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Jeff Green | Sep 25, 2008

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Monarchs have a fascinating life history. They are one of the few species of butterflies found in Canada that migrate south for the winter. Each fall millions of monarchs from across eastern North America migrate many thousands of kilometres to a few spots in the mountains of central Mexico, where they spend the winter. Arriving in Mexico in November and December, they form dense clusters of millions of individuals and remain largely inactive over the winter months.

Once the winter groups break up in March and early April, the butterflies migrate north to the Gulf Coast of the United States, where the females lay their eggs on milkweeds. Several generations of monarchs are produced here in the spring, and it is these offspring of the overwintering generation that continue the migration to their northern breeding range, including our area. The first returning migrants reach southern Canada near the end of May or the beginning of June, and populations will continue to build throughout the summer as long as there are healthy milkweed plants for the caterpillars to feed on.

Monarchs migrating south in the fall build up in large concentrations along the north shores of Lake Ontario and Lake Erie, where they accumulate on peninsulas like Presqu’ile and Point Pelee that jut out into Lakes Ontario and Erie. There they cluster together on trees, forming large overnight roosts before crossing the lakes. These groups may contain a few hundred to several thousand individuals. Monarchs usually form clusters in the same areas year after year.

The monarch has also proved itself to be a highly adaptive species. In the last 150 years there has been a major shift in the North American distribution of the eastern population. This appears to have resulted from the conversion of much of the natural grasslands in the Midwest to cropland, a process that eliminated most milkweeds. At about the same time, many areas of the deciduous forests of eastern North America were cleared for farming, providing ideal conditions for the rapid eastward spread of the common milkweed. The cleared portions of the eastern deciduous forest region now correspond to the main breeding area of the eastern population of the monarch.

There is another interesting feature of monarchs worth noting: Unlike most butterfly larvae, which are coloured to blend in with their surroundings, monarch larvae are brightly-coloured. Their conspicuous appearance serves as a warning to potential predators that the monarchs are poisonous. This is because they ingest poisonous juices from the milkweed plants on which they feed. Any animal unwise enough to eat either an adult or a caterpillar monarch will probably become ill and suffer severe vomiting – and as a result will likely only do it once!

Let’s hope our monarchs have a successful trip to Mexico this fall, so we can welcome their children and grandchildren “home” next spring!