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MILKWEED
by Walter L. Meagher

In the matorral (brushland) of El Charco, to the left of the path descending from La Tienda toward the wall of the dam, is a Pineneedle Milkweed plant, standing less than a meter tall. Perhaps there is more than one plant. At this season it is in fruit, and the fruits are conspicuous. The leaves are thin and needle-like; the plant looks like a little pine tree. Is it a shrub? It seems so. It looks woody, but it is herbaceous, dying back in winter. In Spanish it is called Venenillo (Little Poison), which is apt, as we shall see. I like the Latin name too, Asclepias linaria.

  milkweed
photo Naomi Zerriffi

Asclepius was the Greek god of medicine and health; his snake-entwined staff is the sign of physicians even today. But why name a group of plants, the chief property of which is their poison, after a god in whose powers we no longer believe? Because Linnaeus knew it to have some curative properties; but these are less well known today. Botanists, like Linnaeus, who named plants, drew freely on Greek and Roman deities, and knew Latin and sometimes Greek. For the moment, let us dwell on ‘milkweed’.

If your guide in El Charco breaks a single needle from its branch, or cuts a stem, a milky sap oozes out, as white as the dresses of the little Mexican girls on the day of their first communion. But the sap is not so innocent. Touch it: it is gummy; were an insect to pierce the outer skin of a leaf, its mouth parts would stick together and it would not eat again. It is best to wash your hands after touching the ooze. The sap is sometimes called ‘latex’. While it is poisonous, milkweed latex will not kill an animal as large as a deer or a man. It is not that poisonous. But it tastes bad, and that is enough to deter many or all herbivores.

Let us return to the plant, before turning to the story of the Monarch butterfly. The product of a flowering plant is a fruit. The fruit of Asclepias linaria has an attractive shape: narrow at the tip and wide at the bottom, like a channel marker in a bay, rocking in the waves. Inside are the seeds. The seeds are attached to fine silky threads, a form of seed dispersal we see in the dandelion. From time to time, the seed floss of milkweed has been used as a packing material, especially during World War II, but not the floss of Venenillo, but of the Common Milkweed (Asclepias syriaca) of the United States.

Now the story has an attractive complexity. The Monarch butterfly lays its eggs on a milkweed plant, preferably, but not exclusively, Common Milkweed (Asclepias syriaca) not in El Charco so far as we know, because the Monarch lives mostly above Mexico, as far north as British Columbia and Nova Scotia. But if it did lay its eggs on Asclepias linaria, the result would be the same . Larvae of the Monarch, hatching on the plant, feed exclusively on the milkweed. There are over 100 species of Asclepias in North America; each has the toxin, but in varying amounts and intensities. The most toxic is Asclepias viridis from Florida. Somehow, sometime in the last 50 million years or so, the butterfly evolved a tolerance for the very poison that it then put to work in its own body as a defense against predators, mainly birds. One taste of a Monarch and a jay spits it out; if it swallows the abdomen of the Monarch, the jay vomits. And never again eats a Monarch. One individual has been sacrificed for the good of the species.

Two things the Monarch does. It feeds on milkweed without ingesting too much poison. Severing a stem, close to the leaves, the Monarch caterpillar cuts off the flow of latex within the plant. Soon it may munch leaves free of the toxic steroid known as a cardenolide. The cardenolide is sequestered in the butterfly body in the right amount to make sick every caterpillar-eating and adult-butterfly-eating bird that happens by. In exchange for this benefit, the Monarch, in its adult form, does the second thing: the milkweed plant receives the attention of a faithful pollinator. Hurry down the path from La Tienda to see Venenillo. The fruits will open soon, along a seam, and seeds and floss will float aloft away, to a child’s delight.  

Adapted from Wild & Wonderful: Nature Up Close in the Botanical Garden, ‘El Charco del Ingenio’, San Miguel de Allende, Guanajuato by Walter L. Meagher; photography by Wayne Colony. 2008.

 

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