Monarch Life Cycle

Milkweeds belong to the family Asclepiadaceae, derived from Asklepios, the Greek god of medicine and healing. Though most members of the genus Asclepias are tropical, there are approximately 110 species in North America known for their milky sap or latex contained in the leaves. 

Most species are toxic to vertebrate herbivores if ingested due to the cardenolide alkaloids contained in the leaves and stems. When Monarch larvae ingest milkweed, they also ingest the plants' toxins, called cardiac glycosides. They sequester these compounds in their wings and exoskeletons, making the larvae and adults toxic to many potential predators. 

Vertebrate predators may avoid Monarchs because they learn that the larvae and adults taste bad and/or make them vomit. There is considerable variation in the amount of toxins in different species of plants. Some northern species of milkweed contain almost no toxins while others seem to contain so much of the toxins that they are lethal even to monarch caterpillars.

Monarchs, Milkweed, Nectar Flyways -- in a Nutshell