The poisonous Milkweed is the primary food source for the Monarch Butterfly pupa, or caterpillar (which is classified as a specialist herbivore). Milkweeds are the only source of nourishment up until they are released from their cocoon (chrysalis) and transformed (or undergo metamorphosis) into a butterfly. After transformation the adults drink water and extract nectar from many kinds of flowers. Monarchs do pollinate flowers, but not nearly as proficiently as bees.

The vast majority of all Monarchs in North America make their home in agricultural areas that are abundant with the milkweed plants whose leaves provide a safe place to deposit their eggs, and also to feed their offspring. When returning every spring from their winter migration, the butterflies will stop over at the resurging milkweed and flowering plants and continue northward.