A Shared Existence: Milkweed and Its Myriad Companions

Craig Holdrege

From In Context #24 (Fall, 2010)

Looking at the intimate interaction between milkweed and the many organisms that share in its life, this article is one part of a more intensive study by Craig, with illustrations and photos, of the complex life story of milkweed, “The Story of an Organism: The Common Milkweed.”

We have already seen that common milkweed is an important part of the life of insects that feed on its nectar. Observing nectar feeders on common milkweed, Southwick identified representatives from 15 different orders of insects (and one hummingbird species). Nectar was taken mainly during the day, but also during the night by a variety of nocturnal moths. But these nectar-feeding insects represent only a minority of the insects and other arthropods that interact with milkweed. In the late 1970s Dailey and his colleagues carried out surveys of bugs (Hemiptera) and beetles (Coleoptera) on common milkweed. Over the course of ninety days they found 132 different species of beetles, 18 of which they considered common visitors, since they collected more than 50 specimens of each of these species. They collected 45 species of bugs, 5 of which were common visitors according to the same criterion. Milkweed teems with insect life.

For many insects, milkweed is certainly a small and transient part of their habitat — or speaking functionally, a minor part of their ecological niche. They may nibble on the leaves and flower buds, or drink some nectar and then move on to other plants. As predators they may, like the bug Phymata fasciata, hide in the thicket of milkweed stems, leaves, and flowers, waiting for their prey of flies and small wild bees. And then there are the milkweed specialists, which I will discuss below, that feed almost exclusively on milkweeds. So milkweed provides food and a micro-habitat for a multitude of organisms. Its exuberant growth —  in rhizomes, stems, leaves, flowers, fruits, and seeds — allows abundant insect life to orient around it.

Milkweed’s Specialized Insect Companions

There are at least 10 species of insects that feed only on common milkweed or other closely related milkweeds in the genus Asclepias (see table and photographs). None of these specialist species is a nectar feeder; rather, they feed on milkweed rhizomes, shoots, leaves, flowers, or seeds. The most well-known of these is the monarch butterfly (Danaus plexippus).

The adult butterfly lays its eggs on the leaves of common milkweed, the larvae live from its leaves and the milky sap the plants contain, and the adults drink from the flower nectar, although they are not restricted to milkweeds.

What is fascinating about the monarch and some of the other milkweed specialists is that they do not just feed on the plants, digest the substances, and then build up their own body substances. Rather, they store some of the components of the milkweed sap in their body. When a milkweed stem or leaf is damaged, it exudes a white sap. All you have to do is to scratch the stem with your finger nail and the white sap oozes out and streams down the stem until it gradually hardens. When, for example, a monarch larva bites into a leaf vein or stalk, the sticky (latex-containing) milky sap seeps out and the larva ingests it. It draws out of the sap a particular group of substances known as cardiac glycosides (cardenolides), and instead of breaking them down or excreting them, it stores them in its tissues. The concentration of cardiac glycosides in the tissues of a monarch is substantially higher than it is in the tissues of common milkweed. Interestingly, it is not only the larva that sequesters these substances; they are also retained in the adult, who has gone through the radical metamorphosis from caterpillar to butterfly. So part of the milkweed remains as an essential part of its insect predators.

Cardiac glycosides are bitter tasting and can disrupt the ionic balance of a number of different cell types in animals, including heart muscle, vascular smooth muscle, neurons, and kidney tubules. In high doses they can be fatal to an animal, but in nature this will rarely happen, since they cause vomiting in pre-lethal doses. We would imagine that common milkweed is protected against herbivores by the cardiac glycosides in its sap.

Clearly, however, the sap does not prevent specialist herbivores from feeding on milkweed and sequestering cardiac glycosides, although some of these specialists avoid taking in large amounts of sap while feeding. The monarch and red milkweed beetle are known to bite into a milkweed leaf vein near the base of the leaf, which then exudes sap that flows back out of the more distally-located veins. The insect then crawls to the periphery of the leaf and begins to feed from the part of the leaf that now contains little sap.

Unsurprisingly, researchers believed that by sequestering cardiac glycosides, milkweed predators may be protected against their own predators. Beginning in the 1960s, researchers began testing this hypothesis and, as Malcolm concludes in a review, “much evidence is published to show that many prey species are well defended against predators by the presence of cardenolides.”

So milkweed is helping those insects that prey on it become better protected from their own predators. This is, in a sense, a paradoxical situation in which a plant is providing protection for its predators, which increases the likelihood that there will be more predators to feed on it. Theoretically, one could think that these specialists might eradicate milkweed. But no observations indicate that milkweed populations are significantly harmed by the specialist herbivores associated with them. And it is not as if the monarch or other milkweed specialists have no predators — both monarch adults and larvae are preyed upon at least occasionally by some birds, mice, ants, dragonflies, and wasps, and the larvae can be parasitized by flies and wasps.

Most of the milkweed specialists that sequester cardiac glycosides are brightly colored. (Within a Darwinian framework one interprets such coloring as warning coloration, also called aposematic coloration. The theory is that the bright colors and patterns evolved as a warning sign “keep off.”) Hartman noticed an additional correlation, namely that the brightly-colored, cardiac glycoside-storing herbivores tend to move around the plant a good deal when feeding, eating only small amounts and rarely doing significant damage even to a single shoot. The conspicuous caterpillars of the milkweed tussock moth, in contrast, aggregate on a shoot and can denude it of leaves, leaving only the skeleton of the larger veins. Interestingly, tussock moth caterpillars, which sequester cardiac glycosides, metamorphose into inconspicuous (cryptic) nocturnal moths, that do not sequester appreciable amounts of cardiac glycosides.

As an adult, the monarch butterfly migrates south. The monarchs east of the Mississippi fly as far as 4,800 km to Mexico, where they overwinter. “Amazingly, these butterflies fly from their summer breeding range, which spans more than 100 million hectares, to winter roosts that cover less than 20 hectares, often to the exact same trees, year after year” (Solensky). The expansive extent of the summer range corresponds to the range of common milkweed and a number of other milkweed species. Along the way of their migration, they feed on milkweed nectar and the nectar of other flowers. Their range contracts to the small overwintering area in Mexico, where they are temporally and spatially separated from milkweed. However, they still carry small traces of the plant in their bodies through the cardiac glycosides. The next spring they migrate back north and many of these adults mate, lay eggs, and die in the southeastern U.S. Their offspring feed on southern milkweeds, metamorphose, and the adults fly north to find common milkweed flowering in the northern summer. The life cycle begins anew.

A Milkweed Beetle’s Life

While the life history of an individual monarch can span nearly a whole continent, the life history of a red milkweed beetle (Tetraopes tetraophthalmus) is much more tightly linked to a local common milkweed population. I will describe this relation in some detail. About the time a colony of milkweeds begins to flower, bright red milkweed beetles crawl out of the ground and spread out onto milkweed shoots — an insect version of flowering. They crawl around on the plants and may fly short distances. They generally don’t leave the area of the colony. They begin feeding — on leaves, but mainly on flowers. When a milkweed colony is at a high point in flowering, the red milkweed beetle has its peak in population density. The adults live for about three to four weeks, which corresponds to the main phase of flowering. The synchrony between adult beetle and flowering milkweed is striking. In a colony that flowers later in the year, the beetles emerge later. It could be that the temperature of the soil helps to coordinate this synchrony, since both shoot development in milkweeds and pupation in the milkweed beetle are temperature-dependent.

The beetles mate and the female moves to a nearby grass plant or other hollow-stemmed old-field plant and nibbles a hole in the stem, crawls inside and lays her eggs. This is the one phase of the life cycle that is not dependent on milkweeds. When the eggs hatch, the larvae crawl down into the ground and move to the milkweed rhizomes. There they begin to feed, both on the inside and outside of the rhizomes. They feed exclusively on milkweed rhizomes. They can do considerable damage to short sections of a rhizome, but never have a significant detrimental effect on a colony as a whole. While the colorless larvae are busily feeding below ground on the rhizomes, the fiery red adults have died. The larvae feed until early fall, when they move out of the rhizomes and overwinter in the soil, near the rhizomes, as large pre-pupae. They do not feed during this time. Both milkweed and pre-pupae are quiescent during the winter. Only when the soil reaches a temperature of about 17 to 18 degrees Celsius does the pre-pupa become active — not through movement or feeding, but through metamorphosis. It forms a pupa out of which the adult beetle soon emerges. It breaks through the cocoon and digs its way out of the soil to emerge in a forest of milkweeds, where it begins to feed. The next adult generation begins its short life.

When we reflect on such relationships between two kinds of organisms, a plant and an animal, the boundaries between the two begin to dissolve. We can no longer think of the plant without the animal and the animal without the plant. Normally we think of the plant and the animal that feeds on it as two separate organisms that interact. It is very hard, in fact, not to describe them in such terms. But we can ask the question, “Where do organisms end?” (See the article by that title in the Spring, 2000 issue of In Context.) Clearly, the milkweed is unthinkable without its animal associations, just as the animals cannot be described or understood without the milkweed. Milkweed’s pollination is wholly dependent upon insects just as many insects are dependent upon milkweed for food and reproduction. Therefore, we must transcend the boundaries we construct when we look at an organism from a taxonomical standpoint. We can begin to see organisms as intersecting relationships that are part of the greater web of life. In the case of common milkweed this is especially evident, since even some of its physical substances (cardiac glycosides) remain unchanged as a part of various animal species.

From an evolutionary perspective we need to imagine that the lives of common milkweed and its specialist insects have been related to each other for a long period of time — going back to the mid-Tertiary in the case of the red milkweed beetle. They have co-evolved and have a history together — they belong to each other or are part of each other. One of the key realizations of an ecological-evolutionary perspective is that what appear today to be separate entities are in fact interconnected. As Rausher has stated, “The process of co-evolution between plants and their natural enemies — including viruses, fungi, bacteria, nematodes, insects and mammals — is believed by many biologists to have generated much of the Earth’s biological diversity.”  That this diversity is an expression of the interconnectedness between life forms is what we begin to understand and to appreciate when we concern ourselves with the life histories of intersecting organisms.

Summarizing Picture

When you see an old field, the robust common milkweed plants stand out among the much sleeker grasses, asters, or goldenrods. Common milkweed has thick stems and expansive leaves that in shape and size look more like the leaves of a plant growing in shady woods than in a sunny old field. In the warm summer days of late June and through much of July, the large spherical heads of flowers unfold on the upper part of the stems. The individual flowers are actually quite large for a field plant and they produce large amounts of concentrated nectar. Their scent spreads out into the surroundings. When in flower, a colony of milkweeds attracts — day and night — a great variety and number of insects of all different shapes and sizes. For several weeks in summer milkweed becomes a microhabitat with a singular concentration of insect life.

The flower is highly specialized. Those parts of the flower that normally are in direct contact with the air and insects — the receptive stigma and the pollen grains — are encapsulated, the stigma within the stigmatic chamber that opens to the world only through a narrow slit, and the pollen grains in the pollinia, which themselves are hidden within the chambers. Pollination becomes an intricate process of removal and insertion that is unthinkable without the intervention of insects. Only they can bring the specialized structures into the precise spatial relation the plant needs for fertilization to occur.

While the flower outwardly displays milkweed’s strong specialization in its form, all parts of the plant except the flowers produce the specialized latex sap. (The flowers produce, instead, a sugary nectar.) The latex sap is encountered by animals that feed on the plant. Small insects can become caught in the sticky sap. Others can be repelled by the cardiac glycosides in the sap, while still others incorporate the toxins into their own body. The life of these often vibrantly colored insects is in multiple ways closely bound up with the milkweed.

After the flowers wilt, the fruit pods begin to expand. While relatively few fruits form out of the multitude of flowers, those that do develop grow large — much larger than the fruits of other old-field community plants. The pods swell and orient themselves upward, a contrasting gesture to the globes of flowers. Each pod is full of seeds, seeds that are large and heavy. But they have the light feather-like extensions of the white comas that allow them to be carried away on a breeze when the pods split open. It is almost as if the upward pointing pods are prefiguring what is to come — the upward lift of the coma-bearing seeds that disperse into the larger environment. As with all stages of milkweed, both pods and seeds provide nourishment to insects.

One salient feature that informs milkweed is its exuberant and robust growth. Underground it spreads year to year, forming a network of thick rhizomes out of which the above-ground shoots grow. The thick shoots bring forth large, spreading leaves. All these parts of the plant contain the milky sap, which is continually produced as the plant grows and develops. A marked transformation in substance and form occurs as the many large umbels unfold in the summer light and warmth. As the stems and leaves are rich in milk sap, so are the flowers rich in sweet nectar. Both the leaves and the flowers attract countless insects; milkweed is of fundamental importance to the existence of some of these creatures. In the fall, large pods form, containing many large seeds that spread out into the environment.

Milkweed is effusive and yet it is also specialized. This specialization both attracts and repels insects. Think of the sticky, toxic sap that can also be protective, or the pollination process in which insects are attracted to the nectar, but may become injured or trapped by the flower structure. Milkweed invites life, but also holds it back. There is a fascinating tension in this plant.

References

For the references to this article, see Craig’s in-depth article on Milkweed, “The Story of an Organism: Common Milkweed.” Photos: Craig Holdrege.