Milkweeds (family Asclepiadaceae) are perhaps best known for two reasons: First, they produce a thick, sticky white sap that is appropriate for a plant called “milkweed”. Second, they’re the sole food source for the larvae of monarch butterflies (Danaus plexippus, Nymphalidae). Caterpillars feed specifically on the leaves of plants in this family. Adults feed on the nectar of a more diverse group of plants, but still retain a fondness for milkweed.
Many milkweed species produce cardiac glycoside toxins to prevent herbivory. Monarchs gain their toxicity from eating these toxins, to which they have adapted a tolerance. Monarch toxicity in turn protects them to some degree from predation.
I recently heard a suggestion of planting milkweed in home gardens for the purpose of attracting monarch butterflies. I think this is a great idea, and plan on doing so ASAP.
You’ll notice the photo above is conspicuously lacking a monarch butterfly. The little bugger wouldn’t hold still, and then it flew away. The plant was more cooperative.