COMMON MILKWEED. As the name indicates it is the most common of the milkweeds. According to Naturalist Amy Andrews there are 4 species of milkweed that we have in the park, and this is by far the most common! Plants in the milkweed family are the only plants that can host the caterpillars of the monarch butterfly! Milkweed is the only thing their stomach can handle, so the butterflies lay eggs on the milkweeds, then a caterpillar lives and eats on one plant until it is ready to pupate (form a chrysalis) and metamorphose into a butterfly.