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Nature Notebook – Caterpillar’s Worst Nightmare

The monarch caterpillar we were closely observing stopped moving. As we talked it repeatedly thrust its head backwards. Apparently the caterpillar mistook us for predators and was utilizing a behavioral defense: head flicking.

Parasitoid wasps are a caterpillar’s worst nightmare. The wasps deposit their eggs on or in the caterpillar’s body. The wasp larvae then feed on the caterpillar.

The wasp needs a stationary target for an accurate “strike” so a thrashing caterpillar is less desirable. In addition, that thrashing head is equipped with mandibles strong enough to tear and chew leaves. They can be effective weapons against small enemies such as the wasp.

Some caterpillar species move so violently that even a larger potential predator may be startled enough to rethink this meal. The caterpillars then drop to the ground to disappear within the leaf litter.