The diet of the monarch butterfly changes as it develops, but adult monarch butterflies eat nectar from flowers, as do all other butterflies. The butterfly's mouth has a special design for collecting nectar including a long proboscis located under its head. This long device is hollow like a soda straw and unfurls to allow the butterfly to suck up the sweet nectar from inside the flower.
Many plants produce nectar in their flowers to attract pollinators, including the monarch. These flowers begin secreting nectar as soon as they open. The plant produces nectar in a structure known as the nectarines. The nectarines are generally located in the flower. Nectar is a sugar solution also containing traces of salts, proteins, essential oils and acids. The sugar content of nectar varies from 3 to 80 percent, according to Dictionary.com.
When monarch butterflies are in the caterpillar stage, they eat milkweed plants. These wildflowers are of the genus Asclepias. Milkweed contains a toxin that is harmless to the monarch but poisonous to birds and other predators of the monarch. As the caterpillars eat the different parts of these plants, they store these toxins in their body. When caterpillars go through metamorphosis, these toxins remain in their system, giving them a bitter flavor to predators, according to the National Wildlife Federation.