The annual migration of monarch butterflies (Danaus plexippus) in eastern North America is one of the greatest animal migrations the world over. Up there with the wildebeest and zebras of Africa, sea turtles of Costa Rica and emperor penguins in the Antarctic, monarch butterflies make a long-distance annual migration in order to survive.
Monarch butterflies begin each spring in central Mexico’s Sierra Madre mountains, fly as far north as southern Canada for summer, then reliably return each fall to overwinter in those same Mexican mountains. This cycle includes millions of butterflies spanning four to five generations, distances of up to 3,000 flight miles per butterfly, as well as lots of flower nectar and milkweed (Asclepias).
The monarch is a tropical butterfly so it must move south in fall. Their chosen winter home is in high-elevation forests of oyamel trees (Abies religiosa) 100 miles northwest of Mexico City. At 8,000 to 12,000 feet above sea level it doesn’t sound tropical but average winter temperatures remain above freezing. Their return coincides with the Mexican holiday Dia de los Muertos, the Day of the Dead, and tradition says the monarchs are the souls of ancestors returning to earth for their yearly visit.
The monarchs roost en masse in these large trees so densely that, though each weighs half a gram, their sheer numbers can break a branch. Similarly, when millions suddenly take flight roused by a gust of wind, you really can hear their wings flapping, like the tapping of a quick light rain.
As temperatures warm in late January and early February, the monarchs emerge from hibernation to mate and fly north to lay eggs and spend the summer. Since an egg becomes an adult butterfly in a month and adults live two to six weeks, three to four generations of monarchs will occur during their spring and summer in the north.
Monarchs solely use milkweed (Asclepias) as their host plant to lay their eggs on. The advantage to this lies in the bad tasting, milky “sap” that seeps from damaged milkweed foliage. The caterpillars voraciously eat milkweed, undeterred by this substance, and come to taste like it themselves. Over time, predators have learned this and usually avoid them. While caterpillars need milkweed, adult monarchs feed on flower nectar.
Fall’s cooler temperatures and other seasonal clues trigger a “super generation” to fly back to Mexico and overwinter. Unlike the dispersed journey north, the path south funnels all monarchs east of the Continental Divide into one narrow path through central Texas to Mexico creating a very noticeable phenomenon.
These adults are larger than usual, do not have developed sex organs, and possess more body fat than other generations. They can fly upwards of 1,500 miles, completing more than 50 in a day, and live twice as long. They stop daily for flower nectar and huddle at night for warmth. Ironically, these adults are known to gain weight, instead of losing it, as you might think. This is essential to their survival since they are the ones to mate in the spring and lay the eggs of the first generation of the next migration cycle.
Sonya Anderson is a horticulture specialist in the Pollinator Gardens with the Denver Botanic Gardens. She can be reached at horticulture@denverbotanicgardens.org.
