Monarch Life Cycle

 Director's Note: Our two seasonal naturalists for 2020, Katherine Culbertson and Jack McDonough, are preparing a series of blog posts designed to educate readers about many aspects of monarch biology and related topics.  Here's the third of this series, written by Katherine Culbertson, with photos by Katherine (except chrysalis and Mexico photos by Mark Garland), and titled: 

Monarch Life Cycle

We’re all familiar with adult monarchs – the beautiful black-and-orange butterflies flitting around Cape May right now, sipping nectar out of flowers. But this is only part of their amazing life cycle – and a small fraction of the life of most non-migratory generations. 

Adult female monarch butterfly

Monarch egg

Monarchs start life as a tiny egg laid on a milkweed leaf. Every female monarch will lay around 100-300 eggs in her lifetime, each on its own individual leaf. Milkweeds, plants of the genus Asclepias, are the host plants for monarchs; their caterpillars won’t eat anything else. There are 73 species of milkweeds native to the U.S., of which about 30 are known to be used by monarchs. These plants can vary widely in appearance, ranging from the large broad leaves of the common milkweed to the narrower, pointy leaves of butterfly weed and swamp milkweed, and to the vine-like climbing milkweed.

Newly hatched monarch caterpillar

After about 4-6 days monarch eggs hatch into tiny caterpillars. A newly hatched caterpillar doesn’t have its bright yellow, black, and white banded pattern yet; it also isn’t poisonous yet, as it hasn’t eaten enough milkweed. As the caterpillar grows, its striking colors become more defined. This coloration serves as a warning to birds and other would-be predators that monarchs do not make a good snack – they sequester a toxin from the milkweed they feed on that is toxic to any vertebrates that might try to eat them. A caterpillar’s only job is to eat; and eat, eat, eat, they do! From the time they hatch to when they form their chrysalis – a mere 10-14 days – a monarch caterpillar will increase in size more than a thousand-fold. For comparison, that’s like if a human baby, born at 8lbs, grew to be the size of an Asian Elephant in two weeks! As they grow, monarchs must shed their exoskeleton; that is, they must discard the tough chitinous material that covers their bodies to allow them to continue growing. This process is called molting. Each time a monarch molts, it transitions to a new stage called an instar. Monarch caterpillars molt 5 times during their lives, and thus have 5 instars.

Fifth instar monarch caterpillar

When monarchs have reached their 5th instar, after about two weeks of eating and growing, they are ready to pupate – to form a chrysalis, within which they will go through the process of metamorphosis, transforming into an adult butterfly. Once the 5th instar caterpillars become ready to pupate, they get the urge to wander until they find a suitable, protected place to form their chrysalis. Once they have selected a site, they attach themselves to that spot with silk, and hang upside down in a “J” shape. Soon, they molt a final time, and their appearance has completely changed – gone is the black, white, and yellow striped caterpillar, and in its place, an emerald green chrysalis hangs, decorated with gold “jewels”. The chrysalis may look dormant, but much is happening inside. The various parts of the adult butterfly – legs, wings, and other things – have already started growing within the large, fleshy body of the caterpillar, but now they continue to develop and rearrange within the chrysalis to assemble the adult butterfly. 

Monarch chrysalis 

After about two weeks, the chrysalis will start to change color, shifting from emerald green to a dark, purple-black tone; the bright orange of the butterfly’s wings starts to show through, as well. The butterfly is almost ready to emerge! Soon, the bottom of the chrysalis splits open, and out crawls the fully formed butterfly. At first, the butterfly’s wings are wet and crumpled, and the newly emerged butterfly must wait for them to expand and dry out before taking its first flight. Its proboscis – the straw-like mouth that allows it to suck nectar out of flowers – also isn’t properly formed yet when it hatches out. The two sides of the proboscis must “zip” together after it hatches out. Once the butterfly’s wings have dried, you can tell whether they are male or female – the males have an elongated black patch on both of their rear wings, which produces chemicals used in courtship by many species of butterflies. Females don’t have these patches, and also tend to be slightly darker and smaller than males, with thicker black wing veins.

Adult male monarch

Mating monarchs

Most monarch adults only live about 2-5 weeks, and don’t migrate long distances. Every summer, 3 or 4 generations of these non-migratory monarchs hatch out, migrating relatively short distances as the weather warms and their summer range expands. They mate and lay eggs along the way, parenting the next generation of monarchs. The final generation of the year, however, has a completely different life in store! As the fall approaches, something – no one is sure exactly what – causes this final generation to delay their reproduction and start migrating south. These are the butterflies that will fly hundreds, perhaps more than two thousand miles, and overwinter in Mexico. As they travel, the nectar they drink from flowers allows them to build up enough fat reserves to spend the winter dormant in Mexico. Unlike the previous generations, they will live 6 to 9 months, and in the spring, they will reawaken, mate, and lay eggs as they start to venture north, beginning the first generation of summer monarchs for the next year.

Monarchs starting to head north from Mexico wintering grounds