Milkweed is the life force for all stages of the monarch from egg to butterfly. It is the only plant onto which the female monarch will lay her eggs, and it is the primary food source for the monarch caterpillars until they pupate (form their chrysalis).
Once the milkweed blooms, it becomes a nectar source for the butterflies.
It is very important that you have access to an abundant supply of milkweed as the monarch caterpillars eat NON-STOP until they pupate.
If you don't have milkweed in your yard, you can go for a drive in the country and find it along the roadside in ditches. This is how I first hunted for caterpillars. Later in the summer, the milkweed pods will split and the seeds will fly out and drift away on the wind. If the seeds inside the pod are brown, take the pod home, keep them outside in a container or in your garage so they can stay cold over the winter. (They need to do this to germinate when they are planted) In the spring, plant your seeds in your yard!
When you plant milkweed in your yard, it is so much more convenient to go egg and caterpillar "shopping". You can also keep a much closer eye on new eggs so they don't become a snack for some other insect.
The milkweed plant gets its name from the milky substance that drips from the leaves and stalk if they are torn. This substance is an irritant to eyes and sometimes the skin, so be sure to wash your hands after handling leaves or pods.