Milkweed’s toxins protect butterflies

By Marcia Myers

Common milkweed (Asclepias syriaca)

Common milkweed (Asclepias syriaca)

Not only does this amazing plant act as a nursery, provide nectar, and serve as an important food source, it provides extra protection from predators. Both monarchs (Danaus plexippus) and queen butterflies (Danaus gilippus) store cardenolides, toxins obtained from their milkweed (Asclepias spp.) diet.

Why aren’t the butterflies poisoned? Monarchs and queen butterflies developed enzymes that are “almost entirely immune to cardenolides’ toxic effects”. An article titled, “Butterflies Weaponize Milkweed Toxins” in The Scientist discusses the findings of recent research from Proceedings of The Royal Society B which indicate that the cardenolides-resistant enzymes arose not to specifically encourage the caterpillar to eat more milkweed but to make themselves poisonous to natural enemies.  The milkweed provides the toxins.

Also, other insect species feed on milkweed including the common crow butterfly (Euploea core), which does not absorb the toxins into its bloodstream. Unfortunately, milkweed has been considered a weed to be eradicated by some, thus leading to habitat loss especially for monarch butterflies.

If you want to help, be sure to use locally-appropriate milkweed plants. For more information, visit Milkweed for Monarchs – The St. Louis Butterfly Project.