army worm vs armyworm

army worm and armyworm both are nouns.

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

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

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

Other synonyms of army worm include Pseudaletia unipuncta.

army worm (noun)	armyworm (noun)
noctuid moth larvae that travel in multitudes destroying especially grass and grain	moth whose destructive larvae travel in multitudes
	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