Milkweed Evolves New Toxins to Defeat Monarch Butterflies

By Blake Jackson

The study has discovered that milkweed plants have developed a new way to outmaneuver monarch butterflies in their long-running evolutionary battle. By subtly altering the structure of their toxins, milkweed can now overcome the butterflies’ built-in resistance.

Researchers found that adding a small chemical component containing nitrogen and sulfur to cardenolides-the plant’s toxic compounds-enables these substances to bypass the monarchs’ defense mechanisms.

This highlights an often-overlooked evolutionary strategy: plants don’t just increase toxicity levels, they can also redesign their chemical structures to create entirely new variants of toxins.

“This structural innovation is a new axis for defining chemical toxins in the natural world,” said co-author Christophe Duplais, associate professor of entomology at Cornell AgriTech, in the College of Agriculture and Life Sciences (CALS).

“This very simple modification makes a huge difference in terms of its ecological effect, because now this molecule is toxic to the monarch.”

Monarch butterflies have evolved to tolerate milkweed toxins by preventing them from binding to vital enzymes, even storing them as a defense against predators.

However, scientists observed that monarchs struggled on certain milkweed species. To investigate, researchers analyzed toxins from 52 species, focusing on their molecular composition.

They discovered that the addition of a nitrogen-sulfur ring significantly improves the toxin’s ability to bind to target enzymes.

Surprisingly, nearly 70% of the studied species contained this structure. Even more striking, different milkweed species developed this feature independently, suggesting a widespread evolutionary pattern.

“We demonstrate that this is happening in compounds that have different evolutionary histories - when we discovered that, it was mind-blowing, beyond our expectation,” Duplais said.

“That challenges what we call chemical escalation, the idea of a plant creating toxins and the insect adapts. This structural innovation is showing that lots of iterative changes can happen to complicate that unilateral model of evolution.”

Photo Credit: istock-herreid