See below. Monarch butterflies lay their eggs on the leaves of specific types of milkweed plants; the eggs hatch into larvae/caterpillars which then feed on the milkweed, storing enough energy to take them through the pupal/chrysalis stage of development. Milkweed plants are toxic to birds and mammals, and the butterflies store those poisonous compounds in their exoskeletons, making them distasteful if not outright lethal to likely predators.