Leonard Weber

September 12, 2024

September is a good time to observe Monarch butterflies in Detroit’s Eliza Howell Park. During the first third of the month, the caterpillars can be found relatively easily on milkweed leaves, eating steadily.

Here are three from the past week.

Many of the milkweed leaves are turning brown, and some are starting to fall, but the caterpillars munch away.

This is the overwintering generation, the individuals that will migrate about 2000 miles to the Sierra Madres Mountains in Mexico to spend the winter.

First, though, they need to complete development. The caterpillars I have been seeing look about full size. The next step is to form a chrysalis, where they transform into the adult butterfly in about a week or 10 days.

The caterpillars crawl down and  away from the host plant (milkweed) to select a spot for the chrysalis.

The chrysalis is usually very well hidden, and the above is the only one I have ever been able to locate in the park.

The Monarchs that emerge at this time of the year are quite different, both biologically and behaviorally, from their parents. Those, as adults, lived only a few weeks, mating and laying eggs shortly after emerging.

1) These new butterflies will live much longer (till next spring if all goes well). 2) They will travel much further, flying the roughly 2000 miles to the Mexico wintering ground (traveling 50 to 100 miles a day when migrating). 3) They postpone mating activity until they have started back north next spring.

No individual Monarch butterfly ever makes a round trip. Those that fly south and survive the winter in Mexico start back, but only make it part of the way. They mate, lay eggs, and die on the return trip. It is the next generation that we will see here starting in May 

How do butterflies that have never been to some place know how to “return” there?

I don’t know the answer, but I am watching the final days before the fascinating trip.