The Monarch Butterfly's Annual Cycle
Individual Life Cycle
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.
Annual Life Cycle
The Journey North
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.
Summer Breeding
These first generation monarchs will mate, lay eggs, and die.
Their eggs will start the cycle over again, hatching, growing,
pupating, emerging, mating, laying eggs and dying, again
living up to 8 weeks. Each of three to four summer generations
over the summer lives the same kind of life.
The Journey South
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.
Wintering in Mexico
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.
A New Cycle Begins
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.
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