All plants are involved in an evolutionary battle with herbivores, where plants evolve various traits that deter herbivores and then herbivores evolve the ability to eat them again. This evolutionary struggle has raged on for over 400 millions years, contributing to the massive diversification of both plants and the wide range of herbivores that eat them. So much so that plants and herbivores make up a majority of the diversity of life on earth, at least among organisms bigger than microbes. In some cases this evolutionary struggle results in plants and herbivores that have all sorts of fascinating traits and behaviors.
Milkweeds (genus Asclepias) evolved an impressive array of defenses against herbivores, including toxic cardenolides that stop insects’ hearts, gooey latex which gums up their mouths, and obnoxious hairs which make munching through leaves a hassle. These defenses work really well because most insects don’t even bother trying to eat milkweeds at all. But, unfortunately for the milkweeds, and fortunately for fans of really awesome insects, there are a bunch of herbivores from all across the insect tree of life that evolved to feed on milkweeds and only milkweeds, and they’ve gotten really good at it. Most of them are brightly colored to warn predators of their toxicity (called aposematic coloration). And they are toxic because, amazingly, they can tolerate the toxic cardenolides in the milkweeds and then incorporate them into their tissues. An exceptionally clever adaptation that turns a plant’s defense into a tool to deter predators.
In a recent walk around Sleepy Hallow State Park outside of Lansing, Michigan, I saw a bunch of these milkweed specialist. First is a one we are all familiar with, the caterpillar of the monarch butterfly:
The fruits of some of the milkweeds were coated with large milkweed bugs (Oncopeltus fasciatus):
Then I found one of my favorites, the swamp milkweed beetle (Labidomera clivicollis):
There are two aphid species that commonly feed on milkweeds, the bright yellow oleander aphid (Aphis nerii), which is actually and introduced species in North America, and the more drab-colored milkweed aphid (Aphis asclepiadis). Both are commonly tended by ants who collect sugary honeydew they excrete and then, in some cases, protect them from predators.
I found one more caterpillar that feeds on milkweed, the delightfully fuzzy milkweed tussock caterpillar (Euchaetes egle):
As the sun set I was excited that I found so many fascinating and photogenic animals during a short walk near the lake.
Thinking about the perennial struggle between plants and herbivores reminds me of the last line in Darwin’s Origin of Species “from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved”, which so succinctly summarizes how fulfilling it is to carefully observe, understand, and appreciate the marvelous diversity of life which surrounds us.