Monarch Butterfly Life Cycle
The monarchs that spend the winter in the mountains of central Mexico are the final generation
of a cycle that begins anew each year. Most of the butterflies in this final generation begin their
lives in the northern US or southern Canada, and then migrate thousands of miles to mountaintops
that they have never seen before.
After spending several months in Mexico they return north
beginning in March, starting the cycle again as they lay eggs in northern Mexico and the southern
US. Their parents, grandparents, and great-grandparents have very different lives. These summer
monarchs live only about a month as adults and begin laying eggs when they are only a few days old.
In most years, the total number of monarchs probably increases with each generation. Because the
winter generation must live for such a long time before reproducing, the entire population shrinks
as some of these individuals die during the fall migration and overwintering period. The population
then grows over the spring and summer.
||March to May
||April to June
||Yes, north in spring
||May to July
||June to July
||July to August
||July to August
||July to October
||August to April
||Yes, south in fall and north in spring
Monarchs in Generation 1 are the offspring of the monarchs that overwinter. They are laid from
late March through April in the southern United States and northern Mexico, and fly north as adults.
They do not undergo reproductive diapause.
The first monarch generation of the year begins when females that have spent the winter in Mexico
lay eggs in northern Mexico and the southern U.S. beginning in late March. The last eggs are laid in
late April or early May, farther north. Since it is often cool when Generation 1 larvae are developing,
it may take them up to 40 or 50 days, or even more, to develop from eggs to adults.
Generation 1 adults emerge from late April to early June. They mate and begin to lay eggs about
four days after emerging, and continue the journey north that their parents began, laying eggs
along the way. They begin to arrive in the northern US and southern Canada in late May.
Like all monarchs, this generation begins life on plants in the genus Asclepias, these are species
of milkweed. The most important host plants for Generation 1 monarchs in the southern US are
Asclepias oenotheroides, A. viridis and A. asperula.
Monarchs in Generation 2 are the grandchildren of the overwintering monarchs. They are laid
throughout much of eastern North America from late April through June. They do not undergo
reproductive diapause. Those laid in the southern part of their range continue to migrate north.
Generation 2 larvae are widely distributed throughout the eastern United States, first beginning to
appear in the south in early May, and in the north in mid to late May. Eggs that become Generation 2
may be laid as late as July in the north. These larvae also eat milkweed species; a few of the main s
pecies that they use include A. syriaca (common milkweed), A. incarnata (swamp milkweed), and A.
tuberosa (butterfly milkweed).
Generation 2 adults emerge in June and July, and mate and lay eggs soon after emerging. Most of
those that begin their lives in the south move north as adults, since the southern summers are too
hot and dry for their offspring. Those laid farther north probably do not move far, and can use all of
their energy to produce as many offspring as possible.
Monarchs in Generations 3 and 4 are the great- and great-great grandchildren of the overwintering
monarchs. They are laid throughout the northern part of the range of eastern migratory monarchs
rom late May through July (Generation 3), and late June through August (Generation 4). Some
Generation 3 individuals emerge early enough to reproduce in the northern part of their breeding
range or after moving south (see immature distribution map). However, Generation 3 individuals
that emerge late in August will undergo diapause and migrate to Mexico, as will most Generation 4
Generations 3 and 4 monarch eggs are laid throughout the northern part of their range in July and
August. Some adults move south in late July and August, and may lay eggs as late as October in the
southern part of the US.
Some generation 3 monarchs emerge early enough to produce another summer generation. But those
that emerge later are different from other monarchs in two important ways. First, they will migrate to
and from the overwintering sites in Mexico. Second, they do not reproduce right after they emerge.
In response to decreasing temperatures and shortening day lengths at the end of the summer, their
reproductive organs remain in an immature state.
Instead of mating and laying eggs, they spend their
time drinking nectar and clustering together in nighttime roosts in preparation for their long journey
south. This delayed maturity is called diapause. Most of the monarchs will remain in this condition
until the following spring, when they begin to mate in the overwintering colonies.
During September, October, and early November, migratory adults fly to overwintering sites in central
Mexico, where they remain from November to March. In March, they begin to journey north, laying the
eggs that will become the new Generation 1 along the way.
In early spring, as Monarchs move north through the Texas funnel, they need milkweed on which to
lay the first generation of eggs. They continue to need milkweed through the summer as subsequent
generations reproduce as they move north.
Milkweeds including Common milkweed (Asclepius syriaca), Swamp milkweed (A. incarnata), Poke
milkweed (A. exaltata), Purple milkweed (A. purpurascens), Butterfly milkweed (A. Tuberosa), Prairie
milkweed (Asclepias hirtella), Whorled milkweed (Asclepias verticillata) and Short green milkweed
(A. viridiflora) are some of the milkweed host plants Monarch butterfly depend upon.
Monarch butterflies need floral nectar in the springtime to fuel migration and reproduction. The rate
at which spring-blooming flowers develop is largely temperature-dependent; flowers bloom earlier
in a spring with warmer temperatures. Because adult monarchs are generalists, they are able to eat
nectar from a wide variety of spring flowers. This fact gives them some flexibility. In contrast, monarch
larvae are specialists; they can only eat milkweed. The need for milkweed may determine when and
where monarchs travel because even though adult butterflies don’t depend on milkweed, their
But in the fall when the monarchs are heading south, they don’t need milkweed since they enter
their reproductive diapause, a temporary stage in which they postpone sexual
activities and save their energy to migrate to Mexico. At this point, they need water, ample nectar
sources, trees or other protection at night for roosting, and connected habitats.
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