Monarchs and Milkweed by Anurag Agrawal is subtitled: A Migrating Butterfly, A Poisonous Plant and their Remarkable Story of Coevolution.
The Cover of Monarchs and Milkweed by Anurag Agrawal
This is not the first book I have ever read about the monarch butterfly. It’s the second. The first book I read was when I was six going on seven. It arrived as part of the Weekly Reader book club, and it was called The Travels of Monarch X. I read the book, digested it, but ultimately decided that I did not like it. I liked books about people more. The short lifespan of the butterfly upset me. I didn’t like thinking about death. So I laid the book aside and concentrated on other things for the next fifty years of my life.
The first book I ever read about the monarch butterfly — I was six and had just learned English. It was too hard!
The next time I came to mention the book about the monarch was when I was writing Ping & the Snirkelly People, based loosely on my experiences in first grade. I still did not remember what the book was called or who it was by, but I vaguely remembered reading it in a language I had not quite mastered at the time.
While I have known about the lifestyle of the monarch butterfly since first grade and could recognize a monarch on sight, I wasn’t nearly as interested in them as most people. They were just not at the top of my list, but I still mistakenly assumed that they were useful pollinators, and thought it was a shame if their numbers were dwindling. It was not until I started spotting them on my own property and could see for myself that they were doing nothing for the milkweed plants they depended on that my curiosity was piqued. What was this indolent butterfly contributing to its own upkeep, I began to wonder.
This year, while I had plenty of purple milkweed, the deer on my property ate the flowers before they could even make seed pods.
And when the monarchs came out, it was on the thistle flowers that they seemed to depend.
That’s when I noticed there was a new book out about Monarchs and Milkweed with a beautiful cover illustration. The year was 2017, and the book by Anurag Agrawal was hot off the presses. Somehow,Amazon knew I would be interested, because it showed the book to me in a sidebar, while I was looking for something else. So I ordered the book and started reading it. It was fifty years since the first book about the monarch butterfly.
This is indeed the book I needed to read. It addresses all the points that were bothering me about the Monarch’s reputation and more. Beautifully written and illustrated, it does not talk down to the reader, nor try to sell us a politically correct morality play. This is a book about nature, evolution, scientific inquiry, and also sometimes about politics.
How species involved in interactions evolve and co-evolve depends acutely on the nature of the interaction. If two species are strictly antagonists, then an arms race may ensue. Sometimes, however, species play dual roles, positive under certain circumstances and negative under others. In this case, say if the monarch were both a beneficial pollinator and a herbivorous pest of the milkweed, perhaps the plant would not mount defenses. The benefits of butterfly pollination to milkweed could outweigh the costs of caterpillar herbivory. But here is where I must dispel a widely held myth about monarch butterflies. Milkweeds do not need monarchs, because the butterflies are simply no good as pollinators. Monarchs are strictly pests. (P. 26)
This is exactly what I had come to suspect over the years, watching the monarchs and the milkweeds on my property. It was the Great Spangles Fritillaries that would frequent the milkweed flowers, not the Monarchs. And if we want to see the Monarch butterfly do well, it can’t possibly have anything to do with the great service that it performs for the milkweed, or any other flowering plant, for that matter.
This nonpollinating aspect of monarchs is not widely appreciated. Although monarchs may successfully pollinate some plant species (perhaps the sunflower family…) this phenomenon has not been well-studied, and they are surely unimportant compared with the myriad other flower visitors. Nonetheless, in a recent presidential memorandum… ‘Creating a Federal Strategy to Promote the Health of Honey Bees and Other Pollinators’, Barack Obama singled out the monarch as the only species other than the honey bee… to be named as an important pollinator. Because the monarch is not, however, a good pollinator, the arms race proceeds… (p. 30)
So now that we know that the milkweed plants regard the monarch as a pest and nothing more, we can go back and ponder about the one-sided relationship of the monarch and the milkweed. The milkweed tries to get rid of the monarch by poisoning it, and the monarch keeps coming back, adapting to the poison, and even uses it to keep birds at bay. How did this happen, and can the milkweed do nothing to fight back?
“Well, no wonder the monarch’s habitat is being destroyed!” I kept thinking to myself. “The butterfly is destroying the plants on which it depends and giving nothing in return. It probably deserves to die out.” But that is not the sum total of the story, and so I kept reading to find out more about this crazy, “exploitative” relationship.
The toxins used by the milkweed to thwart the monarch are called cardenolides, and somehow the monarch has found medicinal uses for them against internal parasites, as well as weaponizing them against larger predators. They are related to digitalis, and like digitalis, cardenolides can be both a poison and a medicine for humans, as well. According to Agrawal, traditional medicine has identified the following uses: “subtonic, diaphoretic, alterative, expectorant, diuretic, laxative, escharotic, carminative, anti-spasmodic, anti-pleuritic, stomachic, astringent, anti-rheumatic, anti-syphilitic…” (From Millspaugh, Medicinal Plants, 1892). Humans produce their own cardenolides as a hormone to regulate heart function, which is why cardenolides can be helpful for congestive heart failure. But in plants, cardenolides evolved as a poison for killing herbivores that prey on them. There is no known function of cardenolides for plants except defense. If plants did not use these substances to kill or sicken or keep away their enemies, they would have no use whatever for them.
Many animals avoid succumbing to cardenolides in milkweed by taste aversion alone. They never get sick, because they cannot be induced to eat milkweed. But for monarch caterpillars, milkweed is the only food. How does that work?
Agrawal points out that it is not as if monarch caterpillars are completely immune to the cardenolides in the milkweed. If the dosage is too high — and it varies from milkweed to milkweed — it will definitely kill the caterpillars. But they do have a sodium pump in their tiny butterfly brains that is remarkably unsusceptible to the toxin. The monarch caterpillar can sequester the daily dose of digitalis prescribed for an adult human with a heart problem, which when compared to its size would have killed it had it been a human the size of a butterfly. No, Agrawal does not say it quite this way, but that’s what he means.
The section about the monarch mating cycle reminded me of the time I came upon two monarchs embracing. Was it consensual? Hard to tell, but Agrawal indicates that early in the mating season, females attempt to flee and males force them down. Most butterflies related to the monarch use a different, more consensual strategy of attracting females with pheromones and allowing the female to choose the most desirable males to mate with. That the monarch strategy is different has to do with the fact that a mating male doesn’t just give sperm to the female. He also includes a small gift of enough energy to make the flight back to the egg laying ground. Some of the males are too weak to make it back, and so their only hope of reproducing is to give what energy they have left to a female to carry their genetic material to safety. Females resist early matings, wanting to hold out for stronger males, and also, if they receive too many energy packets, it can kill them. The earlier they mate, the more matings they may end up enduring, and they really only need one for purposes of reproduction. Only thirty percent of attempted forced matings are successful, because the females get away seventy percent of the time. I don’t remember reading about any of this in the book about Monarch X I read as child.
Even though I have never seen a monarch caterpillar, I have spotted a pair mating
The monarch population of North America migrates twice a year, once from the US to Mexico in the autumn and once from Mexico to the US in the spring. They overwinter in a semi-dormant state in Mexico, all their living and mating and birth and death occurs in the US. But it is the cold weather in Mexico that is required for them to know to turn their next migration northward. Minus a cold phase, they always head south. Agrawal is concerned that global warming could cause them to head south away from Mexico into South America in the spring. If that did happen, would they correct course once they encountered cooler weather in the southern hemisphere? It sounds as if they have a single switch in their brain that determines which direction to migrate. Does the switch just tell them to turn around when it gets cold?
Until 1975, it was not even known where the monarchs went during the winter. This means that this information could not have been included in the book that Ping and I read during the school year of 1966/67. A whole group of citizen scientists had been recruited to help tag and keep tabs on the individual butterflies and their comings and goings. Then the question arose: do they make the migration once a year individually, or are there several generations born between migrations? In other words, do the individual butterflies each migrate twice a year, or is their species that does this, so that collectively they appear to be as regular as clockwork? I remember that when I read the book when I was six, I was under the impression that the lifespan of every single monarch butterfly was a whole year. But that is not right. The total time frame from egg to caterpillar to chrysalis to butterfly is only six to eight weeks for those generations that do not engage in autumn migration. The two migrations every year are something the monarchs do collectively.
Cardenolide analysis, bolstered by examining isotopes, has led to understanding of the geographic origins of each butterfly.
… a 2013 study using stable isotopes confirmed that the year’s first new generation of monarchs completed development in Texas and southern Oklahoma, but nonetheless, some first-generation butterflies developed as far north as Missouri and southern Illinois and southern Illinois… In other words, most spring migrants leaving the Mexican highlands lay eggs (and die) in the southern United States, but a few butterflies travel remarkable distances on the return journey from the overwintering grounds — up to 1500 miles… and make it to the Midwest. (P. 80)
When looking for a good place to lay her eggs, a female monarch avoids tall, healthy and well grown milkweed plants. Instead, she choose short, shrimpy milkweeds, either those that have just sprouted or those recovering from a catastrophic event such as having been mowed down. Brush fires create the exact kind of newly sprouted milkweeds that female monarchs are looking for. Why? Because the older and healthier and stronger a milkweed plant, the more potent is its toxin. And while monarchs coexist with milkweeds, the milkweed plant does not need them and is definitely trying to kill them. So it is in the best interest of the monarchs to find milkweed that is young or otherwise vulnerable, so that it cannot afford to invest too much of its energy in creating a potent dose of cardenolides. This is not at all what we have been taught. I personally had believed it was sad that the mowers always came and cut down much of the milkweed at the side of the road before it could even bloom or go to seed. Sad for the milkweed, maybe, but not for the monarchs.
The bee will help pollinate the milkweed, but the monarch will lay eggs that will bring forth parasitic caterpillars that will feed on its leaves.
There is so much about this story that is counterintuitive. Neither the monarch nor the milkweed is looking out for the other. They are not working together — they are antagonists with a serious conflict of interest. But this is not actually unusual in nature. All that peace and serenity and harmony are the result of countless ongoing wars.
Now we have come to Chapter Five, the story of how a caterpillar survives on a diet laced with poison.
The three major stumbling blocks for a monarch caterpillar are trichomes, latex and cardenolides. The trichomes are hairs growing on the leaves of the milkweed. These hairs evolved as a defense against too much sunlight, but they are also useful against insects that want to eat leaves. The caterpillar has to expend a lot of energy shaving off the trichomes before it can eat of the leaves. Latex is that white sap that the milkweed is named for. It can glue the caterpillar’s mouth shut and totally immobilize it. The latex gushes out every time a leaf or stem is injured. It is a defense mechanism pure and simple. Latex serves no other function for the milkweed plant. Sometimes caterpillars build circular dams in order to dispose of the latex. Sometimes they even suction it out in order to move it out of the way. There are many different techniques that are used by different caterpillars in different situations. Finally, cardenolides is the poison contained in the leaves. Different milkweed plants have different doses of this toxin. For instance, the butterfly milkweed has hardly any at all, so it is safe to eat, but the downside is that butterflies who fed on it as caterpillars can be eaten by birds without ill effect. However, the birds don’t necessarily know what a butterfly has feasted on, so the fact that most monarchs are poisonous deters predators even in the case of those that are not. The average female monarch chooses to lay her eggs on milkweed with an intermediate dosage of cardenolides — not so much that it will kill the caterpillar nor so little that it won’t harm the birds. Collectively, they create a defense for all monarch-kind, though that is not something that any particular monarch is thinking about.
The collective benefit to all monarchs from the effort put forth by each is something that happens naturally, despite a complete lack of selflessness on the part of the insects. In fact, if two monarch eggs are laid on the same milkweed plant, the first to hatch will eat the other. That is how unaware they are individually of the “common good.”
But there are also butterflies that are not monarchs who survive by mimicry and telling lies. If you believe that communication can occur without anyone intending to communicate, then it is also possible to lie without saying a word. Sign and signified are learned, and then someone else uses the sign to imply the signified, except that it isn’t so. The Viceroy butterfly announces with its colors: I am a Monarch, beware my poison, and yet it isn’t true. And many, many birds are duped. How were they to know? And all of this happens, without anybody even needing to have a theory of mind. Take that cognitive linguists! But I digress.
Having survived all the traps, the predators and the internal parasites, and the difficulties with overmedicating or undermedicating with cardenolides, the successful monnarch caterpillar leaves the scene of the crime, where all its eating of milkweed occurred, and searches for a safe place to pupate. Evidence of the damage it caused the milkweed might alert predators to its existence as a dormant pupa, so the caterpillar wanders around and finds another spot. Whereas the monarch caterpillar and the butterfly advertise their toxicity with brilliant colors — a practice called aposematic coloration — the chrysalis tries to blend in and not draw attention. While I personally have seen monarch butterflies mating and damaged milkweeds on my property, I have never seen a monarch caterpillar or a chrysalis. I am not sure whether it is because the monarchs on my property are just passing through, or whether it is that the eggs are laid on very low lying milkweed that I am unable to see. I usually spot dogbane plants early in the season, long before I can see the milkweeds. By the time I notice the milkweed plant, it is usually about to bloom. But I don’t see the monarch butterflies until much later in the season. By the time I spot the monarchs, it is usually the goldenrod, bidens and the thistle flower that are blooming.
It is not possible to study butterflies without knowing quite a lot about the plants they depend on. That is why Agrawal devotes so much time in his book to the milkweed plants. In fact, he has a whole chapter called “The Milkweed Village”.
All the species that have milkweed as their food are interconnected, Agrawal points out. As indeed, so are we all.
There are only eleven other species of insects, all of them colorful, that live off the milkweed plant. Most are specialists, eating only milkweed. They include the milkweed tussock moth, Euchaetes egle, the milkweed beetle, Lavidomera clivicollis, the large and small milkweed bugs, oncopeltus fasciastus and Lygaeus kalmi, three species of aphids, Aphis nerii, Aphis asclepiadis and Myzocallis asclepiadis. There’s a fly that eats between the layers of a milkweed leaf: Lyriomaiza asclepiadis. There is even a four eyed red milkweed beetle: Tetraopes Tetropthamlos. There’s a milkweed weevil Rhyssomatis lineaticollis. And there’s a generalist slug that can eat other things, but has been known to also eat milkweed, especially if somebody else has already dealt with the latex flow.
Agrawal is concerned with how few and genetically diverse these milkweed eating insects are, and he speculates about what led them to such a remarkable convergence, most of them using the exact sodium pump de-sensitizing solution to the problem of metabolizing cardenolides. He plans to further explore the issue by manipulating fruit flies into situations that would require genetic changes to cope with environments similar to those of the milkweed eating insects.
Agrawal even mentions the Middle-Eastern milkweed, Calotropis, that I encountered this past year through my friend Dave McClure of Doha, Qatar on Facebook. Even in the Middle East with Calitropis, there is convergence of unrelated insects that have learned to eat the milkweed plant.
One of the seven wonders of the known world is the autumn migration of the monarch butterflies to Mexico. In order to make it to their overwintering site, the butterflies who do so remain virgins. They do not reach sexual maturity, even though they become adult butterflies. This allows them to live much longer than all the other generations of monarchs who breed throughout the summer. Not breeding allows them to extend their normal average lifespan of eight weeks to eight months.
As these summer monarchs emerge, mature and mate during the summer, they do not employ directional flight, but rather they stay in the general locality where they had been caterpillars. Yet, beginning in mid-August, when the last generation of butterflies emerge, they have low levels of juvenile hormone, and this suppresses reproductive maturity. The result is that this last summer generation of monarch butterflies remain virgins, migrate south and wait months to mate. (p. 181)
The chapter on the autumn migration touches on more than the beauty of millions of butterflies overwintering in secluded spots, covering fir trees on mountain peaks with their fluttering wings. It also contains the most dramatic scene about rivalry between scientists. Having found the overwintering grounds in 1975. Professor Fred Urquhart shared the information of the location with National Geographic, which published breathtaking pictures in a 1976 issue of their magazine. But Urquhart refused to tell his colleague Dr. Lincoln Brower, who specialized in milkweed sequestration in monarchs, where the overwintering spots were. Unperturbed, Brower used tidbits of information from the National Geographic article to find the spot, and one day he trekked there only to find Urquhart and his crew tagging butterflies. To say that the meeting was awkward would be an understatement. The two never could get along.
Which brings me to the politics of science and also the politics of art. It was Urquhart who was behind that 1966 book that was sent to me all those many years ago as part of a Weekly Reader book club that my father subscribed me to. I was expecting an interesting story, and instead got a meandering paean to a migrating butterfly. I was not impressed. The story had no discernible plot. But though the book was written by Ross E. Hutchins, it was part of Urquhart’s grand plan to recruit small children to help him track the monarch butterflies to their overwintering grounds. At that point in my life, I only ever read the story in a book, and not the preamble or the afterword. But if I had read that part, I would have found this plea:
ATTENTION Would you like to work with Dr. Urquhart as a research associate in his interesting research on butterfly migration? If so, you should write to: Dr. Fred A. Urquhart, Associate Professor, Department of Zoology, University of Toronto, Canada. He can supply you with information and special marked tags to be attached to Monarch butterfly wings. Perhaps in this way you can make an interesting contribution to the study of Monarch migration and the flight paths they follow. Perhaps, too, one of your tagged Monarchs might fly even farther than Butterfly X.
He should probably have added: “But don’t share any of this top secret information with my nemesis, Dr. Lincoln Brower.” It would have made a great thriller!
Speaking of dark plots, the very last chapter of Agrawal’s book reminds us of the roots of the environmental movement, and how Richard Nixon contributed to some of the most dastardly government agencies that are a daily part of our life today.
The environmental movement began to take hold in the decades preceding the discovery of the monarchs’ overwintering grounds in 1975. Prior to this, monarchs were simply a quiet icon of nature. It was the beauty behind metamorphosis from caterpillar to butterfly, the science of mimicry and sequestration of toxins, and the mystery behind their migration that captured people’s attention. Nonetheless, milestones of the era included the publication of Rachel Carson’s Silent Spring in 1960, and Richard Nixon’s double boon of creating the Environmental Protection Agency and advancing the Endangered Species Act of 1973.
Today, it is under the Endangered Species Act that through the declaration of U.S. Fish & Wildlife that domesticated chimpanzees are an endangered species, PETA brings lawsuits to harass chimpanzee owners and accuse them of a “take”, as if their American born and hand reared chimpanzees had been taken out of nature only yesterday, when they had been bred locally for generations. But for tenured faculty at major universities, all this is seen as a good thing.
When people talk about monarchs and milkweed today, they ask what government should do to protect these species against the incursion of man. Is there not enough milkweed for the monarch to exploit for its own survival, while the plant tries to get rid of this pest? Have too many trees been cut down, so there is no place to roost at night? Are there too many cell phone towers up, confusing the butterfly when it is trying to navigate?
Agrawal states that the declining number of milkweed plants is currently the most popular explanation for a decline in the number of monarchs. That there is a decline is undeniable.
While in the early 1990s there were some 400 million butterflies, roosting each winter in Mexico, in the past few years , the number has hovered at 100 million butterflies, or a 75 percent decline… In Fenruary 2016, as I write this chapter, news was just released that there were 200 million monarch butterflies in Mexico (well over triple the number last year.) Still, any way you slice it, there has been a persistent and long term decline. (p. 213.)
Agrawal discusses different methods of taking a census of the monarch butterfly population and their divergent results. He also explains the difference between the survival of the species and the survival of the migration. The monarch butterfly species developed in a tropical climate and began to migrate later, because of population pressure on the milkweed it fed on. Agrawal does not believe the decline in monarch populations in North America is due to a limitation in the availability of milkweed, despite the popularity of this explanation. Agrawal and his group of scientists believe the decline has more to do with what happens in Mexico in the overwintering sites. But even this does not necessarily spell doom for the monarch. Migration is just one strategy for survival.
Monarch butterflies as a species are not at all endangered, but due to logging that has affected the overwintering sites, and due to other fluctuations in climate, the annual migration to Mexico is endangered. This is not a problem that government can solve, as outlawing logging in the areas affected has not prevented illegal logging, and climate change is not necessarily man made or under our direct control. Monarchs have survived climate fluctuations in the past, long before humans knew much about them. Hopefully, they will survive without our intervention in the future, even if it means they will need to change course or alter some of their habits.
Here is a video of the author, Anurag Agrawal, speaking about possible reasons for the decline in monarch butterfly populations.
Copyright 2017 Aya Katz
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