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 overall milkweed plant family contains over two thousand varieties. The Monarchs lay their eggs on several varieties, but seem to prefer the native, instead of hybrid, varieties. Many kinds of milkweeds contain poisons and are toxic enough to harm livestock and other animals that have not learned through evolution to avoid them.
On the predator side, two species birds, the Black Back Oriole and the Black Headed Grosbeak are not affected by the Monarch’s toxins, and are this butterfly’s main adversary.
Varieties of milkweed are found abundantly, and although the United States government has not labelled the plant as poisonous or noxious, farmers consider the plant a weed and a real nuisance to their livestock and crops. Farmers often apply herbicides to large acreages to kill unwanted vegetation. This has resulted in a very sizable decrease of the Monarch Butterfly’s source of the milkweed plant, especially throughout the mid-western agricultural areas, thereby affecting their summertime migratory behaviour.
Scientists are now able to determine the migratory paths of a Monarch Butterfly. An adult can be analyzed to determine exactly which species of milkweed it fed upon earlier in its caterpillar state.
By Jim Pratt