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armyworm

[ahr-mee-wurm]
noun
  1. the caterpillar of a noctuid moth, Pseudaletia unipuncta, that often travels in large numbers over a region, destroying crops of wheat, corn, etc.
  2. any of the larvae of several related moths having similar habits.

Origin of armyworm

An Americanism dating back to 1735–45; army + worm
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for army worm

army worm

noun
  1. the caterpillar of a widely distributed noctuid moth, Leucania unipuncta, which travels in vast hordes and is a serious pest of cereal crops in North America
  2. any of various similar caterpillars
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012