the caterpillar of a noctuid moth, Pseudaletia unipuncta, that often travels in large numbers over a region, destroying crops of wheat, corn, etc.
any of the larvae of several related moths having similar habits.

Origin of armyworm

An Americanism dating back to 1735–45; army + worm Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019