By late spring most have left the Gulf coastal states and are breeding farther north. By the beginning of June they have reached Minnesota and are far into the northeastern states. They continue to breed in the northern states and southern Canada all summer, greatly augmenting their numbers. Decreasing daylength and colder temperatures tell Monarchs born in the early fall not to breed and instead to begin the immense journey across North America to overwintering grounds in the high fir forests of central Mexico approximately 100 miles west of Mexico City. They arrive at the overwintering areas in November, remain until March of the following year, and then return to the southern United States to lay eggs on freshly sprouted milkweeds.
Monarchs are insects whose life history involves complete metamorphosis.
The immature Monarch looks drastically different from the familiar adult. The newly oviposited creature spends a week or so as an egg, cracks open its shell, and then eats it before venturing out to try some milkweed leaf tissue. Its only business in life is to eat and grow, which it does in remarkably short time. When a suitable spot is found far from its feeding place so that parasites and predators will not find it in its vulnerable immobile chrysalis stage, it attaches itself to a silken pad that it has spun, sheds its skin, and forms an emerald shroud that it decorates with golden spots. In about a week, a totally different creature emerges.