This week, I’m excited to bring you an episode I’ve been looking forward to for a long time. My guest, Dr. Anurag Agrawal is the author of Monarchs and Milkweed, an engrossing look at the intricacies of the monarch life cycle and the role a single genus of plant plays in their migration and survival. Suffice it to say monarchs have a precarious relationship with milkweed that is literally a matter of life and death.
Dr. Agrawal is a professor in the Department of Ecology & Evolutionary Biology and Department of Entomology at Cornell University. He was fortunate to spend a lot of his childhood exploring the great outdoors and those experiences led to his fascination with plant ecology and species evolution. I, for one, am grateful that he gathered some of his vast knowledge into a book exploring the relationship between milkweed and the monarch species it sustains.
The monarch is often one of the first butterflies we learn about at an early age, which may be why this species seems to hold its own beloved place in our collective appreciation of nature. Organizations like Monarch Watch attract citizen scientists across the country and from all age groups to join in the effort to monitor the migration and population of these colorful creatures.
There is still a lot we don’t know about monarch behavior, but I’m betting at least a few of the facts Dr. Agrawal shared will surprise you.
The Monarch Lifecycle
During the span of a year, four generations of monarchs are hatched and mature to fulfill their unique role in the epic migratory cycle of this delicate insect.
It all begins in the mountains located about 50 kilometers from Mexico City. Clustered in the branches of Oyamel fir trees, 200-600 million hibernating monarch butterflies ride out the winter months. There, the temperatures remain in the low 40’s (Fahrenheit), which is the ideal range for hibernation.
As the temperatures warm in February to above 55 degrees and the days grow longer, these throngs of monarchs begin their migration north, traveling about 800 miles. Most of them cross over the Rio Grande into Texas. Some of the millions move into surrounding states, but most of the colorful monarch mob remain in the Lone Star State.
Once there, the monarchs mate, lay eggs and die. The eggs are considered the first generation of the year. By the 2nd or 3rd week of March, they hatch as a caterpillar. It will take another 2-3 weeks for them to form a chrysalis and another 10 days to transform into a butterfly.
Once they have their wings, this first generation heads mostly north to northeast again. They make their way into northern states – like Indiana and Michigan – and to areas east of the Appalachian mountains where they also mate, lay eggs and die. In total, this generation will only live 4-6 weeks.
The second generation hatches and goes further north, congregating in Ontario by June. There, this generation dies after mating and laying eggs for the third generation. The third generation is hatched and matures in the same timeframe. You might call this the “slacker generation,” because they are the only group which remains within the same general area. However, their lack of ambition doesn’t increase their lifespan. Like the previous two, this third-generation only survives 4-6 weeks.
The fourth generation is the group born to endure. They hatch and mature just like their preceding generations, but they have a hero’s journey in their future. Once they become butterflies in August, they don’t mate. They head south – beginning the 2-3,000 mile migration back to the mountains of Central Mexico.
Follow the Milkweed
Although it takes three generations to reach their northern habitat range, the fourth-generation monarchs will make the entire journey back to Mexico on their own. They travel only by day, landing to feed on nectar-producing plants in the late afternoon and resting overnight.
Having just been born, what drives this fourth-generation south in the first place? Scientists believe that shortening days and cooling nighttime temperatures of August trigger an instinct programmed deep within the monarchs’ genetic code.
Another factor pushing them south is the lifecycle of another species – milkweed. Varieties of this plant are the only safe harbor for the eggs and early life stages of monarch butterflies. In fact, each generation moving north was following the emergence and availability of milkweed.
Milkweed is the only food source for monarch caterpillars. Without it, the insect won’t survive. Spider milkweed emerges and matures in Texas by March, where the eggs of the first generation are lain. During this early spring month, milkweed is still sleeping in states further north. So, monarchs procreate in Texas and surrounding areas where milkweed is available for their offspring.
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, varieties of this unique plant develop in Canada just in time for the second generation to arrive there and lay eggs for Generation Three.
Since milkweed is the only food source for monarch caterpillars, females will only lay their eggs on that species. The newly-hatched caterpillars don’t have the ability to find their own food – unless it’s the egg of their own kind. A monarch caterpillar will eat a monarch egg, which is why females also avoid laying an egg on an already-occupied milkweed plant.
Think about that for a moment. Not only is milkweed the only food source, but each plant in our country typically sustains the life of just one insect in a season. That means, hundreds of millions of milkweed are necessary to sustain our current population of hundreds of millions of monarchs. I don’t know about you, but that leaves me in awe.
By August, milkweed across the continent is fading, and its foliage becomes less edible. There isn’t sufficient milkweed in late summer to support an additional generation of monarch caterpillars. So just as the availability of milkweed guided monarchs north, its absence pushes the fourth generation to travel back to their southern hibernation habitat.
What Doesn’t Kill Them Makes Them Stronger
Monarchs’ reliance on milkweed means that life is tough from Day One. The first thing a tiny, newly-hatched monarch caterpillar eats is its eggshell, after that the only item on the menu is milkweed – until the mature butterfly stage.
Unfortunately for monarchs, milkweed has developed some serious obstacles to protect itself from feeding insects. It’s the only plant providing the monarch with life, but it also holds the power of death.
Milkweed is tough stuff. Before the young caterpillar can feed on a leaf, it must shave off spiky leaf hairs, one by one. It doesn’t eat these hairs – also known as trichomes. It clips each one off at the base and brushes it aside to clear off an area where the caterpillar can sink its teeth (yes, monarch caterpillars have teeth-like chompers called mandibles!) into the leaf surface.
Named for the milky sap or latex it excretes, milkweed fights back when its surface is broken by a hungry caterpillar. The toxic latex is pressurized within the plant, so once the small caterpillar finally punctures the tissue, it’s met with a gush of gluey toxin powerful enough to knock the creature off of the plant completely.
The caterpillars which can hold on, continue to feed and focus their efforts on cutting the veins of the plant which transport the latex. It takes a long time for a caterpillar to chew its way along the vein, but the project creates a small sap-free island within the leaf space. There, the caterpillar can feed without suffering mouthfuls of the milky poison.
All this takes place within the first week of life. At that point, the caterpillar has grown to about an inch in length, and it requires more than a small patch of feeding area. So, it crawls to the leaf petiole (the stalk that attaches the leaf to the stem) and gnaws on that until the structure is weakened, and the leaf hangs low without completely separating from the plant. As a result, the latex stops flowing into the leaf, and the caterpillar can feed away.
Although the sap makes it difficult for the caterpillar to feed, there’s a long term benefit. The toxins in the latex become sequestered within the body of the monarch, making the insect toxic to other predators – like birds.
Monarchs in Decline
With a complete dependence on milkweed in order to reproduce, it’s easy to understand why the monarch population ebbs and flows with the availability of the plant. There are good and bad years for milkweed and, subsequently, for the monarch species.
Severe drought experienced across the country a few years ago stunted or even prevented milkweed growth. As a result, the monarch population was hit hard. The numbers have bounced back in recent years, but overall, Dr. Agrawal reports that the monarch species has been in a steady decline since the mid-1990s.
That is largely due, of course, to the decline in milkweed.
The effects of urban sprawl ripple across links of the ecological food chain in ways that we are still striving to understand. The natural habitat spaces which remain have become increasingly fragmented, and that forces monarchs to travel further to find what they need.
How do they find milkweed anyway? When you stop to consider their ability to find this lone species among vast spaces of territory, it’s pretty mind-blowing. A couple of years back, I had the privilege of observing that remarkable ability in action.
I had just come across a newly-sprouting milkweed plant in the leaf litter on my five-acre GardenFarm™ property. It was the first I had found that spring, and believe me, I’m always on the watch for it. Just a minute later, a distant flutter of orange caught my eye. I watched, breathless, as this delicate creature flew around for several minutes and over a large area of the property before finally coming to land on the small milkweed sprout at my feet.
Scientists still don’t know how monarchs manage to track down the location of a milkweed plant like I watched take place that day. It may be that monarchs can pick up visual cues, or the plant might emit a chemical signal that the powerful olfactory senses of the butterfly can detect. No one is sure. I don’t know about you, but I often enjoy being humbled by the lingering mysteries of our natural world.
The Long Journey
Another great monarch mystery is their ability and drive to return to the mountains of central Mexico year after year. The fourth-generation which makes the long journey has never been to Mexico – or to any of the stops along the way.
It will take them 2-3 months to reach their destination of rest in the Oyamel fir trees. En route, they fuel up on nectar from nearly any nectar-producing flower they come across.
These creatures fly between 35 and 50 miles every day, and they often rest overnight in the same spots as the monarchs of previous migrations. A Washington Post article from the early 1900s featured the reports of one gardener who had observed that hundreds of monarchs would rest on the same branch of the same tree on her property every year. Although there were similar trees in the area, the monarchs always clustered in exactly the same place.
How could that be, since each new generation of migratory butterflies had never been to her property? That remains another monarch mystery.
As the millions of fourth-generation monarchs arrive in Mexico, they all head for the same cluster of mountaintops to gather on the Oyamel firs there for winter. This fourth-generation will remain in place until February when, at the ripe old age of 8 months, they begin their journey north and the whole amazing process begins again.
There’s no milkweed in the region where they hibernate, so why the area has always been their winter home remains another unknown.
For many years, the area was at risk from logging and mining. After all, the local people have relied on the region’s natural resources for their own survival. Fortunately for the monarchs, this small area is now considered a UNESCO World Heritage Site, and the Mexican government takes great care to preserve it.
That said, area residents often have no other way to support themselves beyond traditional reliance on the natural resources which border the monarch habitat on all sides. So, wildlife vs. human conflicts continue, including recent violence against some of the workers affiliated with the monarch preserve.
No doubt, this challenge – added to all the other many difficulties monarchs face each year to continue as a species – means the population of this iconic butterfly will remain at risk for decades to come.
All in all, Dr. Agrawal reports that, when compared with historical averages, the monarch population has shrunk by 50-70% during the past 25 years.
Here’s the good news: We as gardeners can each play an important role in supporting these beautiful creatures. So, let’s take a look at some positive steps any gardener can implement:
Wherever you garden, add some milkweed to your property and encourage those around you to do the same. There are many varieties and, while they do make life difficult for feeding caterpillars, these plants are easy to care for and beautiful to look at. Your other pollinators will enjoy feeding on their nectar too. However, do keep in mind that milkweed can be deadly toxic to livestock, so if you plant it, be sure to keep it out of reach of grazing animals.
If you’re interested in starting milkweed plants from seed, they can be notoriously tricky, but you can increase your success rate by using a winter sowing method or by stratifying the seeds before planting.
Plant Nectar-rich flowers
The early life stages of a monarch are dependent on milkweed, but the adults require nectar from a wide diversity of plants. Spring and summer bloomers will help support the adults moving north, but it’s the fourth generation in late summer and fall that could use the biggest boost.
They need fuel for the journey south, and many gardens lack late season blooms. Consider adding native plants, like goldenrod and aster, which are vital food producers for adult monarchs. Do a little research to learn which flowers are good nectar producers in your area beyond summer and well into fall.
Add a Meadow
Speaking of adding more nectar-producing flowers to your landscape, consider converting a little lawn area into a meadow. A meadow space is easy to create, incredibly low maintenance and can offer a powerhouse of diversity in color for you and ongoing bloom for pollinators from early spring through early winter.
A meadow garden in a small container or a corner of a raised bed can provide an important food oasis for migrating butterflies – especially if you get your neighbors to add a little space as well. All those little gardens add up to fill in the habitat gaps created by urban development.
Even a gardener with the best of intentions can inadvertently cause harm through the use of pesticides in the landscape. There are more good bugs than bad bugs out there, so although our culture has been ingrained with a “spray it away” mentality, I encourage you to pause first. Learn about the unintended consequences of the product you are considering using. Exercise a little more tolerance for what is usually minimal damage from the bad guys.
Check out two powerful podcast series I’ve shared that explain more about safe methods of control and the good guy bugs that will come if you just give them time. The links to all of those are in the Links & Resources section below.
If you can’t contribute through your plant choices, consider supporting any number of organizations working to support monarchs and other important wildlife – like the World Wildlife Fund, National Wildlife Federation or the Xerces Society. Private donations are critical to their ongoing work.
I hope you’ll listen in to this episode by scrolling to the top of the page and clicking the Play icon in the green bar under the page title. My conversation with Dr. Agrawal was so fascinating, I know it will definitely be worth your time. Have you checked out Dr. Agrawal’s book, Monarchs and Milkweed? If not, why not? With lots of great photos, interesting details and good science to back it all up; you might just find it as hard to put down as I did.
Links & Resources
joegardener Online Academy Three popular online courses on gardening fundamentals; managing pests, diseases & weeds; and seed starting!
Master Seed Starting – My newest online course teaching you how to master the art of starting your own plants from seed and seeding care! Registration closing soon, so don’t miss out!