How Do Monarchs Know When to Migrate?

Migrating monarchs
How do monarchs know when it's time to migrate?. Flickr user Anita Ritenour (CC license)

The monarch butterfly is a true miracle of nature. It's the only butterfly species known to complete a round-trip migration of up to 3,000 miles each year. Each fall, millions of monarchs make their way to the mountains of central Mexico, where they spend the winter hunkered down in the oyamel fir forests. Just how do the monarchs know when it's time to migrate?

Differences Between Summer Monarchs and Fall Monarchs

Before we tackle the question of what makes a monarch migrate in the fall, we need to understand the difference between a spring or summer monarch and a migrant monarch.

A typical monarch lives just a few weeks. Spring and summer monarchs have functional reproductive organs soon after emergence, allowing them to mate and reproduce within the constraints of a short life span. They're solitary butterflies that spend their brief days and nights alone, with the exception of the time spent mating.

The fall migrants, however, go into a state of reproductive diapause. Their reproductive organs aren't fully developed after emergence, and won't be until the following spring. Rather than mating, these monarchs put their energy into preparing for the arduous flight south. They become more gregarious, roosting in trees together overnight. The fall monarchs, also known as the Methuselah generation for their extended life span, need lots of nectar to make their journey and survive the long winter.

Three Environmental Factors That Trigger the Monarch Migration

So the real question is what triggers these physiological and behavioral changes in the fall monarchs?

Three environmental factors influence these changes in the migrant generation of monarchs: the length of daylight, the fluctuation of temperatures, and the quality of milkweed plants. In combination, these three environmental triggers tell monarchs it's time to take to the skies.

As summer ends and fall begins, days grow gradually shorter.

This steady change in the length of daylight helps to trigger reproductive diapause in late season monarchs. It's not just that the days are shorter, it's that they keep getting shorter. Research at the University of Minnesota showed that monarchs subjected to a constant but short amount of daylight would not go into reproductive diapause. The daylight hours had to vary over time to cause the physiological change that makes a monarch migrate.

Fluctuating temperatures also signal the change of seasons. Although daytime temperatures may still be warm, late summer nights become noticeably cooler. Monarchs use this cue to migrate as well. University of Minnesota scientists determined that monarchs reared in a climate of fluctuating temperatures were more likely to go into diapause than those reared at a constant temperature. Late season monarchs that experience changing temperatures will suspend reproductive activity in preparation for migration.

Finally, monarch reproduction depends on an adequate supply of healthy host plants, milkweed. By late August or September, the milkweed plants begin to yellow and dehydrate, and are often covered with sooty mold from aphids. Lacking nutritious foliage for their offspring, these adult monarchs will delay reproduction and begin migration.