Monarch Butterflies in September: The Amazing Migration Begins

Each year I tend to pay more careful attention to Monarch Butterflies in September than I do in August or July. I am curious about when I will see the last one of the year and I am fascinated by the very long migration they are undertaking.

On New England Aster

Those that are present in Eliza Howell Park now are part of the last brood (or generation) of the year. The life of this generation is quite different from that of the Monarchs I was watching here earlier in the year.

On Goldenrod

These individuals, emerged from their chrysalises in (late) August or early September, are beginning about a 9-month lifespan; the lifespan of the previous generation can be measured in weeks, not months.

The previous generation developed from eggs laid here in Michigan this summer and, as adults, quickly began to produce more young. These September Monarchs have a whole different mission. Their sexual maturity is delayed until after they complete the long, long migration to Mexico and survive the winter there.

Their role now is to fuel up and head south.

On Ironweed

I find it helpful to have a visual image of the migration. This map is taken from monarchbuttterflyusa com.

Most will arrive in their Mexican wintering grounds in November. There they hang out in the fir tree forests, where they cluster on the tree trunks, staying in the region for the rest of the winter.

In March they head back north, now sexually mature. They stop to lay eggs (on milkweed plants, of course) in northern Mexico or the southern U. S., leaving the rest of the trip to the next generation. This new generation, which has never seen Michigan, arrive here usually in May.

Between the Monarchs arriving in the spring and those now departing to the south, there is a generation, perhaps two, that live their whole lives here.

Now my focus is on the bright new generation that is leaving us in order to survive the winter, so that others can return.

Neither the departing Monarchs nor those that will arrive here in the spring are making a trip that they have taken before. In another of nature’s wonders, they know where to go.

Some butterfly species survive the winter as eggs, some as larvae, some in the chrysalis stage, some hibernate as adults. Monarchs have a different strategy, migrating thousands of miles south and then back north.

I am sure that I am not the only one who looks forward to seeing the next generation next spring!

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