The Great Monarch Migration

In mere weeks, a humble insect will begin an incredible journey. The current generation of monarch caterpillars, Danaus plexippus, is preparing to transform and make their annual migration. The butterflies take to the wind as the end of summer draws close, bound for warmer weather in Mexico. Most of the monarchs born in our stretch of the Blue Ridge will depart by mid October, though we may continue to see populations from further north passing through until the first frost. Once in Mexico, hundreds of millions of individuals crowd flowers and hang from trees in enormous roosts- also called bivouacs- while they wait out the winter weather. 

Some of the butterflies in our collection. The three in the bottom left box are monarchs, Danaus plexippus. The middle butterfly is a male, evidenced by the pheromone-producing black spots on the vein of the hind wing called his “pouches.” The top and bottom are females, who tend to have thicker black lines along the hind wing veins and lack these pouches. The box above them holds three commonly confused, but still very cool, Viceroy butterflies- Limenitis archippus. They mimic the coloration of monarchs and are similarly distasteful to predators. Notice the continuous black arc on the hind wing that runs parallel to the wing border present only on viceroy wings.

A female monarch visiting milkweed in our rain garden, likely selecting a suitable plant to host one of her young

When temperatures start to rise and days grow longer in northern Mexico, the monarchs prepare to make their return flight by increasing nectar consumption and beginning to mate. Come March, the butterflies depart- following the spring warmth as it stretches north. On their return flight, the butterflies fly low and stop often to feed and lay eggs. A female monarch may carry more than 300 fertile eggs at a time, and will continue finding new mates as she journeys onward. Mother monarchs are very particular about where they will leave these eggs. While the adult butterflies forage nectar from a variety of flowers, the caterpillars exclusively feed on milkweed species. She carefully examines each milkweed plant she finds and, should it pass her inspection, lays a single egg on the underside of a leaf near the top of the plant. This ensures her caterpillars have plenty of food to grow quickly without competing with others or decimating the plant. 

Monarch caterpillars feed exclusively on milkweed- a plant that is toxic to many other animals. The monarch stores these toxins in its own body so predators learn not to eat them.

Many of you are aware of the monarch butterfly migration; and we all are familiar with the four stages of their complete metamorphosis from egg to caterpillar, then chrysalis and finally adult butterfly.  However, the way these two life cycles interact may come as a surprise. In our region, D. plexippus experiences 4 generations every year. We have two generations of summer monarchs composed of small, short-lived individuals who will only know this land as home. The successors of the second summer monarchs are significantly larger, stronger, and longer-lived (Up to 9 months as a butterfly!) It is this generation, the fall monarchs, that will make the entire flight to Mexico and return more than halfway back, dispersing their eggs the entire return flight. From these eggs emerge the spring generation, who will complete the migration past the US’s northern border, recolonizing their entire native range.

Poster created by Paul Mirocha for Monarch Watch. This stylized map shows the seasonal ranges of monarch migration over North America. Our fall monarchs fly to Mexico before winter, then fly up into the South East in the spring. The next generation continues moving north, recolonizing the northern half of the continent. Summer generations breed in the north until the rise of the next fall generation, who will make the entire migration south to Mexico and halfway back.

Despite being one of the most extensively studied butterfly species, there are still many mysteries entomologists are working to solve. A community science project called Monarch Watch has volunteer participants help generate data about monarch populations during the fall migration. Unfortunately, the winter roosts in Mexico have been rapidly shrinking in recent decades, sparking much of the research and conservation efforts going on now. Leading theories of causes for this decline include habitat loss and resource scarcity. Much of their historic breeding and feeding lands have been converted to farms, making the butterflies’ journey even more perilous.

Monarch Watch is a community science project that began in 1992. Participants catch and tag fall migrating monarchs with a sticker that has a unique code. Data is recorded including the tag code, date, location, and information about the individual, before it is released. In the winter, butterflies are recaptured in Mexico and searched for tags. The data generated is crucial for estimating monarch population change over time as well as predicting migration times, routes, and survival rates.

Helping monarch populations can be an easy feat for anyone with the lawn space to grow a few native plants. Research some local long-blooming wildflowers, especially those that provide plenty of nectar and bloom through fall. Asters, columbine, vervain, mountain mint, and many others are fairly low maintenance native wildflowers that are great nectar sources for pollinators, including monarchs. Milkweed prefers to have a fair amount of space and well drained soil that gets plenty of water. However, there are several varieties that prefer different conditions, so researching a variety that would do well in your area is a great way to support the monarch population.

At our latitude of around 36° 50’ N, monarch tagging begins at the end of the month- August 29. Peak abundance usually occurs in late September, and the last of this year’s monarchs might be seen into early November. As we look forward to watching another generation of butterflies follow in the wingbeats of their great-grandparents, let these beautiful creatures remind you of the enormous cycles that make up nature.