Source: http://butterfly-photo.blogspot.com/2009/09/helping-monarch-butterflies.html
Many people have heard about the great migrations Monarch butterflies undergo each winter. But I had no idea what they did the rest of the year. It turns out that entire generations live and die before we see the next wave of migrants sweep through California.
The Monarchs that we see during the winter migrations were all born in about August/September. The Monarchs west of the Rockies migrate to California, while the Monarchs east of the Rockies head down to Mexico to overwinter.
Come February, the Monarchs mate, and the females fly off in search of milkweed to lay their eggs on. Monarch caterpillars will only eat milkweed, so mom needs to find a good patch to leave them in. She lays one egg on each plant, and will lay between 100 to 300 eggs.
The eggs hatch about 4 days after they’re laid, and the young spend about 10-14 days being caterpillars. Then they form a chrysalis, where they will take about 10-14 days to metamorphose into the adult butterflies that we are all familiar with.
The new adults fly off, chasing new blooms of milkweed north. They mate, lay eggs, and die in about two to five weeks. Monarch populations do this all summer, until the generation that is lucky (or unlucky?) enough to be born in late summer.
That generation is different. The changing day/night cycle as winter approaches delays the development of their gonads, so they can’t mate and produce viable offspring.
In order to avoid freezing up north, they migrate south to warmer climes. These adults are the ones we see in the groves and forests around California.
When late February rolls around, their gonads develop and they are able to mate.
And thus, the cycle begins anew.
And I didn’t know until recently that Natural Bridges State Park is one of the places the Monarchs winter. It’s amazing to see hundreds of them resting on a single tree branch.