

Herbivores and Plants
A Co-evolutionary Arms Race

When a predator attacks, the prey runs, flies or swims to safety. What do plants do to avoid herbivores? They don't just stand there--although that's how it may appear at first glance. A closer look at plant/herbivore interactions reveals an astonishing array of defenses and counter defenses.

Monarchs and milkweed are a case in point. "A co-evolutionary arms race is operating in this plant-herbivore system," says Dr. Lincoln Brower. Let's look at both sides of the "battle":

Two Milkweed Defenses: Being Sticky and Poisonous
Here are two chemicals that any potential milkweed herbivore will confront:

  • Latex: A milky white sap that becomes sticky and coagulates when exposed to air. (Latex contains particles of rubber. It is found in a variety of plants, including rubber trees.)
  • Cardiac glycoside: As the name implies, this chemical affects the heart. To various degrees, it is toxic to herbivores with hearts (birds and mammals). Monarchs and several other arthropods that eat milkweed have a tolerance for cardiac glycosides, although evidently not at the high levels found in some milkweed species.

A Counter Attack: Leaf-notching Larvae

In this video clip, watch the monarch cut the petiole of the leaf before beginning to eat it. This "leaf-notching" behavior cuts off the supply of latex. Notice how careful and thorough the caterpillar is. Amazingly, he walks down the very leaf he clipped, which seems to be "dangling by a thread."

Many Don't Make It: Little Larvae Stuck in Latex
A study by Zalucki, Brower and Alonso studied the negative physical and chemical effects of latex and cardiac glycosides on first-instar monarch butterfly larvae. They found that, among other observations, 27% of the 1 instar larvae became mired in the sticky latex and died. The scientists used forceps to nibble through the petioles of milkweed leaves, mimicking the behavior of mature monarch larvae. Here are other observations from their publication:

Physical Defenses
Here are two common examples of physical plant defenses.

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