Milkweed is the host plant for the monarch butterfly. As a host plant, it provides the monarch larvae and adult butterflies with a food source. Milkweed belongs to the genus Asclepias, which has many different species spread out through America and Mexico. Asclepias line the migration path for these butterflies as they travel back and forth from Mexico to the northern states. It takes multiple generations to make the trip up north and only one generation to make the trip back to Mexico. A trip of that distance is going to require a lot of fuel!
Milkweed can be found in many different variations depending on where it is found. There are many different species native to California such as Narrow-Leaf Milkweed (Asclepias fascicularis), Showy Milkweed (Asclepias speciosa) and Indian Milkweed (Asclepias eriocarpa). There are also many non-native varieties of milkweed that grow well here in southern California. Tropical Milkweed (Asclepias currasavica) has many variations, but is commonly found in red/yellow or yellow.
The parent butterfly will lay its egg on the underside of a milkweed leaf. The egg looks like a little green ball of slime, about the size of a pinhead. The caterpillar emerges immediately hungry. This larvae will eat only the sappy leaves of milkweed. The white milky sap of milkweed contains alkaloids, which cause it to be toxic when ingested. But Monarch caterpillars are able to ingest this toxin and use it to its advantage. The alkaloids become present in the body of the caterpillar, making it toxic to predators. Monarch larvae are strikingly colored with yellow, black, and white. The adult is also strongly colored with vibrant orange wings on dark black lines. Larvae or adult, they always present a clear colorful warning to predators. After the caterpillar eats its fill of milkweed leaves, it will wander a short distance to find a safe place to start its chrysalis.
Milkweed is also known to get a few other insect visitors in addition to the butterflies. They will get visited by a true bug called Milkweed Bug. This black and orange bug is relatively harmless to the plant. Milkweed bugs feed on the seed pods, and might hinder your seed production at most. You will probably also encounter orange Oleander aphids on your milkweed and they are also relatively harmless to the plant and caterpillars.
Caring for Milkweed is easy. If you are gardening here in southern California, I’d recommend planting Narrow-Leaf Milkweed (Asclepias fascicularis). It has supported butterflies in our area long before we got here and has the proper alkaloids to keep the caterpillars toxic to predators, while the tropical milkweeds do not provide the same alkaloids. Narrow-leaf Milkweed is drought tolerant, and thrives here. It reseeds itself easily and requires little to no care after the initial planting. Every winter this species will go dormant, and so will the butterflies.