Monarchs and Milkweeds From East to West
In the summertime Eastern Monarchs can be found all the way from Mexico to southern Canada and from the Rocky Mountains to the Eastern seaboard. And, all across this huge land area, four or sometimes five generations of Monarchs are born in a single season.
Then, starting in September and October, all Eastern Monarchs, (with the exception of a few that use the Atlantic seaboard) undertake a single prodigious migration to reach their over-wintering destination, the Monarch Butterfly Biosphere Reserve, in Southern Mexico.
And, during this migration, these tiny creatures fly the most astonishing distances—those that begin their journey in the Canadian Maritime provinces will travel nearly five thousand miles—to reach their winter destination in Central Mexico. Butterflies begin their fall migration in September or October, and finally reach their overwintering sanctuary in November.
Then the following March those same butterflies set out on their northbound trip. However no individual butterfly flies all the way north. At successive stops along the way the female monarchs lay eggs on there milkweed plants, and the resulting caterpillars initiate the new generation of butterflies that continue the journey northwards. It is estimated that, in the course of the entire summer, eventually four or five generations will be raised.
But, in early fall, the final generation of females, instead of producing more eggs, is programmed to store their food supply in preparation for their long flight back to Mexico. And these butterflies are also programmed to fly south or south-west depending on their starting points, to reach this single destination.
Check out this wonderful interactive map to see the flight paths taken by the butterflies, both during their trip south in the fall, as well as how the successive generations that are raised on the way north in the course of a single summer.
Thus, if humans are to halt the decline of the Monarchs, we need to ensure that milkweeds and other nectar producing flowers are available for the butterflies at each breeding point along their journey.
Like their eastern counterparts, the Western Monarch butterflies also have a migratory life-cycle. Their summer territory is to the west of the Rocky Mountains, where typically they will hatch three generations at successive points along those fly-ways.
Then every fall, the third generation of these butterflies is programmed head south-west to reach one of several over-wintering destinations along the west coast of California and western Mexico.
Unlike the Eastern and Western populations, the American Monarchs that are found in Florida and along the gulf coast do not migrate. Instead this populations remains in place on a year-round basis.