Science for all newsletter Milkweed is a toxic treat for monarch butterflies

Canopied with vibrant little star-shaped flowers, the tropical milkweed shrub is a favourite of millions of migrating monarch butterflies in America, which lay their eggs on them, feed on their leaves and stems as caterpillars, and then as strikingly patterned butterflies, feast on the flowers’ nectar, among other plants.

The plant does more for the monarch than feed it: it makes the caterpillars and butterflies toxic to predators thanks to the chemicals in its sap. As with many other plants, milkweed toxins evolved as a chemical defense against herbivores.

A new study now finds that ‘cardenolide’ (a type of steroid) mixtures in milkweed is a toss-up for monarch caterpillars: it reduces growth and sequestration (storage) while helping the creatures fend off enemies. 

Although the benefits of plant defensive chemistry are well established, why plants produce such a diversity of compounds has long been a mystery, said the paper. “Are individual compounds targeting different plant attackers, or do mixtures act as a more effective defense than individual compounds alone?,” the authors pondered.

They found that there is a cocktail of cardenolide toxins that are involved in this relationship, including some rare nitrogen- and sulfur-containing containing cardenolides, which seem to slow down growth, feeding and sequestration, making it a trade-off for the monarch.

The scientists tested the effects of the diversity of phytochemicals (compounds produced by plants to defend against pathogens, herbivores, and stress) on the monarch caterpillar, with a special focus on cardenolide toxins. They found that the nitrogen- and sulphur-containing cardenolides in milkweed reduced caterpillar performance and sequestration more than other related cardenolides.

“Coevolution,” that is, when two or more species reciprocally affect each other’s evolution, can produce highly specialised defense molecules such as nitrogen- and sulphur-cardenolides in milkweed that adversely affect sequestering in herbivores, said the paper.

Similar dynamics likely shape many predator-milkweed and butterfly-milkweed plant relationships in India, Aswathi Asokan, a nature educator and naturalist, who keenly follows the life-cycle of butterflies, told The Hindu. We have our own species of milkweed plants in India, such as Calotropis gigantea, along with the tropical milkweed that serve as host plants for various species of our own milkweed butterflies, she said.

“A comparable example could be of butterflies, such as the plain tiger and striped tiger, and their long-standing relationship with milkweed plants.” These toxin-producing plants and butterflies are in a “long-term evolutionary negotiation,” said Ms. Asokan, adding that “plants keep diversifying their chemical defenses, while their consumers, such as butterflies, evolve ways to tolerate and repurpose these toxins to their benefit.”

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