Slow monarch arrival?

Monarch cluster photo by Connie Masotti.

Monarch cluster photo by Connie Masotti.

Monarch overwintering season has officially begun, and most of the monarch butterflies in the country have found their way to overwintering sites up and down the California coast, or high in the mountains of Michoacán State in Mexico. At least that is what we should be seeing.

What we are seeing in both the eastern and western populations (eastern being the monarchs that go to Mexico in the winter, and western being those that come to Pacific Grove and other coastal California sites), is that the migration seems to be delayed.

Our most up-to-date counts show 3,000 to 4,000 monarchs in the Monarch Grove Sanctuary. That number falls short of the more than 7,000 that were counted at the same time last year. While the leading edge of the eastern monarch migration has reached sanctuaries in the Mexican mountains, many remain as far north as the Great Lakes and as far east as the Atlantic coast. So what is going on?

Record-breaking warm weather across much of the United States, especially through October and into November, may be to blame for the delay:

It is thought that, in a typical year, monarchs receive their signal to begin migrating from a few observable factors:  #1) the shortening of days as the season changes from summer, to fall, to winter, and #2) the dying-off of milkweed.

For #1, we have a constant. No matter what the weather is doing during September, October, and November; the amount of daylight will slowly decrease (in the Northern hemisphere) beginning on June 21st, the summer solstice, and ending on December 21st with the winter solstice. It is thought that monarch butterflies can detect this shortening period of daylight and use it to begin their journey.

For #2, however, the dying-off of milkweed is much more variable. Milkweed is a heat-loving plant, and is also the plant that monarch caterpillars need to survive. As weather begins to cool in the fall, milkweed dies off, taking with it the habitat needed to feed monarch caterpillars and sustain the next generation of butterflies. Scientists believe this decreasing supply of milkweed in the late fall is another signal, telling the migrating generation of monarchs to hit the trail.

So, if the monarchs that would normally be the “super-generation” of migrators is emerging to find healthy milkweed and unseasonably warm temperatures, might they just stay put? Why not leave the migrating to the next generation? Or maybe they’ll just sip nectar for a few weeks, and begin their journey in late October instead of late August or September. What’s the harm?

Well, when late migrators eventually set out on their journey, they will have a higher chance of encountering damaging storms, hard frosts, and strong winds, lowering their chances of completing their incredible journey and reaching the safety of their overwintering site. This could be what we are seeing and it could spell trouble for a species that is already facing a host of challenges, including human-induced climate change and habitat loss.

There is still a chance that we will see a strong late migration, and that the monarch population in Pacific Grove will continue to increase over the next few weeks. For now, we at the Museum will continue to monitor and marvel at this tiny insect which, year after year, flies many hundreds of miles to spend the winter in our little town. For your best chance at seeing the most monarchs in Pacific Grove, visit the Sanctuary around Thanksgiving or in early December. You can also check our website for up-to-date monarch numbers, or visit the Museum to talk to a staff member or view our monarch exhibit.