What determines the length of a monarch butterfly’s life?

[Professor (female)] Good afternoon, everyone. Q12 Today we’re gonna discuss the development
   of butterflies and how--as I’m sure most of you already know--they progress
   through four distinct stages: egg, larva, pupa, and adult. It’s a rather unique
   lifecycle, one that sets butterflies apart from many other organisms. Um, you
   may not be aware of this, but there’s a term that describes this kind of lifecycle--
   we call it "holometabolism." Q17 Holometabolism--literally
   meaning, um, "total change"--refers to the complete metamorphosis that
   occurs in organisms like butterflies, as they progress through the four stages
   of their lifecycle, starting as an embryo within an egg and ending as an adult
   imago...
[Student A (male)] What was that you said...imago?
[Professor] Is that term new to you? Imago. In insects, the imago is the last stage of
   development. In butterflies, it’s the stage where they become sexually mature
   and develop fully functional wings--the stage where they actually become a...a
   butterfly. Got it?
   OK. Q13 So let’s just jump right into the topic then, starting with the stage of
   development where the butterfly is an egg. Uh, I think I’ll talk specifically about
   the lifecycle of monarch butterflies--to represent the lifecycle of butterflies
   in general. So, the whole process of development in monarchs--from egg to
   adult--takes about a month. It all begins when an adult monarch lays a single
   egg on the leaf of a milkweed plant. um, usually it’s on the underside of the leaf,
   near the top of the plant. We aren’t exactly sure how many eggs an adult female
   will lay during the course of her lifetime, but scientists suspect that it may be
   from 100 to 300. Anyway, after about four days, these tiny little eggs--they’re
   about a millimeter long--these tiny little eggs hatch. Q13 And once they’ve
   hatched, monarchs enter the larval stags.
[Student A] Is this the stage where they’re known as caterpillars?
[Professor] That’s right. Q14(C) This is the stage where the monarch exists as a caterpillar--
   caterpillars are butterflies in larval form. Q14(E) Um, you could basically say that
   caterpillars are little more than eating machines. That’s pretty much all they
   do--eat. Eat and grow, I should say. That’s because during the larval stage,
   butterflies do all of their growing. For two weeks, caterpillars just sit around
   on milkweed leaves and eat and eat and eat. Q14(B) Periodically, monarch
   caterpillars actually outgrow their skin! So they molt--that is, they lose their
   skin and grow a new one. At the beginning of the larval phase, monarch
   caterpillars are about 2 to 6 millimeters long, but by the end of it, they’re
   anywhere from 25 to 45 millimeters in length.
[Professor] Q13 After the larval stage, the monarch proceeds into the pupal stage. Uh, let
   me show you some slides so you can see what this really looks like. So, uh, first
   here’s the egg...then, this one shows the larval stage...and here, this is the slide
   I wanted you to see. It’s in what we call the pupal stage. Quite a transformation,
   isn’t it?
[Student B (male)] That’s also got another name, right? Uh...chrysa...chrysalis!
[Professor] Right. It’s also called the chrysalis stage. So, about two weeks have passed since
   the monarch hatched from its egg, and now it’s in the chrysalis--or pupa--
   stage. Um, to enter this stage, the caterpillar hangs upside down from a branch,
   um, gripping it with its two hind legs. Next, it molts again, covering itself with a
   green exoskeleton that’ll protect it while it undergoes the changes that’ll turn it
   into an adult. For the next ten days or so, there’ll be very little movement from
   inside the protective casing. Q15(C) Because it’s relatively defenseless during
   this stage, the chrysalis is usually well camouflaged to help protect the pupal
   monarch from predators. Q15(A) When it’s fully developed--and this happens
   about 28 days after hatching from its egg--the monarch will emerge from its
   shell and expand its wings for the first time. Q13 It’s reached the imago stage
   and is now an adult.
   The most important task for adult monarchs is to mate and produce offspring
   that will start the whole lifecycle all over again. Q16 Adult monarchs actually
   have a pretty short lifespan--two to five weeks for generations born in the
   summer. However, the last generation of the summer, which matures in late
   summer or early fall...they live a lot longer--eight to nine months--and they
   migrate south to warmer places like Mexico or California. After the winter is
   over, they return and reproduce before dying.