Annual Life Cycle
Generation 1 | Generation 2 |
Generations 3 & 4
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 kilometers to mountaintops that neither they nor their parents
or grandparents have ever seen before. After spending several months in Mexico they
return north, 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.
Table 1. Summary of Monarch Annual Cycle
|
Generation # |
Timing of immature stages* |
Timing of adult stage* |
Migrates? |
Overwinter? |
|
1 |
March - May |
April- June |
Yes, north in spring |
No |
|
2 |
May - July |
June - July |
No |
No |
|
3 |
July - August |
July - August |
Some |
Some |
|
4 |
July - October |
August - April |
Yes, south in fall and north in spring |
Yes |
Information for the following pages on monarch migration are taken from papers by
S.B. Malcolm, B.J. Cockrell, and L.P. Brower and from data collected by the Journey North website.
Generation 1
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.
Life Cycle Summary
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 United States 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 milkweed family.
The most important host plants for Generation 1 monarchs in the southern US are
Asclepias oenotheroides, A. viridis and A. asperula.
Generation 2
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.
Life Cycle Summary
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 late May.
Eggs that become generation 2 may be laid as late as July in the north.
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.
Many members of Generation 2 eat milkweed species including A. syriaca (Common
Milkweed) and A. incarnata (Swamp Milkweed) as larvae.
 |
 |
|
A. incarnata |
A. syriaca |
Generations 3 & 4
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 from late May through July (Generation
3), and late June through August (Generation 4). Those in Generation 3 do not undergo
reproductive diapause or migrate, but members of Generation 4 that emerge late in
August will undergo diapause and migrate to Mexico.
Life Cycle Summary
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 daylengths 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.
Generation 3 and 4 undergo huge increases in size, taking advantage of the abundance
of milkweed and favorable temperatures in the northern US and southern Canada.