Monarch larvae appear to feed exclusively on milkweeds in the genus Asclepias and several other genera of viny milkweeds in North America. Milkweeds are perennial plants, which means an individual plant lives for more than one year, growing each spring from rootstock and seeds rather than seeds alone. In the Midwest, milkweeds were historically common and widespread on prairies, but habitat destruction has reduced their range and numbers.
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.
Flowers are significant in plant identification because flowers are intricate structures with many features that can be assessed, counted, and measured. Affinities among closely related species can be shown because floral morphology is conservative. Leaves and stems have features shared across many groups and lack unique combinations of features for species identification.
Like other flowers, milkweeds have floral whorls of sepals (collectively referred to as the calyx) and petals (collectively called the corolla). Flowers of milkweeds are interesting because they have an third whorl of five hoods each of which encloses a horn (modified filaments of the anthers). Together, hoods and horns are referred to as the corona. The horns of some species are long, while the horns of others are reduced to the point they cannot be seen.
Milkweeds have a unique and fascinating pollination mechanism in which the plant relies on Lepidoptera (butterflies and moths) and Hymenoptera (bees, ants, and wasps) for pollination. Hundreds of pollen grains are packaged into two connected sacs or pollinia, which is collectively referred to as the pollinarium [see SEM photo at right]. When a foraging insect lands on a flower, the pollinarium can easily attach itself to its leg. Once removed from the flower, the pollinia actually re-orient as the translator arms bend as they dry. Upon landing on another flower, the properly oriented pollinarium is deposited into a receptive stigmatic groove where the pollinia breaks down and the pollen germinates, growing pollen tubes through the stigma to the ovules in the ovary.
The milkweed fruit is a follicle, commonly referred to as a pod, which splits at one suture to release many seeds, sometimes hundreds, depending on the species.