Monarch Migration Begins

I see them everywhere now. Deep orange with black trim, monarch butterflies dance among the wildflowers we planted for them. Two, then three, then four! Are they fighting or loving as they bang their delicate wings? Then, calm for an instant, they alight on the many-petaled clusters of butterflyweed and sip its abundant nectar.

I look closely while they are still. One, at least, is a male. I can see the slender black lines with a swollen black dot on each of his hindwings as he unfurls his slim proboscis and drinks deeply through its hollow center. This amazing tube is made of two long, C-shaped channels, which the butterfly joined together with tiny hooks and fringes after he emerged from his chrysalis. Even now, if his proboscis gets clogged with sticky nectar, he can unhook the two channels to clean them.

As a female – with robust black lines – explores a nearby butterflyweed, I try to watch her feet. Butterflies taste through their “toes,” identifying their platform as a milkweed, and also determining its quality as baby food. While this low, bushy plant looks nothing like common milkweed, it is a close cousin, and one of the few host plants that can nourish monarch caterpillars. Each female lays an average of 700 pinhead-sized eggs during her two-to-six-week lifespan.

Leaning in closer—hoping to catch her in the act of egg laying—I startled her into flight instead. Monarchs, with their compound eyes, have almost 360 degree vision, and use it to detect predators even with their head in a flower.

The eggs that are laid now, in mid-August, will become butterflies who live longer and fly farther than their parents. The eggs laid now will become the migratory generation, and take part in an amazing, two-month-long journey. Monarchs produce three to four generations per summer, and the population grows with each new generation. By early fall, monarch numbers are at their peak.

The shortening days and cooler nights of fall trigger many seasonal changes in the northwoods. Leaves begin to show their brilliant fall colors, birds stop singing and begin to migrate. Many insects enter an overwintering phase of their life cycle. Monarchs are no different. The onset of fall triggers the larvae and pupae of monarchs to enter "reproductive diapause." They metamorphose into pre-pubescent butterflies who cannot yet reproduce, but who are ready to journey south.

This happens all across the monarch’s habitat at slightly different times. Canadian butterflies begin their migration earlier, swirling through Wisconsin at about the same time that our butterflies are ready to head south, too. Eve Depew, the seven-year-old scientist from Hayward, WI, who has been raising monarchs all summer, has more than a dozen chrysalids that are close to emerging. She will name and photograph each monarch, then send them out to start their journey.

The building wave of peak monarch abundance hits us here in early September. The surge reaches Houston, Texas, in mid-October, and the butterflies’ destination – the Transvolcanic Mountains of central Mexico – in mid-November.

These young butterflies have never seen their destination–nor did their parents, grandparents, or even great-grandparents. Flying only by day, they navigate with some combination of sunlight (the ability to see polarized light lets them know the position of the sun, even when hidden by clouds), and magnetism. Tiny “Johnston’s organs” at the base of each antenna are sensitive to stretching. They are used to detect wind and gravity, and maintain balance and orientation during flight. They may also be able to detect the Earth’s magnetic field.

Even with such sensitive instruments, it is a mystery how these tiny (and thus tiny-brained) butterflies even know where they are supposed to end up.

Watch for the change in monarch flight patterns as fall approaches. Their erratic fluttering from flower to flower is replaced by directional flight. Heading south, they rise on thermals of warm air, and soar on wind currents just like birds. Like a hot-air balloon pilot, they fly at the altitude with the best wind direction. And they avoid flying on days with a strong headwind. Scientists using a model butterfly have discovered that the forces from tiny eddies of air created during each wing flap generate lift. By using all of those efficiencies and by feeding regularly, monarchs are actually able to gain weight on their two-month-long trip, and can travel at least 265 miles in a day!

Entering the winter with plenty of fat stored in their abdomens, millions of monarchs prepare for a long wait. This migratory generation must face a host of perils not encountered by their parents. And the continuation of the species rests on their tiny shoulders. (More about this next week.)

Do you have monarch eggs, caterpillars, or butterflies in your yard, too? We may be in the presence of the greatest generation.

Monarchs that are born at the end of the summer don’t fully mature. Instead, they enter “reproductive diapause,” and live longer than any other generation – long enough to fly to Mexico for the winter. The thicker black lines on the butterfly mark her as a female. Photo by Larry Stone.