Monarch butterflies will only lay their eggs on milkweed because it is their hostplant. A hostplant is the plant that the caterpillar will eat when it hatches from its egg. So, the monarch caterpillar will only eat milkweed, and the monarch butterfly will only lay its eggs on milkweed. The monarch depends in the milkweed, the milkweed does not depend on the monarch butterflies.
What plants Monarch butterflies do not like is not all that important, though no specific plants that they do not like are known. The most important thing is that they not only like, but need specific kinds of milkweed for their survival. They lay their eggs on milkweed and the caterpillars feed on the plant.
It's not symbiotic; the monarch caterpillar defoliators the milkweed plant, which harms the plant and gives the insect protection. The adult monarch is not a significant pollinator of the milkweed flowers either, as compared to bees and other more common small butterflies. The milkweed plant would be much better off without the monarch, but the monarch cannot survive without the milkweed.
The monarch butterfly must lay their eggs on milkweed plants (Asclepias ), and the swan plant is listed on wikipedia as a type of milkweed. Most plants in the milkweed family produce flowers which the monarch can feed on. The caterpillars of the monarch butterfly can only eat leaves from plants in the milkweed family and will die without this food source. The milkweed plant is known for it's white milky sap, which contains alkaloids…
The monarch caterpillar is at a high risk when they eat Milkweed. Milkweed got its name because its full of a sticky milk colored liquid. Many caterpillars get stuck in it and die. The caterpillars that eat milkweed are immune to a special toxin in the milkweed but still are at risk of dying in the sticky liquid. Butterflies do not eat milkweed. They drink nectar from flowers or juice from fruits.
Caterpillars others than the monarch... there are only four types of butterflies that lay their eggs on milkweed: the Monarch Butterfly (Danaus plexippus); the Queen (Danaus gilippus); the Tropical Milkweed Butterfly (Lycorea cleobaea); and the Soldier Butterfly (or "Tropic Queen"; Danaus eresimus). Another caterpillar that I was desperately trying to find the name of (which brought me to this page) is Euchaetes egle, the milkweed tussock caterpillar or milkweed tiger moth.
It is believed that the Monarch Butterfly originally evolved in South America and then was split into different subspecies. Others believe it evolved in the "New World". Fossils of butterflies have been found that are believed to be 35 million years old. However, the Monarch and other milkweed butterflies aren't represented in the fossil records. This means there is no record of exactly where the Monarch butterflies evolved from.
According to the WWF and the IUCN, Monarch butterflies are not currently endangered. However, their migratory patterns are at risk because their routes are endangered due to habitat loss. Illegal deforestation of their overwintering grounds is the greatest current threat. Logging of their traditional wintering grounds means the butterflies have fewer places to rest during winter. Not only that, but such habitat loss reduces their supply of milkweed, a key component of their diet. There…