Butterfly monarch butterflies facts. The monarch butterfly is sometimes called the "milkweed butterfly" because its larvae eat the plant. In fact, milkweed is the only thing the larvae can eat! If you'd like to attract monarchs to your garden, you can try planting milkweed (if you live in the right area). You can purchase milkweed seed online from Butterfly Encou nters ( close window when done purchasing to return to this screen ). Adult female monarchs lay their eggs on the underside of milkweed leaves. These eggs hatch, depending on temperature, in three to twelve days. The larvae feed on the plant leaves for about two weeks and develop into caterpillars about 2 inches long. After awhile, the caterpillars attach themselves head down to a convenient twig, they shed their outer skin and begin the transformation into a pupa (or chrysalis), a process which is completed in a matter of hours.
