We turn now to another non-native plant that the Monarch utilizes: Tropical Milkweed (Asclepias curassavica). Tropical Milkweed is vilified not just by invasion ideologues but by many (though definitely not all) Monarch butterfly advocates. The typical narrative of exotic vs. native is a shade different here as the conflict is plant vs. insect rather than plant vs. plant, but the black and white presentation is the same. Tropical Milkweed is accused of being harmful to the beloved and imperiled butterfly in a number of ways, even though some localized populations of Monarch have come to utterly depend on it. The true tale of the Tropical Milkweed is decidedly nuanced. It is certainly not guilty of all charges leveled against it—which are marred by the usual conflations of correlation and causation, for example—and the Monarch clearly benefits from it, yet it might have effects are not entirely benign. Much of that is about interpretation. All this being said, it’s early days yet in the research of A. curassavica and questions definitely outnumber answers. It’s certainly too soon to make the categorical statements that are common.
Tropical Milkweed is a perennial plant up to three feet high with narrow leaves that are set in opposite pairs along its smooth stems, which exude a milky latex when cut or broken. The inflorescence is quite striking, made up of clusters of umbels of up to a dozen flowers each with the distinctive "hooks and hoods” arrangement particular to Milkweeds. Five red or orangey-red pointed petals, the “hoods,” are reflexed backwards and surround five bright yellow curved “hooks” which are fused anthers. Head on, an individual blossom looks like a star placed in a star, and in profile like a fiery yellow comet with a red tail. It’s quite a showy plant, which is one reason it’s so popular in flower gardens. Another reason is that, unlike the native Milkweeds of the US, which are herbaceous—meaning they die back in the winter—Tropical Milkweed stays green all year round and can bloom anytime in warmer areas like California, Florida and the Gulf Coast.
Tropical Milkweed is native to the western hemisphere, from Mexico through Central America into South America. Besides being introduced to the US, it was also brought to Australia, China, India and other parts of the world. Though it has medicinal uses going back to pre-Columbian times for a variety of ailments including skin conditions, venereal disease, fevers and pneumonia—and is currently reputed to have anti-cancer properties—its human-induced spread seems to have been merely as a garden plant or accidentally. It prefers warm climates with at least seasonal moisture, including rain forests and savannas but will tolerate deserts and steppes. It is also a pioneer species, and thrives in disturbances such as roadsides and railroad tracks, farm fields and animal pastures, managed timberland, urbanized areas and seasonally flooded zones. Its flat, lightweight, tufted seeds are wind dispersed, greatly aiding in spread after establishment.
The Asclepias genus contains cardiac glycosides, aka cardenolides, which are steroids that are toxic to most animal and insect life. Monarchs, however, have adapted to them and use them for their own defense, harmlessly uptaking them into their own bodies, which are then bitter-tasting to predators like birds.
The eastern Monarch population’s main food source is the Common Milkweed, Asclepias syriaca, which is native to North America east of the Rockies and north of the 35th parallel, in what happens to be the farm belt. Monarch populations have been adversely affected by the spraying of herbicides that kill A. syriaca, which is considered an agricultural weed. Herbicide use in that part of North America has increased since the late ‘90s with the widespread introduction of “Round-up Ready” corn and soybeans, which are genetically modified to tolerate the herbicide glyphosate, so that it can be applied without risking the crop. Eastern Monarchs also take advantage of Tropical Milkweed where it grows, in the southern part of the US.
The historical diet of the western Monarch population in California has been presumed to be any of the native species of Asclepias in the state, of which there are over a dozen growing in a variety of habitats. Indeed, Monarchs currently use these various plants as larval hosts, but they also visit A. curassavica for both these purposes, and in some cases have been observed to prefer it (as have honey bees, “syrphid flies, carpenter bees, bumble bees, leafcutter bees and assorted other insects”i). Given that the western Monarch is a threatened species, isn’t it beneficial that one more species of Milkweed is available, especially one that is grown in city gardens and which thrives in the human-made disturbances that mar so much of California?
No, insist invasion ideologues and some Monarch advocates, who level two charges at Tropical Milkweed: first, that it puts Monarchs in greater danger of being infected with a particular, harmful parasite than native Milkweeds do, and secondly, that it “disrupts” the species’ migratory activity.
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