The monarch butterfly (Danaus plexippus) is a tropical species that has expanded its range into temperate regions following milkweed's distribution. To escape harsh winter conditions, North American monarchs make a long-distance two-way migration, flying south each Fall and then north again each Spring. This spectacular multi-generation migration is key to the success of the monarchs’ annual life cycle.
The fall migrating/overwintering generation, followed by a spring generation and three or four summer generations together comprise the monarch's annual cycle. Although one generation makes the long journey south to overwintering sites, it takes multiple generations of monarchs to repopulate habitat across North America. Nectar plants, milkweed, water, and the protective forests in winter are all crucial to continue the monarch’s annual cycle.
Learn more about the monarch in each season below.
Beginning in late August and continuing through September and October, individual monarchs fly up to 2,000 miles to reach their winter home. They drink nectar along the way and to conserve energy, catch southward air currents that allow them to soar as they go. Fall monarchs look like other monarchs, but they are physiologically different, emerging from the pupa in a state called reproductive diapause (see below). Instead of mating and laying eggs, the fall monarch puts all of its energy into migrating to a climate in which it can survive until springtime. We call these monarchs the migrating generation. Migration and delaying reproduction allow them to stay alive until the next spring when they can fly north, resume reproduction, laying eggs as they encounter milkweed.
Monarchs east of the Rocky Mountains migrate to overwintering sites in Mexico, whereas monarchs in the western states migrate to sites along the California coast. Monarchs in the southwest fly either southeast to Mexico or west to California. We know this from long-term tagging programs like Monarch Watch and Southwest Monarch Study. Regardless of their destination, it is an incredible journey.
Did You Know: Diapause is a period of suspended development produced by changes in the insect's hormonal levels. A variety of signals from nature combine to induce this diapause condition, including shortening day length, changes in temperature between day and night, and even the age of the milkweed plants that they had eaten as larvae.
Eastern population monarchs arrive at their overwintering sites in the Transvolcanic mountains of central Mexico around the beginning of November (often coinciding with dia de los muertos). Once there, monarchs find south-facing slopes of high mountains and settle on branches and trunks of oyamel fir trees, forming dense clusters. Temperatures are low at these elevations, allowing the monarchs to go into torpor (very similar to hibernation), conserving their energy until warmer spring weather returns.
After spending several months in these overwintering sites (from early November until March), lengthening days signal that it’s time to become active again. In late February and March, monarchs end reproductive diapause, becoming ready to mate and lay eggs as they move northward. Once reproductive, the butterfly typically lives only another few weeks.
|Did You Know: monarchs of this fall/overwintering generation live up to 8 or 9 months?!
A similar spring migration occurs in the west where monarchs that overwintered along the California coast migrate north and east in search of milkweed habitat.
Monarchs leave their southern winter home in in Mexico in March, flying northward to find milkweed in northern Mexico and southeastern United States (i.e., Texas, Louisiana, and Florida) with some venturing to the western United States. They use their last bit of energy after the long winter to fly north in search of milkweed. Eggs laid in these regions by these migrating monarchs mark the start of the next generation of monarchs.
As spring progresses into summer, this new generation of monarchs continues to fly north, arriving in northern U.S. states and southern Canada shortly after milkweeds appear there, laying eggs as they encounter milkweed. Their offspring become the first of the summer generations of monarchs.
Summer generation monarchs spend much of their time mating and reproducing. When not nectaring, female monarchs search for milkweed to lay their eggs on, and male monarchs patrol milkweed patches in search of females.
Breeding monarchs may live two to five weeks. The development time of eggs, larvae, and pupae depends on temperature. Cool or very hot conditions result in longer development times. The survival rates of the immatures are low, with less than 5% surviving to become fifth instars. A variety of predators, including ants, spiders, true bugs, beetles, and lacewing larvae cause much of this mortality.
Eggs laid by the female monarch start the life cycle over again (see the Monarch's Life Cycle) and signify a new generation. Depending on location, there can be three to four generations throughout the summer.
As summer conditions wane, a final generation of monarchs emerges that shifts from reproduction to migration, heading to warmer climates to survive winter. The annual cycle repeats.