Annual 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 kilometers to mountaintops that neither they nor their parents (and likely their grandparents) have ever 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.
Table 1: Summary of Monarch Annual Life 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|
*Months during which each generation exists.
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.
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 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.
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 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 species 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.
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). 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 individuals.
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.