About the Monarch
- Individual Life Cycle
- The Journey North
- Summer Breeding
- The Journey South
- Wintering in Mexico
- A New Cycle Begins
Each monarch begins life as an egg, laid underneath a milkweed leaf. That egg hatches after 3-5 days to reveal a first instar larva. Over a period of 9-15 days, that larva increases its body mass about 2000 times as it grows, molting, or shedding its skin five times to allow for this rapid increase in size. The period between each molt is called an instar; monarch larvae undergo five instars. It then pupates, and spends another 9-14 days as a chrysalis, or pupa. When fully developed, the adult butterfly emerges from the pupa casing, pumps fluid from its abdomen into its wings, and flies off to nectar, mate, and (if a female) lay its own eggs. Adult butterflies that don’t migrate live another 2-6 weeks. The development time of eggs, larvae and pupae depend on temperature, with cool or very hot conditions resulting in longer development times. Their survival rates are low, with only about 5% surviving to become fifth instars. The others are killed by a variety of predators, including ants, spiders, true bugs, beetles, and lacewing larvae.
Each spring, the first monarchs arrive in northern and eastern US states, and southern Canada shortly after milkweed first appears in an area. These first monarchs in the northern breeding range came from eggs laid by females who likely spent the winter in Mexico and flew to find milkweed in northern Mexico and the southeastern US (like Texas, Louisiana and Florida) using the last bit of energy they had after their long winter. We often think of these first monarchs we see in late spring as “the first generation” of the new year.
The monarchs that emerge as adults at the end of the summer can’t stay in colder climates for the winter. They aren’t adapted to handle the cold temperatures and lack of moisture. They undergo the same life cycle as the generations before them, until they are adults. At that point, their lives are different from all other broods. Instead of mating, and laying eggs, they put all of their energy into migrating to a climate that can sustain them until springtime. This allows them to stay alive until the next spring, when they’ll be able to fly north and lay eggs. This migration is the key part to success of the monarchs’ annual life cycle. These fall monarchs look exactly like all other monarchs. However, they are physiologically different, and emerge from the pupa in a state called reproductive diapause. Diapause is basically a period of suspended development; these 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 daylength, changes in temperature between daytime and night time, and even the age of the milkweed plants that they ate as larvae! Beginning in late August and continuing through September and October, these individuals fly up to 2,000 miles south to the volcanic mountain ranges of central Mexico. They drink nectar and catch warm air currents that allow them to soar instead of using powered flight as they go.
In Mexico, monarchs find the south facing slopes of the mountains and settle on branches of oyamel fir trees. The temperature and moisture allow them to go into a torpor (very similar to hibernation, only not as deep a “sleep”). This way, they can reserve their energy until the warmer spring weather returns.
The monarchs that spend 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 them that it’s time to be active again.
At the end of the winter, monarchs end diapause, becoming ready to mate and lay eggs as they move northward. Once they become reproductively active, they’ll only live another few weeks. Their eggs then mark the start of another annual life cycle, as the first generation of monarchs is born again. Unlike the summer generations, which only live for 2-5 weeks as adults, winter butterflies live 7-9 months – surviving a long southward migration, several months in Mexico, and a northward migration back into the southern US. This cycle of the a first generation of monarchs, followed by three to four summer generations, and a final fall migrating/overwintering generation together makes up the annual life cycle of the monarch species. Monarchs have evolved to live this way, with special adaptations that allow them to survive seasonal changes to their habitats. Nectaring plants, milkweed, water, and the oyamel fir forests in Mexico are all crucial to allowing this annual life cycle continue. Throughout each year, in whatever country they find themselves, monarchs compete with human needs for land and resources. Efforts to provide for monarchs are coupled with addressing the needs of human residents, particularly in the constrained overwintering area. It can be done, and MBF works to that end.
Much more information about monarch biology and conservation can be found in the North American Monarch Conservation Plan (link).