Milkweed & Monarchs: Asclepias a Poisonous Plant.(eBook Fragment)
Milkweed Plant— Butterfly— Seeds
The Milkweed Village
Summer in a milkweed patch is a colorful place. Not only are the flowers beau- tiful, but fragrances waft by, and bees are buzzing around. And you might see a monarch butterfly perched on a flower or find one of its caterpillars grazing leaves — as we know, they are not shy. And as you watch patiently, or perhaps as you turn over leaves, you are likely to be rewarded. Several species of other insects, with different shapes and sizes, but all with a familiar bright coloration also make milkweed their home. Two species of seed-eating bugs share many characteristics with monarchs. Not only are they aposematically red and black, but they also sequester cardenolides from the plant and have the same genetic substitutions in their sodium pumps. Turn to another plant, and bright yellow aphids may be sucking the milkweed’s sap, or one of several beetles that are red and black may be climbing the stem. If the insects are chewing the leaves, you are likely to see them cutting the latex-delivering veins before they feed. It’s a village or “community” of insects that are hitched together via a common resource.Their bed and breakfast is milkweed.
The coevolutionary arms race between monarchs and milkweeds is thus an exemplar of a widely repeated phenomenon. Looking into the milkweed com- munity reveals a parallel set of species that have convergently evolved color- ation, behavior, and sequestration. They participate in the arms race as a com- munity. And as discussed earlier in this book, convergent evolution is a hallmark of adaptation, because multiple species have independently evolved to look or behave in similar ways (in this case specializing on milkweed). In this chapter, I will take a community ecologist’s perspective on insect-milkweed coevolu- tion, exposing how what begins with monarch butterflies extends to shape how multiple species interact. Some of milkweed’s defenses are general and work to keep most of the community under control. Other defenses are highly specific and target particular insect species.
Summer in a milkweed patch is full of insects eating all the different parts of the plant, and in addition to sharing a history of battle with milkweed, they themselves interact in various ways. Drawing on John Muir’s observation above, we will ultimately consider how the species that share milkweed as their food are interconnected.
List of Illustrations vii
Welcome to the Monarchy 1
The Arms Race 22
The Chemistry of Medicine and Poison 43
Waiting, Mating, and Migrating 63
Hatching and Defending 90
Saving Up to Raise a Family 119
The Milkweed Village 148
The Autumn Migration 178
Long Live the Monarchy! 210
Image Credits 271
A MILKWEED BY ANY OTHER NAME
Our understanding of convergence and its importance for evolution are rein- forced by the depth and breadth of the evolution of insect traits. By depth, I mean how similar are the convergent traits: do the species simply look the same, or did they evolve by the same molecular mechanisms? The latter was certainly the case with the sodium pump evolution of milkweed insects, which shows deep molecular convergence among several species. And by breadth, I mean the extent to which highly unrelated organisms have converged on the same solution. Again the milkweed insects across many evolutionary lineages have certainly converged. But convergence can be considered at many scales. For example, are there parallel universes, or at least villages, of insects on other milkweed species? For the many Asclepias species across North America, there is a shared and parallel insect community. Some species, such as mon- archs, A. nerii, and Lygaeus, are relatively promiscuous and occur on many milk- weed species, while others species, like Tetraopes and Euchaetes, although host- specific, have close relatives in the same genus that have branched out onto other milkweeds. But what happens when we go further afield — specifically, if we consider an Old World relative of milkweed called Calotropis, a latex-rich plant that grows natively in the Middle East and India? The insect community on Calotropis reveals an assemblage that has converged and parallels that of the common milkweed of North America.
Before we get there, however, I must explain a mystery that relates to the story of Calotropis. The story has to do with how common milkweed, a native of eastern North America, got its scientific name, Asclepias syriaca. The species name might suggest, confusingly, that the plant is native to Syria. To under- stand this appellation, we need to dig back to the 1630s, when a French bota- nist and physician, Jacques-Philippe Cornuti, was studying plants imported from North America to the Paris Botanical Garden. Cornuti examined the plant we now call common milkweed and concluded, in his 1635 book, Ca- nadensium Plantarum Historia, that it was the same species then called “Beidel- sar,” a plant known to the Middle East that had been classified as Apocynum syri- acum by Carolus Clusius in 1601. Unfortunately, Cornuti did not see the two plants’ flowers, which are quite different. More than a hundred years later, in his 1753 opus Species Plantarum, Carolus Linnaeus kept the species name syriaca for common milkweed, although he put the plant in the new genus Asclepias, which he named after the Greek god of medicine. Linnaeus recognized that A . syriaca was a New World plant native to eastern North America, but he decided to keep the old species name, probably for consistency. Additionally, he named Beidelsar Asclepias gigantea (which would later be renamed in the genus Calo- tropis). However, Linnaeus could have straightened things out, had he realized that Cornuti had confused the two plant species as one. But, Linnaeus was unaware of Cornuti’s mistake, and unaware that Clusius had already classified Beidelsar as Apocynum syriacum. In my humble opinion, Linnaeus should have kept the species name syriaca for Beidelsar, retaining some historical prece- dence and a geographically correct name, but he apparently did not check his sources closely enough.
Philippe Cornuti, was studying plants imported from North America to the Paris Botanical Garden. Cornuti examined the plant we now call common milkweed and concluded, in his 1635 book, Ca- nadensium Plantarum Historia, that it was the same species then called “Beidel- sar,” a plant known to the Middle East that had been classified as Apocynum syri- acum by Carolus Clusius in 1601. Unfortunately, Cornuti did not see the two plants’ flowers, which are quite different. More than a hundred years later, in his 1753 opus Species Plantarum, Carolus Linnaeus kept the species name syriaca for common milkweed, although he put the plant in the new genus Asclepias, which he named after the Greek god of medicine. Linnaeus recognized that A . syriaca was a New World plant native to eastern North America, but he decided to keep the old species name, probably for consistency. Additionally, he named Beidelsar Asclepias gigantea (which would later be renamed in the genus Calo- tropis). However, Linnaeus could have straightened things out, had he realized that Cornuti had confused the two plant species as one. But, Linnaeus was unaware of Cornuti’s mistake, and unaware that Clusius had already classified Beidelsar as Apocynum syriacum. In my humble opinion, Linnaeus should have kept the species name syriaca for Beidelsar, retaining some historical prece- dence and a geographically correct name, but he apparently did not check his sources closely enough. tiid moths on Calotropis. There are some four species of aphids on Calotropis, including Aphis nerii, which parallels the group of aphids on common milk- weed. Calotropis also supports a seed-feeding weevil that is only a distant rela- tive of its North American counterpart, but is similarly colored. Other special- ist seed predators of Calotropis include two aposematically colored flies in the genus Dacus. It is quite a convergent village of insects.
Among the major insect groups, what has been conspicuously missing from the milkweed faunas we have discussed so far are grasshoppers (Orthoptera) and wasps (Hymenoptera), two major groups that include herbivorous insects. Grasshoppers have colonized Calotropis, and in both North Africa and Asia there are species in the genus Poekilocerus that have specialized on cardenolide- containing plants. Yes, these species are aposematically colored, sequester cardenolides, and have largely insensitive sodium pumps. As far as we know, herbivorous wasps have not colonized milkweed, but a sawfly wasp in Europe, Monophadnus latus, has colonized toxic Helleborus plants, which produce toxins very similar to cardenolides. And, yes, this wasp has converged on the same genetic sodium pump substitutions as the monarch and several other milk- weed insects. During the past 350 million years of evolution, as insects di- verged into six major groups that include plant feeders (see fig. 7.5), each group independently evolved species that would specialize on these toxic plants.
That is a lot of convergent evolution. Not only do similar forms with simi- lar behaviors and physiologies evolve to utilize the same host plants, but in distant places, where similar plants have been evolving independently for a long time, there too have similar insect communities evolved. The extent to which there are similarities and distinctions in the respective arms races across continents is an open question, and one that would be fascinating to unravel. The final aspect of milkweed’s insect community that I want to dis- cuss is that of the entangled bank of species, and how they are ecologically interconnected, despite representing more than 350 million years of inde- pendent evolution.
ONE FINAL MEMBER OF MILKWEED’S COMMUNITY
Homo sapiens have found many uses for milkweed. The plant has long been used medicinally. Thomas Edison showed that milkweed’s latex could be used to make rubber. The oil pressed from the seed has some industrial applications as a lubricant, and perhaps even some value in the kitchen and as a skin balm. As a specialty item, acclaimed for its hypoallergenic fibers, milkweed coma (the seed fluff that carries milkweed seeds in the wind) is currently used to stuff pillows and comforters. Perhaps more surprising, the same fluff is highly flammable and is used by survivalists to start fires. It is also so highly absorbent of oils, that it is now being sold in kits to clean up oil tanker spills. The fibers from milkweed stems make excellent rope and were used by Native Americans for thousands of years. More than two hundred years ago, the French were using milkweed fibers to make “cloth and velvets more lustrous than silk.” This is a diverse plant with a lot to offer.
But thus far, most of what I have written in this book would likely (hope- fully!) deter you from feeding on milkweed as a food. Native Americans and early French Canadians did collect nectar from milkweed flowers and use it as a sweetener, as milkweed nectar is mostly sugar and water. Yet for a common and abundant plant, what can one to do to make the vegetation palatable as a food resource? We humans, with our highly sensitive sodium pumps, do the one thing that milkweed insects don’t do. We cook. In fact, the invention of cooking foods has been deemed one of the greatest advances in human evolu- tion. Cooking certainly reduces the time spent chewing and digesting. But, perhaps more important, cooking opens up much of the botanical world — toxic as it may be — for human consumption, because heat breaks down many toxins, and hot water can also be used to leach out poisons.
Euell Gibbons, author of the 1962 classic, Stalking the Wild Asparagus, and famed proponent of wild plant edibles, was a huge advocate of eating milk- weed. In his article titled “How to Milk a Milkweed,” he wrote:
The young shoots can be gathered in late spring when they are from four to eight inches high. Rub the natal wool off them, then cook and serve like asparagus. . . . If you get there too late for the shoots, don’t despair. Gather the tender, young, top leaves and prepare them like spinach. . . . When flower buds that appear in the axils of upper leaves become greyish-green hemispheres an inch or so across composed of crowded, bead-like buds, they can be gathered and cooked in the same manner as broccoli. . . . Finally, when the warty pods are only about two inches long, firm in texture, and the silk and seeds are still underdeveloped, they can be gathered and cooked like okra. . . . All four of these vegetables are bitter — so bitter that few people can enjoy them unprocessed. . . . Fortunately, the processing to tame that bitterness down to palatable levels is easily done.
Gibbons recommended pouring boiling water over the vegetables in a pot, then heating only to regain the boil, and pouring off the water before sautéing. From my own experience, rinsing the vegetables with cold water several times tames the bitterness and preserves the burst of flavor produced during sauté- ing (fig. 7.9). “Season with salt, pepper and butter. Serve proudly.”
At the end of summer, many herbivores have enjoyed the benefits of eating milkweed. Together, as a community, they have imposed natural selection on the milkweed and are joined together in a coevolutionary arms race. But while all others die off or go into diapause anticipating winter, monarchs are just preparing for an epic journey. Although the first summer generations of mon- archs each live for about one month, the final “Methuselah generation” will live for eight months. The transition begins with the signs of autumn and a migra- tion of thousands of miles by an insect that weighs less than a dollar bill.