While the entire life cycle of the monarch butterfly depends on milkweed, milkweed doesn’t need the monarch at all! In fact, did you know there has always been, and continues to be, an all-out war between milkweeds and monarchs?
Truth be told, war isn’t just for humans. Plants actually originated “chemical warfare,” which is understandable since they can’t run away or hide from those that try to eat them. With the monarch notorious for only eating milkweed…from the milkweed’s point of view, the monarch is a pest!
Oh, but you say, monarch butterflies do help milkweed by accidentally pollinating as they drink nectar from milkweed flowers. But to be honest, they are not very good pollinators for this particular flower. Milkweed pollen isn’t powdery, so it won’t casually adhere to a butterfly body part. Milkweed packs its pollen in sacs called pollinia which attach to an insect’s legs when they slip into a special slit in the flower. Pollinia are pretty heavy, too. A strong insect can remove pollinia and take them to another flower. A weaker insect can become trapped and die. Most milkweed plants are pollinated by the larger bees, wasps, and beetles…not monarchs who just sip the sweets, lay their eggs, and fly on their way.
But getting back to war…milkweed is a really awesome plant to study because it has so many defensive traits. A milkweed plant uses all defenses in its arsenal to repel the monarch caterpillar. In fact, after the monarch egg hatches…it’s an all-out war! Most monarch caterpillars do not make it past the first day of life because of adaptations in place to defend the milkweed plant.
First there are the trichomes, or hair-like structures on its stems and leaves. Fortunately, newly-hatched caterpillars get a decent first meal from a protein-packed eggshell. Then right to work, “mowing the lawn” of inedible trichomes. This takes time and exposes the caterpillar to predators. Monarchs prefer to lay eggs on less “hairy” varieties. (Some milkweeds have a waxy layer, rather than trichomes, to be removed. This presents a “slipping hazard” which requires the caterpillar to make a silken button to hold it in place.)
The next and most dangerous hazard is the pressurized latex “hydrant” that spurts out at first bite. This actually presents two distinct problems. First, the sticky, quick-drying latex can quickly engulf and terminate the hungry caterpillar. Observations of more than 3000 eggs in one research study showed 60% of lives were lost in this latex! Overcoming this hazard often involves making a “circle trench” in the leaf to alleviate pressure and provide a nutritional island of food. As the caterpillar grows, it is more able to sever veins in the leaf to reduce latex pressure, and can even snip the stem/petiole of a leaf, collapsing the leaf downward to provide a real smorgasbord as well as a safe hideout!
Secondarily, the non-nutritious latex contains chemical poisons which were created to fight off those trying to enjoy milkweed for lunch. However, monarchs are able to turn these poisons to an advantage, by incorporating them into their own bodies instead to protect from predators. Each type of milkweed has a different “cocktail” of toxins, some even too strong for most monarchs, who often take a bite, choke, and die! Fortunately, this is not the case with milkweeds common in the Midwest!
As you enjoy the Monarch butterflies now appearing, and breeding in SW Missouri, think of all the natural history you are witnessing. This did not happen all at once, but over evolutionary time through mutations which allow a monarch‘s body to handle milkweed toxins. A female monarch may choose where to lay eggs, but that is just the beginning of the ecological drama! Always remember, milkweeds do not produce toxins for the benefit of monarchs!