Monarch Life Cycle
The life of a monarch begins when a female monarch lays an egg, usually on the underside of a milkweed leaf. The egg hatches after 3-5 days to reveal a very small larva (caterpillar). Over a period of 9-15 days, the larva increases its body mass about 2,000 times as it grows and molts (sheds its skin) five times to allow for the rapid increase in size. The period between each molt is called an instar; monarch larvae undergo five instars, so the largest caterpillars are fifth instar larvae. Each larva then pupates and spends another 9-14 days as a chrysalis or pupa. When fully developed, an adult butterfly emerges from the pupa casing. It pumps fluid from its abdomen into its wings, and in a short time the wings dry and harden. The new monarch then flies to nectar and soon mates. If it is a female it starts laying eggs.
Breeding monarchs may live two to five weeks. The development time of eggs, larvae, and pupae depend 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.
The Journey North
Each spring, the first monarchs arrive in northern U.S. states and southern Canada shortly after milkweeds first appear. These first monarchs in the northern breeding range come from eggs laid by females who likely spent the winter in Mexico and flew to find milkweed in northern Mexico and the southeastern United States (like Texas, Louisiana, and Florida) and to a lesser extent the western United States. They use their last bit of energy after the long winter to begin the migration and reproduce. We often think of these first monarchs we see in late spring as the first summer generation of the new year. A similar spring migration occurs in the west from the California coastal overwintering sites.
The first summer generation monarchs mate, lay eggs, and die. Their eggs start the cycle over again, hatching, growing, pupating, emerging, mating, laying eggs, and dying, living up to 6 weeks. Each of three to four summer generations continues this cycle.
The Journey South
Monarchs that emerge as adults at the end of the summer are different because they aren’t adapted to survive freezing temperatures. Instead, this generation of monarchs undergoes the same life cycle as the generations before them until they are adults. Then their life is different from all other broods. Instead of mating and laying eggs, they put all of their energy into migrating to a climate in which they can survive until springtime. We call these monarchs the migrating generation. Migration allows them to stay alive until the next spring when they can fly north and begin laying eggs.
The fall migration is key to the success of the monarchs’ annual life cycle. Fall monarchs look like other monarchs. They are physiologically different, however, emerging from the pupa in a state called reproductive diapause. Diapause is a period of suspended development produced by changes in their hormonal levels; fall individuals do not have the mature internal sex organs (testes and ovaries) that the summer generations have. 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. Beginning in late August and continuing through September and October, these individual monarchs fly up to 2,000 miles south to the Transvolcanic mountain range of central Mexico. They drink nectar and conserve energy, catching warm air currents blowing to the south that allow them to soar instead of using powered flight as they go.
Wintering in Mexico
In Mexico, monarchs find the south-facing slopes of high mountains and settle on branches and trunks of oyamel fir trees, forming dense clusters. The low temperatures allow them to go into torpor (very similar to hibernation, only not as deep as “sleep”). This way, they can conserve their energy until warmer spring weather returns.
Monarchs that pass the winter in the mountains of central Mexico are the final generation each year. After spending several months in Mexico (from early November until March), warm, lengthening days signal that it’s time to become active again.
A New Cycle Begins
In late February and March, monarchs end reproductive diapause, becoming ready to mate and lay eggs as they move northward. Once they become reproductively active, they live only another few weeks. Their eggs then mark the start of another annual life cycle, as the first summer generation of monarchs is born again. Unlike the summer generations, which live for 2-6 weeks as adults, the overwintering generation of butterflies lives 7-9 months – surviving a long southward migration, several months in Mexico, and a northward migration back into the southern United States.
The first generation of monarchs, followed by three to four summer generations, and a final fall migrating/overwintering generation together comprise the annual life cycle of the monarch species. Monarch butterflies have evolved to live this way, with special adaptations that allow them to survive seasonal changes to their habitats. Nectar plants, milkweed, water, and the oyamel fir forests in Mexico are all crucial to continue this annual life cycle. Monarchs compete with human needs for land and resources wherever they are. Efforts to provide suitable habitat for monarchs are linked to supporting the needs of the local residents who have shared the same region with the butterflies for many generations. supporting the monarch life cycle is difficult, but it can be done, and MBF works to meet this challenge.
Much more information about monarch biology and conservation can be found in the 2016 Monarch Conservation Implementation Plan.