The Monarchs of Milkweed

Concerns regarding reduced numbers of North America’s iconic monarch butterfly have been blamed on the reduction of milkweed thanks to increased use of agricultural herbicides.

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Consequently, gardeners have been urged by environmental organizations to welcome the plant into their perennial borders. It turns out it may not be that simple. Recent research has revealed that Monarch butterflies, Danaus plexippus, lead a lot more complex life than was previously thought. Their larvae, caterpillars, feed exclusively on the foliage of milkweed. In doing so, they not only consume nutrients, but also toxins that render them poisonous to predators. This, like the adults’ famed annual migration, is well established. Now, new research has revealed that female adult Monarchs search for particular plants that contain other toxins that control a microscopic parasite of these lovely Lepidoptera. Of course, since this is Mother Nature, life gets more complicated.

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Even the most reclusive urbanite is likely to be familiar with Common Milkweed, Asclepias syriaca. The complex blooms may be various shades of dusty rose, lavender, even a dull purplish purple borne on 3- to 5-foot plants found from southern Canada down through eastern and central North America. The warty seedpods that follow the flowers contain a mass of silky fibre, used for filling hypoallergic pillows and to clean up oil spills The entire plant is known to contain cardioactive compounds. Despite this possible toxicity, it has found wide use in folk medicine. Indeed, the botanical name derives from Asclepius, the Greek god of healing. But the Common Milkweed, Asclepias syriaca, is not alone, not be any means. There are at least 30, perhaps as many as three dozen other species known from northeastern and north-central North America. Then there are more in the Rocky Mountains and on the west coast, on down through Mexico into Central and South America for some 140 species in all. So Monarch caterpillars have something of a diversity to nosh down on. Some have been shown to contain compounds, cardenolides, effective against a protozoan parasite, Ophtyocystis elektroscirrha (OE). Gravid female butterflies sick with the debilitating OE will flit from milkweed plant to milkweed plant to discover which have the most cardenolides – a toxic steroid – to reduce infection they would otherwise pass on to their offspring. Researchers recently revealed in the Proceedings of the Royal Society B that a mycorrhizal soil fungus affects how much cardenolide a plant stores. There is yet another fly in the ointment – or perhaps spore in the soil – that affects the potency of cardenolides. The fungus also controls the uptake of the essential plant nutrient phosphorus. Changes in the levels of phosphorus appear to cancel out the benefits of foliar chemistry, the scientists discovered. Just how the female Monarch butterfly detects increased levels of the biocontrol, allowing her to self-medicate her offspring, is unknown. Researchers speculate that it concerns certain volatiles emitted by the Milkweed plants that allow her to make the decision where to lay her eggs. Experts are warning of the risks involved in planting non-native Milkweeds. In fact one tropical species, Scarlet Milkweed or Bloodflower, Asclepias curasavica introduced into gardens in Texas has been shown to increase the infection levels of OE.

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