Why Do Caterpillars Poke Holes In Leaves Then Leave?

Have you ever looked at some tree leaves and noticed that caterpillars would nibble a small tunnel through a leaf, then crawl over to a fresh new leaf and do it again. Wouldnt it be much easier if the caterpillar just stayed at the same leaf and continue munching away at it?

Plants are not passive

Plants are generally not as passive as we think they are. Many species have self-defense mechanisms in place, such as toxins or other chemicals that are foul to the insect. When damaged, the plant will condense these chemicals in the damaged area to prevent further damage and increase sugar flow to repair the damage. Plants also respond by releasing all kinds of anti-herbivore chemistry (not just “toxins”) and that will drive most caterpillars off. Basically, the food goes bad as they eat, and so they will take a few bites and wander off.

Some caterpillars are specialized though and are resistant to the toxins. In fact, they like them, so making the poisons just encourage feeding. There are specialist caterpillars on tobacco which have been well studied. The tobacco plants have evolved and produced supplement defensive protein which inhibits digestion. This makes the plant even less valuable to the caterpillar. If it tries to compensate by eating more it will have to eat more toxin to do it, and since there resistance to the toxins is limited, it’s a somewhat effective defense.

No clues for predators

Another cool example of that is the milkweed plant. It releases a thick latex that will cover and suffocate it’s primary threat – the monarch caterpillar. However, the monarchs circumvent this by chewing off the leafs main vein, allowing it to consume the leaf in relative safety. Also, when it reaches adulthood the monarch butterfly is the milkweeds main pollinator!

Additionally, predatory and parasitic insects will actually use plant cues to hunt for their prey. This includes visual cues (a partially nibbled leaf) and chemical cues (the volatile compounds released by a damaged plant). Predators will even hunt caterpillars by searching for their poop. Some caterpillars have actually evolved behaviors to “clip off” partially-eaten leaves at their stem. This prevents them from sticking out and attract visual hunters. They will also “fling” their poop away from them so that it can’t be used to track them.

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