armyworm vs army worm

armyworm and army worm both are nouns.

Word	Noun	Adjective	Verb	Adverb
armyworm	Yes	No	No	No
army worm	Yes	No	No	No

As nouns, armyworm and army worm are synonyms defined as:

armyworm and army worm: noctuid moth larvae that travel in multitudes destroying especially grass and grain

Other synonyms of armyworm include Pseudaletia unipuncta.

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