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Monarch Reproduction & Feeding
Monarchs go through complete metamorphosis, meaning they have four distinct life stages: egg, caterpillar, chrysalis, and adult. It takes approximately a month for an egg to become an adult butterfly.
Learn more about Monarchs: Creating Habitats | How to Help Monarchs
Monarch Eggs
​A monarch egg is about the size of a pinhead. It is off white with ridges running top to bottom. The egg remains glued to the underside of the milkweed leaf for three to five days depending on the temperature.
A healthy female will produce on average four hundred eggs in her lifetime. How many of those she successfully lays depends on how long she lives and the availability of milkweed to deposit them on.
Monarch Caterpillars
Caterpillars, especially when young, will eat the leaf and tender new growth, as seen in the top and bottom photos. When older, they will pierce the stalk of the plant and suck out the sap as you see in the image on the right.
A tiny caterpillar (2/10th of an inch) about the size of a grain of rice emerges into the world. The tiny hatchling will eat its eggshell before starting on its host plant, the milkweed.
Caterpillars spend most of their time eating. In approximately two weeks, a single caterpillar will eat several hundred times its weight in foliage growing 2000 percent of its size as a hatchling.
During the caterpillar stage, monarchs will only eat milkweed plants. Toxins in milkweed are stored in monarchs making them distasteful to predators.
Caterpillars, like adult insects, have exoskeletons. These are hardened structures that protect the inner organs and muscles but do not expand to accommodate a growing caterpillar.
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To grow, the caterpillar must shed its exoskeleton or molt. Each stage between molting is called an instar and can be used to identify the caterpillar’s “age.”
Chrysalis (Pupation)
After the fifth and final molt, the caterpillar transforms into a chrysalis or pupa. Often, but not always the caterpillar wanders away from the milkweed plant onto surrounding woody vegetation or other structures where it attaches itself in place with a silk pad.
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You know the transformation has begun when the caterpillar hangs itself upside down in a J shape.
It sheds its exoskeleton one last time to reveal a green casing that hardens into a protective shell.
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For another two weeks, a magical transformation is occurring.
Though it is not fully understood, we do know that the transformation is regulated by a series of hormonal changes that result in a complete cellular rearrangement of the caterpillar’s body transforming it into an adult butterfly.
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Even with this cellular rearrangement, monarchs carry the protective chemicals they acquire as caterpillars from eating milkweed into adulthood as an adult butterfly.
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Just before the adult emerges (known as eclosing) the chrysalis usually changes color and then breaks open. The butterfly crawls out and pumps their wings. Then within a few moments, the monarch takes its first flight.
Non-breeding, fall migratory monarchs live up to nine months.
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Breeding monarchs typically survive two to five weeks as adults. Roughly half of their life is spent as a caterpillar and half as a butterfly.
Given this brief existence, the availability of nectar and milkweed plants at the right time is existential to monarch survival.
Milkweed: a Monarch’s life begins with a plant
There are approximately 75 milkweed species native to North America, all classified together in the genus Asclepias. Rush Milkweed (Asclepias subulata) is the most common in the Coachella Valley. We also have Desert Milkweed (Asclepias erosa) and Whitestem Milkweed (Asclepias albicans). Both of the latter are locally native but less common.
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Milkweed is the host plant for monarch caterpillars. This means that monarch caterpillars can only eat milkweed.
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The massive feeding that monarchs do as caterpillars isn’t just about powering their larval growth. Caterpillars also need to eat for the butterflies they will become.
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This is particularly true for the production of eggs, which are made almost entirely of protein.
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As adult butterflies, monarchs feed on sugary flower nectar as a source of carbohydrate fuel giving them the energy to keep flying during their long migration.
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Much of the protein they need to make eggs comes from the feeding on milkweed they do as caterpillars and the protein reserves they store up in that life stage.
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Milkweed supplies caterpillars with protective cardenolides. This is the toxic compound that makes them unpalatable to predators.
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Over millions of years of shared adaptation and evolution, milkweeds have produced these bitter compounds as a defense against being eaten. Monarchs have countered by developing a tolerance to the toxins. Adult monarchs advertise the toxicity they gain from the milkweeds with their bright orange and black coloring — warning predators that they are poisonous.
Nectar Plants
Adult butterflies feed on nectar from a wide range of blooming plants. Nectar which is usually around 10 to 70 percent sugar provides critical carbohydrates, water, and low levels of lipids, and proteins as well as vitamins and minerals. This fuel source provides the energy for breeding, migrating, and overwintering.
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The quantity and quality of nectar sources play an important role in the size of the monarch population. You want to make sure you have a wide range of plants blooming when monarch butterflies are in your area.
Monarchs have a broad visual spectrum and true color vision. This allows them to locate nectar plants in the landscape.
You may have heard that monarchs prefer yellow, red, and orange flowers but in truth, they are opportunistic feeders nectaring on whichever flower provides the better nectar reward.
What not to plant
Lantana is a popular shrub and ground cover in the Coachella Valley and Southwest for both humans and butterflies.
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When we choose which varieties to plant we need to be conscientious of which ones can become invasive. West Indian Lantana, Lantana camara, is considered one of the most invasive plants in the US and is on the California Invasive Plant List.
Learn more about Monarchs: Creating Habitats | How to Help Monarchs