The annual monarch butterfly migration south starts this month, so I thought it would be a good time to revisit my conversation with ecologist and evolutionary scientist Dr. Anurag Agrawal, an expert on monarch butterflies and milkweed — the only type of plant that monarchs lay their eggs on.
Anurag was born and raised in Pennsylvania, surrounded by fields of milkweed with monarchs fluttering by, and he spent his childhood exploring the great outdoors. Today, he is a professor in the department of ecology and evolutionary biology at Cornell University in Ithaca, New York. He thrives on solving scientific mysteries and loves plants of all kinds. His research and fieldwork investigating the relationship between monarch butterflies and the milkweed species that sustain them is the subject of his book “Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution.”
Dr. Anurag Agrawal spent years collecting information for his book on the relationship between monarchs and milkweed. (photo: John Lynch)
Anurag’s book is fascinating — it’s truly an amazing story. I couldn’t put it down. If the book was just about monarchs or just about milkweed, it would be compelling enough, but what makes the book so extraordinarily engrossing is its explanation of how the monarch species and the milkweed genus co-evolved.
For a comprehensive recap of my conversation with Anurag, see the show notes from the original airing. Or read on for a quick refresh and updated Links & Resources.
The Monarch Butterfly Lifecycle
The migratory cycle of the monarch butterfly is a remarkable natural phenomenon.
Each year, between 200 million and 600 million monarchs hibernate in the Oyamel fir forests of central Mexico. They stay in this cool environment, where temperatures hover just above 40°F, from late fall through winter — the ideal range for hibernation.
As temperatures rise above 55°F and days lengthen in February, the monarchs begin their migration northward. They travel approximately 800 miles, primarily crossing into Texas and surrounding states. Once there, the monarchs mate, lay eggs and die. The eggs are considered the first generation of the year. The eggs hatch within two to three weeks, and the resulting caterpillars will grow for another two to three weeks before forming chrysalises. After another 10 days, those caterpillars complete the metamorphosis into an adult butterfly and emerge from the chrysalis.
It takes three generations of monarch butterflies to journey north, but just one generation to return to Mexico, where the cycle begins anew.
A monarch butterfly larvae, aka caterpillar, on a milkweed leaf. (photo: Amy Prentice
Milkweed: The Sole Food Source
Monarch caterpillars can only eat one genus of plants to survive, Asclepias, commonly known as milkweed. There are about 200 species of milkweed but their ranges vary. Their maturity times are staggered, aligning with monarch migration.
Spider milkweed emerges and matures by March in Texas, where monarchs lay the eggs of the first generation. During this time of year, milkweed is still dormant in states further north.
It’s not until May that milkweed begins to show itself in the northern states, which corresponds with the second monarch generation being lain in those areas. Finally, milkweed varieties develop in Canada just in time for the second generation to arrive there and lay eggs for generation three.
Monarch butterflies exclusively lay eggs on milkweed and aim to lay only one egg per plant, avoiding the plants where an egg has been laid already. That means hundreds of millions of individual milkweed plants are necessary to sustain the current population of hundreds of millions of monarchs.
Monarch butterflies avoid laying an egg on a milkweed plant if another egg is already in residence. So it takes millions of milkweed plants to support the laying population of butterflies. This butterflyweed is one of many milkweed varieties. (photo: Dr. Anurag Agrawal)
Milkweed Makes Monarchs Tough
When a monarch caterpillar hatches, the first thing it eats is its own eggshell. Then the caterpillar eats milkweed, and nothing else, until it forms a chrysalis and becomes a butterfly.
Before a caterpillar can feed on a leaf of a milkweed plant, it must shave off spiky leaf hairs, called trichomes, one by one. The caterpillar doesn’t eat these hairs but clips each one off at the base and brushes it aside to clear an area where it can sink its mandibles into the leaf surface. But those trichomes are only the first line of the milkweed’s defense against insects that would like to eat it.
Milkweed is named for the toxic, milky and sticky sap or latex that oozes through its veins. The latex is pressurized within the plant, so once a caterpillar punctures the tissue, it’s met with a gush of gluey toxin powerful enough to knock the creature off the plant. Caterpillars that can hold on will continue to feed and focus their efforts on severing the veins of the plant that transport the latex. Once it breaks through the vein, a caterpillar can enjoy the leaf without encountering gushes of the toxic substance.
The caterpillars inevitably consume some of the toxic latex, and those toxins, called cardiac glycosides, are sequestered in the caterpillar’s body. As a result, monarch caterpillars and monarch butterflies are toxic to predators like birds.
Immediately after hatching, the monarch caterpillar sets to work to create an area free of milkweed sap. (photo: Dr. Anurag Agrawal)
Monarch Butterfly Decline
The monarch population ebbs and flows with the availability of milkweed. There are good and bad years for milkweed and, consequently, for monarchs.
Severe drought can stunt or even prevent milkweed growth. But the bigger issue monarchs have is urban sprawl. The destruction of wild habitat so that more residences and commercial buildings can be constructed results in the loss of natural spaces where milkweed naturally occurs. Areas where monarchs can find sustenance for their offspring have become increasingly fragmented, and that forces monarchs to travel further to find what they need.
A monarch butterfly chrysalis. (photo: Amy Prentice)
What Gardeners Can Do to Help Monarch Butterflies
Gardeners can play an important role in supporting monarch butterflies. Here are some of the positive steps to take:
Plant Milkweed: Add milkweed to your garden to support monarch caterpillars. These plants are low-maintenance and attract other pollinators, but keep them away from grazing animals as they can be toxic. For best results, start seeds using winter sowing or stratification techniques.
Grow Nectar-Rich Flowers: Monarch adults need a variety of nectar sources, especially in late summer and fall. Incorporate native flowers like goldenrod and aster to provide essential fuel for their southward migration. Do a little research to learn which flowers that are native to your area will be good nectar sources well into fall.
Create a Meadow: Convert part of your lawn into a meadow for a diverse, low-maintenance garden that blooms from early spring to early winter. This helps fill habitat gaps caused by urban development and supports migrating butterflies.
Avoid Pesticides: Use pesticides cautiously, as they can harm beneficial insects. Learn about safer alternatives and give natural predators a chance to manage pests.
Support Organizations that Support Monarchs: If gardening isn’t an option, consider donating to organizations like the World Wildlife Fund, National Wildlife Federation, or Xerces Society to aid monarch conservation efforts.
That little dot is a monarch egg. (photo: Amy Prentice)
I hope you enjoyed my conversation with Dr. Anurag Agrawal on monarch butterflies and milkweed. If you haven’t listened yet, you can do so now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title.
What do you do to support monarchs in your garden? Let us know in the comments below.
Links & Resources
Some product links in this guide are affiliate links. See full disclosure below.
Episode 023: Attracting and Protecting Pollinators with Eric Mader
Episode 049: When Good Bugs Eat Bad Bugs: The Business of Beneficial Insects
Episode 050: Organic Pest Control: Beneficial Insects and Beyond
Episode 067: Predatory Beneficial Insects: Feared Foes of Garden Pests, Pt. 1
Episode 068: Top Predatory Beneficial Insects and How to Attract Them, Pt. 2
Episode 103: How to Create a Backyard Meadow: Simple Steps for Success No Matter the Space
Episode 130: Winter Sowing: A Simple Way To Successfully Start Seeds Outdoors
Episode 230: Monarch Rx: The Prescription for Healthier Butterflies
Episode 234: Converting Lawn Into Meadow
Episode 262: Garden for Wildlife: Accessing the Right Native Plants, with the NWF
Episode 331: The Ecological Garden Blueprint: 10 Essential Steps That Matter Most
Episode 354: How Pesticide Regulations Fail Pollinators, with the Xerces Society
Episode 374: The Insect Crisis: How Losing the Little Things Creates Big Problems
joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.
joegardener Online Gardening Academy Organic Vegetable Gardening: My new premium online course. The course is designed to be a comprehensive guide to starting, growing, nurturing and harvesting your favorite vegetables, no matter what you love to eat, no matter where you live, no matter your level of gardening experience.
joegardener Online Gardening Academy Master Seed Starting: Everything you need to know to start your own plants from seed — indoors and out.
joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.
joegardener Online Gardening Academy Growing Epic Tomatoes: Learn how to grow epic tomatoes with Joe Lamp’l and Craig LeHoullier.
joegardener Online Gardening Academy Master Pests, Diseases & Weeds: Learn the proactive steps to take to manage pests, diseases and weeds for a more successful garden with a lot less frustration. Just $47 for lifetime access!
joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.
Earthbound Expeditions: Discover South Africa with Joe Lamp’l
Growing a Greener World® Episode 804: Gardening for Butterflies & Other Beneficial Insects
Growing a Greener World® Episode 1011: Creating a Meadow Garden, Anywhere Around Your Yard
“Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution,” by Anurag Agrawal
Milorganite® – Our podcast episode sponsor and Brand Partner of joegardener.com
Heirloom Roses – Our podcast episode sponsor and Brand Partner – Enter code JOE20 at checkout for 20% off all roses until 12/31/24.
Pure Protect Deer Repellent by Heirloom Roses – Our podcast episode sponsor and Brand Partner of joegardener.com
Dramm – Our podcast episode sponsor and Brand Partner of joegardener.com
Farmers Defense – Our podcast episode sponsor and Brand Partner of joegardener.com
Disclosure: Some product links in this guide are affiliate links, which means we get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us, and compensation is not an influencing factor on their inclusion here. The selection of all items featured in this post and podcast was based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Corona Tools, Milorganite, Soil3, Greenhouse Megastore, Territorial Seed Company, Earth’s Ally, Proven Winners ColorChoice, Farmer’s Defense, Heirloom Roses and Dramm. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.
