The monarch butterfly depends on milkweed to survive, the plant being the caterpillars’ only food source. It’s a system that seems really simple ‒ but that’s not the case at all! A number of studies have demonstrated that the interactions between these two organisms are much more complex than they look. Generation after generation, milkweeds have developed defensive strategies against herbivores, while monarchs have developed exploitation strategies to foil the these tough plants’ defenses. Thus, for millions of years, they’ve been participating in a veritable arms race (from an evolutionary viewpoint, of course).
An unappetizing plant
Most plants possess defense mechanisms against herbivores. Not being able to flee when they’re being eaten, the plants have acquired other interesting strategies for protecting themselves. In the case of milkweed, the plant relies on three major defense mechanisms: 1) the presence of trichomes (hairs) that act as a physical barrier; 2) the production of cardenolides (steroid molecules) that are toxic for a large proportion of animals; and 3) the production of latex, a thick whitish fluid that is both a physical barrier (latex dries out and become sticky when exposed to the air) and a toxic substance (because here too we find an important concentration of cardenolides). These three mechanisms have been successful in dissuading a large number of animal species from feeding on milkweed. Cardenolides can be fatal if consumed in large quantities (which has been the case with a number of ruminants in livestock production like cattle, sheep and horses). However, certain species are able to get past this plant’s defenses, including the monarch!
An essential food source for the monarch, but still a gamble
Milkweed constitutes the monarch caterpillar’s sole food source, but there’s a risk involved. This butterfly has managed, over the many years of its evolution, to find the means to breach the milkweed’s defenses, going so far as to incorporate cardenolides in its own tissues so as to benefit, in turn, from a protection thanks to those toxins (read more about it on the Mission Monarch website ). As for the hairs present on the leaves, the monarch caterpillars will start by “mowing the lawn” before feeding on the plant’s tissues. They won’t eat the hairs, just remove them in order to move about and feed more easily. Although those defenses don’t kill the caterpillars, they harm them and slow them down, exposing them to other dangers. The latex in milkweed plants can also prove to be lethal, especially for young caterpillars. On some species of milkweed, 30 percent of just-hatched caterpillars die stuck in latex when trying to feed. To avoid that sad fate, caterpillars have developed a few tricks. They’ll create a circular trench around themselves to cut off the flow of latex. That way they can feed with no fear of getting swamped by the viscous fluid. Older caterpillars, meanwhile, use a different strategy: they cut off the flow at the source by severing part of the stalk at the base of the leaves. It’s truly impressive how the monarch has succeeded, in the course of its evolution, in adapting its behavior and its metabolism to be able to exploit milkweed.
A fascinating relationship still being studied today
Several scientists are still working on trying to better understand the complex interactions that exist between the monarch and its host plant. The more we discover about the two species, the more we understand how herbivores and plants interact together. There’s nothing trivial about this, because that knowledge helps us make better informed (and more environmentally friendly) decisions in such areas as forestry, agriculture and even the conservation of species and habitats. In protecting the monarch and milkweed, we’re also protecting a wide range of knowledge. We thank you for contributing to monarch protection by taking part in the Mission Monarch program: when you do that, you’re helping protect knowledge!