Monarch Generations
Which Generation Goes to Mexico in the Fall?

Spring is the beginning of the monarch's breeding season. In March, the monarchs migrate north from Mexico. They breed in the southern United States in March and April, and then they die. At that time they are 7-8 months old. They were born as early as August, survived fall migration, a long winter, and spring migration to the southern U.S. In contrast, during the summer breeding season, monarchs live for only 2-5 weeks! Several generations live and die each summer, and it's the final generation that migrates to Mexico in the fall. (See Monarch Annual Cycle slideshow.)

1) Using the worksheet below, you can figure out which generation migrates to Mexico. Here is the question: Of the monarchs that spent last winter in Mexico, do you think this fall's migratory generation will be:

  • their children?
  • their grandchildren?
  • their great-grandchildren?
  • their great-great-grandchildren?
  • their great-great-great-grandchildren?
  • their great-great-great-great-grandchildren?
  • have the idea, now see if you can calculate the answer!

2) First you need to know how long it takes for one monarch generation to develop:

Development Time for a Monarch Generation

# days

Egg to Larva


Larva to Chrysalis


Chrysalis to Adult


Days before adult female is old enough to lay eggs


Total number of days between generations

25-37 days

Average number of days

_____ days

3) Use the chart below to calculate the development date for each generation. Assume the first eggs are laid on March 15th, by the monarchs that overwintered in Mexico.

4) Fill in the blanks for each new generation by adding the average number of days for a generation to develop.

5) Monarchs stop laying eggs in the north in late summer, when the days begin to get shorter, the temperatures drop and milkweed stops growing. For this example, use August 15th as the date the eggs of the last generation are laid. That is, when you get to August 15th on the chart below--stop! This is the generation that will migrate to Mexico!

Generation Number Generation
1st Date this generation ready to lay eggs


Over-wintering generation (monarchs from Mexico).

March 15


Children of monarchs from Mexico












Great-great-great grandchildren





Important Note: These are estimates! In reality of course, the time for a generation to develop varies, based on temperature and other factors. This question is only intended to help estimate the timing of monarch generations.

1) There are many assumptions, averages and estimates in this activity. Each one affects the answer you get. How many can you find? If you change some of the assumptions, averages and estimates, how does it affect the answer?

2) If you live in the monarch's summer breeding range, you can use this chart to keep track of the generations that develop this summer where you live.

  1. Watch for eggs on your milkweed plants.
  2. Bring one or two inside, where they're safe from predators.
  3. Count how many days it takes for them to develop. (Be sure to keep temperatures the same as outdoor normal temperatures, so the development time will be the same. A screened porch is ideal!)

3) A female monarch can lay eggs throughout her 2-5 week lifetime, and a single female lays hundreds and hundreds of eggs. What does this mean about monarch generations? (Answer: There is a great deal of overlap between generations.)