibbling on the leaf and strategically avoiding the veins where the latex flows. In some cases, they might even make tiny cuts to drain the sap before feasting on the “safe” parts of the leaf.

The study by a trio of German entomologists from University of Hohenheim has revealed an unusual transformation that occurs as these caterpillars grow.

Transformation of monarch caterpillars

Scientists closely followed monarch caterpillars from hatching all the way to when they formed their chrysalis. They found something unexpected: once the caterpillars reached a certain size, they stopped carefully avoiding the latex on the milkweed plant and instead actively ate it.

The change in behavior is a critical turning point in their development. They go from being weak and defenseless to strong and protected. This surprised the researchers and challenged what we previously thought we knew about the connection between monarch caterpillars and milkweed.

To understand this better, the team gave the caterpillars small amounts of the toxin-filled latex directly. Instead of avoiding it, the caterpillars ate eagerly. In the words of the researchers, observing the caterpillars’ behavior was “like watching a cat lap up milk.”

Monarch caterpillar’s metabolic detoxification

The secret to the monarch caterpillar’s amazing transformation is its ability to handle the milkweed’s poisons. These poisons, called cardenolides, are deadly to most animals because they mess up the balance of important salts in their bodies, leading to death.

But as the monarch caterpillars grow older, they gain the ability to tolerate these poisons in a process called “detoxification” or “metabolic detoxification.” This special skill lets the older caterpillars keep the poisons inside of them, turning what could have been a danger into a powerful weapon against predators.

This clever trick protects monarch caterpillars from predators as they grow and eventually become butterflies.

Study significance

The monarch caterpillar’s life cycle is an excellent example of how creatures can adapt and overcome challenges. By finding ways to turn a problem into an advantage, the caterpillar teaches us about resilience in the face of difficulty.

More about monarch caterpillars

Monarch caterpillars are well-known for their distinctive black, white, and yellow striping. They go through several stages, known as instars, during which they grow rapidly before pupating into the adult butterfly. Here are some key points about Monarch caterpillars:


Monarch caterpillars feed exclusively on milkweed plants. The milkweed provides all the nutrients the caterpillars need to grow and also supplies them with cardenolides, toxic compounds that make the caterpillars (and the adult butterflies) poisonous to most predators.

Growth and development

Starting as tiny eggs laid on the underside of milkweed leaves, once hatched, the caterpillars go through five growth stages or instars. They shed their skin at each stage. 

The entire process from egg to chrysalis can take about two weeks, depending on temperature and food availability.


Monarch caterpillars are easily recognizable by their bright stripes. These colors are thought to be aposematic, or warning colors, indicating their toxicity to predators.

Role in the ecosystem

As part of their life cycle, monarch caterpillars play a critical role in their ecosystem, particularly in pollination. Their feeding activity on milkweed can influence the health and distribution of these plants. 

Furthermore, as a food source for some predators that can tolerate their toxins, they contribute to the biodiversity of their habitats.


While the caterpillars themselves do not migrate, they are a crucial stage in the life cycle of the monarch butterfly, which is famous for its long-distance migration. 

The butterflies migrate from North America to central Mexico and California for the winter. The generation that migrates is usually several generations removed from the ones that made the journey the previous year.


The monarch butterfly, including its caterpillar stage, is considered an indicator species for monitoring the health of ecosystems. Their numbers have been in decline due to habitat loss, pesticide use, and climate change. 

Conservation efforts focus on protecting their habitat, especially the milkweed plants that the caterpillars need to survive.

Understanding and protecting monarch caterpillars is crucial for conserving the species and maintaining the ecological balance in their habitats.

The research is published in the journal Proceedings of the Royal Society B.

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