Milkweeds are not the most colorful of our spring flowers. Next to the brilliant yellows and reds, pinks and blues and snowy whites of other blooms, the overall pale green of the common Antelope-Horns milkweed tends to fade into the background. It’s the structure of the big, globular flower clusters that draws one’s attention, and, on closer inspection, the curious form of the flowers themselves. But as most naturalists know, they play an important role in the ecosystem, with significance for much of North America.
Two similar species of milkweeds that bloom in central Texas during the spring are commonly given the name “Antelope-Horns.” Both apparently earned the nickname for the resemblance to the headwear of a pronghorn “antelope,” of the paired, inward-curving seed pods (properly termed follicles by botanists).
The species called simply Antelope-Horns is Asclepias asperula, which primarily ranges westward from Austin into the desert Southwest. The other species, Asclepias viridis, called Green Antelope-Horns or just Green Milkweed, has broader, less pointed leaves, and ranges from central Texas eastward to Florida. In both species the pale green flowers are tightly packed into the flower heads, though in A. asperula the heads are more spherical.
Both species have flowers characteristic of the milkweed genus. The five petals form a shallow bowl, hiding the small sepals beneath them, and cupping the stout column of the reproductive parts – the stamens and pistils – in the center of the flower. So far, this is all very similar to other kinds of flowers.
Now a variation: the central column is formed by the fusion of the top parts of the stamens and pistils. This structure is unique to the milkweeds. Called the gynostegium, it takes a close look to see well, but is important in the plants’ pollination.
The partial fusion results in a tiny slit, between adjacent stamens, that catches the legs, or hairs on the legs, of insects drawn to the flower’s nectar. When the insect pulls its leg away, it often extracts a tiny “saddle” fitted with pollen in the two “saddlebags.” This structure is termed a pollinium. If things go right for the milkweed, the insect then carries the pollinium away to be deposited in the next milkweed flower.
Some insects get their legs caught in the slit and are not strong enough to pull out. This occasionally happens to the nonnative honeybee. Native bumblebees are strong enough to pull themselves free; they and other native bees may once have been the principal pollinators of milkweeds. Nonetheless, most of the insects found to be “saddled” with pollinia in recent studies have been honeybees.
Though the gynostegia are difficult to see well enough to appreciate, other structures of the milkweed flower are often conspicuous, and distinctive to these interesting plants. Cupped inside the petals are a set of five knobby structures arrayed around the central column. These features, called hoods, contain the flowers’ copious nectar. They are often contrastingly colored – in the Antelope-Horns species they are usually a dark purple and white – to guide insects to them. Pollinators often land on them, and, while they are maneuvering to get to the nectar, a leg will slide into a gynostegial slit to be fitted with a pollen saddle.
In some Asclepias species, a claw-like point will project from each hood (or near it) toward the center of the flower. These structures are called horns. Ironically enough, the two species of Antelope-Horns can be distinguished from other milkweeds by the lack of these horns. (For a good illustration of a Common Milkweed flower, with parts labeled, see here.)
Milkweeds are perhaps best known for being the food plant for caterpillars of the renowned Monarch butterflies (Danaus plexippus). Most insects and other herbivores do not eat milkweed leaves because chemicals contained within the milky sap, or latex, are distasteful, and can be toxic when eaten in quantity. Milkweeds evolved this chemical defense against the hordes of herbivores that might otherwise feast on them.
Botanists have determined that the toxic chemicals are steroids called cardenolides, and that the concentration of these chemicals varies greatly between milkweed species. Some milkweeds, such as the bright orange Butterflyweed (A. tuberosa), have little or none of the toxin. Both species of Antelope-Horns average high toxin concentration, but it also varies greatly among individual plants.
Monarch evolution has outmaneuvered the milkweeds’ chemical defenses, at least somewhat. The caterpillars are able to sequester the cardenolides into their own bodies, which, in turn, makes them distasteful to would-be predators. As the caterpillars grow and metamorphose into adult butterflies, the chemicals remain in their bodies, so the adults are also protected. A bird that samples a Monarch is unlikely to try one again.
Even Monarchs, though, have difficulty dealing with the sticky latex. Small caterpillars, in earlier growth stages, tend to avoid severing the leaf veins; the resulting rapid flow of latex can gum up their mouthparts to the point that they cannot eat. Larger caterpillars sometimes circumvent this problem by chewing quickly through the midvein to stop latex flow to the leaf before they start chowing down. The larvae also have trouble digesting high concentrations of the toxic cardenolides.
Female Monarch butterflies may be able to sense the concentrations in the plants. One experiment concluded that they preferentially laid eggs on plants of the species that contained higher levels of the toxin. They may also avoid plants with extremely high concentrations. Only 27 of the more than 100 species of North American milkweeds (in the Asclepias genus) have been found to host Monarchs.
The Monarchs that appear in central Texas in spring are the same individuals that sailed through here in fall. They are now returning toward their northern birthplace after spending the winter in Mexico. But they are near the end of their lives as they push northward. So northbound females will stop to lay eggs on milkweeds here. In most cases, the host plants will be Antelope-Horns, since they are the most widespread milkweeds available on the Monarchs’ migratory path, and usually contain suitable levels of cardenolides.
Another generation of Monarchs, raised on milkweeds in Texas and other southern states, will complete the circuitous journey begun by their parents (or grandparents). They will lay eggs in the northern states and Canada, and their descendants will be the ones to migrate to Mexico. So the plants here are critical to the life cycle of these amazing insects.
Alarms have been raised over the precipitous decline of the Monarch population, at least partly attributed to increased pesticide use in the Monarchs’ breeding range. The loss may be linked both to direct poisoning of the butterflies and to dwindling numbers of the milkweed host plants.
Where herbicides are sprayed to control weeds, drifting droplets can be blown a quarter-mile or more (if applied negligently on windy days), where they can damage or kill unintended targets in natural areas. Milkweeds are frequent colonizers of old fields and the edges of croplands, so may be especially vulnerable to drift of these herbicides. The risk is greater where crops that have been genetically modified to withstand specific herbicides are sprayed with more toxic concentrations.
A campaign to save the Monarch by planting milkweeds is well underway. This effort can be beneficial in some places, but not everywhere.
In rangeland, for instance, planting is probably unnecessary. Where livestock are the primary crop, milkweeds may be more numerous than on un-grazed sites. Herbicides are not often sprayed widely, and the milkweeds’ chemical defenses dissuade grazers. If livestock show signs of poisoning from eating milkweed, the problem is that they have too little else to eat. The solution, for a wise land manager, rather than spraying the milkweeds, is to reduce grazing pressure so that the grasses rebound.
In urban and suburban areas, milkweeds and other native plants have been replaced by buildings, asphalt, and tidily-manicured lawns. For a female Monarch trying to find a site to lay her eggs, the ever-expanding cities can be hostile country. Planting native milkweeds in parks and gardens can provide oases for them. But a patch of milkweed next to a highway may do more harm than good, if the butterflies drawn to it end up on a windshield.
In landscapes dominated by row-crop agriculture, milkweeds planted in spots distant from carelessly-applied herbicides may help to replace those lost to poisoning. However, in the long run, the real solution will be to reduce our dependence on increasingly toxic pesticides.
Other references not cited in this article included “Evolutionary and ecological implications of cardenolide sequestration in the Monarch butterfly” by S. B. Malcolm and L. P. Brower in Experientia, vol. 45; “Insect pollinators of 12 milkweed (Asclepias) species,” by Robert F. Betz, et al, in Proceedings of the Thirteenth North American Prairie Conference; and the University of Minnesota’s “Monarch Lab” website here.