THE SMALL WORLD OF MILKWEED • B Y MARIE-EVE ANDRÉ AND LUCILE PIC
Over the course of their evolutionary history, plants and insects have woven close relationships. Milkweed, of which there are a dozen or so species in Canada, is an amazingly effective plant in the way it adapts to curb the impact of herbivores on its growth. Sticky latex, toxic molecules—every means possible is used to avoid being eaten! In turn, some insects have adapted to foil its defences. This is how milkweed and its mini-community influence each other: a complex co-evolution that has developed over millions of years.
LATEX
TOXIC COMPOUNDS
PHOTO Espace pour la vie (Sonya Charest)
As a means of protection, milkweed produces a sticky, viscous latex that oozes out copiously at the slightest harm to its leaves or stems. For example, if a milkweed plant is gnawed by an insect, an abundant flow of latex immediately leaks out onto the plant-eater, sticking to its mandibles or legs—making it hard for the insect to really chow down! As a result, many insects simply opt to look elsewhere for their next meal.
Milkweed tissue contains toxic compounds called “cardenolides.” Besides their bitter taste, these molecules have unpleasant effects on animals that ingest them: heart palpitations, nausea, slower growth, etc. Cardenolides are found in various concentrations in several parts of the plant, from the roots to the leaves, by way of the stems. But it’s the latex that contains the most! That’s what makes it even more effective at repelling leaf-eaters.
