Milkweed Herbivores
Milkweed has evolved some interesting defenses to protect itself against the herbivores – insects that eat leaves, flowers, and stems. First, you’ll notice that if you break a leaf or stem, it will start “bleeding” a milky sap. This substance can glue an insect’s mouth parts or adhere to wings, killing the insect. Next, the leaves and stems have hairs that detract many species. And finally, milkweeds contain toxins – cardenolides – that can sicken or kill wildlife if ingested. It’s so potent that it can even poison grazing livestock - which is one reason milkweed has been broadly destroyed in agricultural areas over the past century.
With these defenses, it’s actually amazing that milkweeds have herbivores. But many insects have co-evolved with milkweed to the point of not only developing special chemistry to avoid the defenses, but to ingest the toxins. That in turn makes them poisonous to predators. Many of the milkweed herbivore specialists have co-evolved to have similarly bright orange/ black color markings (Mullerian mimicry) to warn potential predators that they’re all poisonous. That color pattern is understood to mean “don’t eat me!”. While monarch caterpillars lack those colors, the adult butterfly – that has inherited some of that poison – also has the color scheme. The strategy works so well, that there are many non-poisonous insects that mimic the coloration pattern just to trick predators into leaving them alone too (Batesian mimicry).