The leading edge of the 2006 monarch migration has begun passing through Chicago. It's been a good summer for monarchs so far here in the Chicago area, so I'm hoping for a strong migration this year. There have been three distinct local generations of monarchs this year: one in late June, one in early August, and one that is just now emerging from the chrysalises. Typically, there are small numbers in the earliest generation, with more abundant second and third generations. This year, the number of first generation monarchs was typical of what one would see later in the summer. The second generation was as abundant as we typically see during the migration. It’s too early to tell what will be seen with the current generation, though if the pattern holds, it could be very large.
Adults from generations I and II live for about two weeks, which is typical of adult butterflies in this part of the country. The current generation is very different from the previous two, however. The shorter day length at this time of the year stimulates the butterflies to produce a substance called juvenile hormone. This compound sends the butterflies into reproductive diapause. In other words, they stop breeding. It also increases their lifespans, and they will live eight to nine months. Their circadian clocks stimulate the onset of migratory behavior.
By the middle of next week, the peak of the migration should be passing through Chicago. The butterflies will continue to fly southward, stopping along the way to take nectar from flowers and to roost for the night. By early November, they will be near Morelia, about 100 miles WNW of Mexico City, where they will spend the entire winter. The most remarkable part of the journey is that the butterflies find the same tiny refuges year after year, yet they are at least four generations removed from the last butterflies to make the trip. We are learning more and more about how the butterflies navigate. Still, our level of understanding is analogous to knowing how a compass works, when the question at hand is how a pilot navigates to a distant airport.
These monarchs were photographed in Lincoln Park in Chicago. They are on late boneset, a plant that is used heavily by migrating monarchs in northern Illinois.