Adapted from a question originally answered on Quora.
The beloved monarch butterfly is an important icon in many cultures. People throughout North America look forward to their stunning migration back to Mexico every fall, and the Mexican holiday Dia de los Muertos (Day of the Dead) coincides with the butterflies' return to their winter roosting grounds, symbolizing the return of the spirits of loved ones to celebrate life and death. Unfortunately, these spectacular bugs are facing extinction. Predation combined with loss of habitat and pesticide use has taken a large toll on the monarch population, and the changing climate has disturbed their life cycle and migration timing.
Since monarchs are becoming a rare sight, many butterfly gardeners lament the discovery of monarch predators in their gardens. Parasitic wasps and flies have no qualms about laying their eggs inside monarch caterpillars, and ants, mantids, and other carnivorous insects will gladly pluck the eggs and small larvae from milkweed leaves. It is estimated that only about 3% of monarch eggs laid in the wild survive to adulthood! Monarchs sadly don’t have stingers or claws or sharp teeth to defend themselves, but they do have a few things keeping them out of the mouths of predators.
They are poisonous!
Both the monarch caterpillar and the adult butterfly are toxic to most animals because of the poisonous milkweed that is their only host plant. Milkweed contains chemicals called cardenolides that affect the heart muscles of many animals. This makes the monarchs taste really bad, and most animals that eat them once won’t ever go after one again!
They are brightly-colored
The caterpillars are striped white, black, and yellow, a classic combination that, in many animals, screams “DANGER: POISONOUS!” The butterflies are bright orange, another typically “unsavory” color. These warning colors are called aposematism and can be found in many different animals. Predators are much less likely to try to eat animals with these colors in general, regardless of whether or not they are actually poisonous.
They can fly!
Well, the adult butterflies can, anyway. This seems pretty obvious, but the ability to take off quickly is important when many of the predators that want to eat you can strike quickly!
Although diligent butterfly hobbyists who raise monarch larvae in captivity can have a survival rate as high as 99%, individual raising efforts have no significant effect on the overall monarch population. However, their passion for helping monarchs along with education about habitat preservation luckily seems to have increased the overwintering population in Mexico. Monarchs are still a very threatened species, and need our help to reverse the damage humans have done to their habitat. Visit the Monarch Joint Venture to learn more about what you can do to help save the monarchs!