The Collaborative International Dictionary of English v.0.48:

armyworm \armyworm\, Army worm \Ar"my worm`\ (Zool.)
   (a) 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.
   (b) The larva of a small two-winged fly (Sciara), which
       marches in large companies, in regular order. See Cotton
       worm, under Cotton.

   Syn: army worm
        [1913 Webster]

   2. the larva of the fungus gnat; -- they march in large
      companies in regular order.
      [WordNet 1.5] Arna

WordNet (r) 3.0 (2006):

armyworm
    n 1: noctuid moth larvae that travel in multitudes destroying
         especially grass and grain [syn: armyworm, army worm,
         Pseudaletia unipuncta]
    2: moth whose destructive larvae travel in multitudes [syn:
       armyworm, Pseudaletia unipuncta]
    3: larva of fungus gnat that feed on cereals and other grains;
       they march in large companies in regular order when the food
       is exhausted