Milkweed and its varieties are the ONLY plants that Monarchs eat. If you aren't sure that a plant you see is milkweed, simply break off a leaf. If a thick white milky liquid runs out, BINGO! :-) The milky sap is very sticky and dries quickly to a gummy texture. It does wash off hands and clothes with soap and water. I often have to scrub my scissors that I use for cutting the leaves with a scratch pad and hot water to keep them from gumming up.
Milkweed refers to anything in the Asclepias family of plants. Here is the description for Common Milkweed (Asclepias syriaca) from Wikipedia including a bit about Monarchs:
Common milkweed is a clonal perennial herb growing up to 2.6 m tall. Its ramets grow from rhizomes. All parts of common milkweed plants produce white latex when broken. The simple leaves are opposite or sometimes whorled; broad ovate-lanceolate; up to 25 cm long and 12 cm broad, usually with entire, undulate margins and reddish main veins. They have very short petioles and velvety undersides.
The highly fragrant, nectariferous flowers vary from white (rarely) through pinkish and purplish and occur in umbellate cymes. Individual flowers are about 1 cm in diameter, each with five cornate hoods and five pollinia. The seeds, each with long, white flossy hairs, occur in large follicles. Fruit production from selfing is rare. In three study plots, outcrossed flowers had an average of about 11% fruit set.
Monarch larvae consume only milkweeds, and monarch populations may decline when milkweeds are eliminated with herbicides.
Efforts to increase monarch butterfly populations by establishing butterfly gardens require particular attention to the butterfly's food preferences and population cycles, as well to the conditions needed to propagate milkweed. For example, in the Washington, D.C. area and in the northeastern United States, monarchs prefer to reproduce on A. syriaca, especially on young, soft leaves. As monarch reproduction in that area peaks in late summer when A. syriaca leaves are old and tough, the plant needs to be cut back to assure that it will be regrowing rapidly and producing young leaves when monarch reproduction reaches its peak. In the Washington, D.C. area one can have such leaves in July, August, and early September during the main oviposition period in three ways. First, one can grow seedlings. Second, one can cut large shoots to about half their height in June and July before or after they bloom. Third, one can cut large shoots to the ground in June and July. Cut plants often produce new shoots from their rhizomes. It is advisable to let some large, mature shoots remain in summer and fall because large monarch larvae, milkweed tiger moth larvae, and other native species feed on mature leaves. Milkweed bugs commonly feed on follicles. Monarch larvae can consume small seedlings to the ground. To save seedlings, one can transfer larvae from seedlings to larger shoots.
So where should you look for milkweed if you haven't been growing it yourself already? It seems to be able to grow just about anywhere, but the best places to look are on the edges of forested areas, road-sides, and fields that have been let go (unmowed). Rural areas with large farm fields often have milkweed, just make sure you are not trespassing.
If you want to grow it yourself you can either collect seeds from Milkweed pods in the fall and scatter them in soil, or pull up a root ball and soak it in water for a few days until small shoots start to appear. Then plant in a spot that is partly shaded. It can take time to get milkweed established so don't worry if it doesn't seem to be working at first. It took 2 years for mine to really take hold.
Be careful! ONLY collect milkweed from areas that you KNOW do not spray pesticides or use a systemic pesticide. Even if they say it is a "Natural, organic" pesticide it will still harm or kill your caterpillar! One way to tell that a plant has been treated: there will be NO insect life and the leaves will all be perfect.
A Monarch caterpillar that was leaking green liquid
I think my caterpillar is sick. What can I do?
How to tell if your caterpillar has been poisoned: It will begin vomiting green liquid and become inactive after a few bites. What to do: If it is a larger caterpillar you can pick it up gently and cupping it in your hand you can run lukewarm water over it under a faucet. Then clean the container it is living in and put fresh, untreated for sure leaves in. Smaller caterpillars will most likely die. Sometimes these caterpillars will expel green liquid if they are feeling threatened, but it takes quite a bit to upset them that much.
How can I keep this from happening again?
Grow your own milkweed, either from root cuttings or from seed. Or locate a local friend who you know does not treat their plants that you can collect from. Always thoroughly rinse your leaves before giving them to the caterpillars, although this does not ensure there is not poison in them. Don't give up though!