Armyworms (genus Spodoptera) are dull-colored (greenish-brownish to black) hairless caterpillars which can wreak havoc with many grain crops (especially corn) and grasses such as timothy when their populations are high. Its name is derived from its feeding habits.  They will eat everything in an area and once the food supply is exhausted the entire ‘army’ will move elsewhere in search of an additional food source.  An infestation is hard to detect as the caterpillars migrate to new feeding areas in the cool and darkness of the night.  When the caterpillars near maturity, they can lay waste to an entire crop in a few days.  Fall armyworm larvae (Spodoptera frugiperda) have been reported to feed on more than 80 species of plants, and although they favor grasses in particular, they frequently damage many crops as well such as: alfalfa, barley, clover, corn, oat, rice, ryegrass, sorghum, timothy, wheat and many others.

Additional Information and photos (including the moth stage):