armyworm

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various moth caterpillars, especially of the genera Pseudaletia and Spodoptera, that move in large groups and are destructive to crops and other vegetation.

from The Century Dictionary.

• noun A name given to the larva of Leucania unipuncta (Harris) on account of the immense numbers in which it sometimes marches over a country, completely stripping it of all the grasses and young grain in its way.

from the GNU version of the Collaborative International Dictionary of English.

• A lepidopterous insect, which in the larval state often travels in great multitudes from field to field, destroying grass, grain, and other crops. The common army worm of the northern United States is the noctuid moth (Pseudaletia unipuncta, formerly Leucania unipuncta. The name is often applied to other related species, as the cotton worm.
• The larva of a small two-winged fly (Sciara), which marches in large companies, in regular order. See cotton worm, under cotton.
• the larva of the fungus gnat; -- they march in large companies in regular order.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The larva of various species of noctuid moth, which typically feed in large, destructive groups

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun noctuid moth larvae that travel in multitudes destroying especially grass and grain
• noun larva of fungus gnat that feed on cereals and other grains; they march in large companies in regular order when the food is exhausted
• noun moth whose destructive larvae travel in multitudes
• noun noctuid moth larvae that travel in multitudes destroying especially grass and grain