University of Illinois

No. 16/July 10, 1998

Fall Armyworms: Many Southern Illinois Cornfields Are Infested

Fall armyworm infestations are causing many producers throughout southern Illinois to "wrestle" with some tough management decisions in replanted cornfields. For many producers that I visited with this week, plants in their fields were no taller than 12 to 18 inches. Typically, fall armyworms infest cornfields that are in the late-whorl to early tassel stages of plant development. Not this year. Our economic threshold for fall armyworms on whorl-stage plants is to consider a rescue treatment when 75 percent of the plants display injury and armyworms are continuing to feed. Because many southern Illinois cornfields are not at the whorl stage of development, the economic threshold becomes very blurry. We suggest a rescue treatment when 25 percent of the plants are being injured by true armyworms on seedling corn. Both our true armyworm and fall armyworm thresholds are based upon corn that was planted at more optimal planting dates in the spring. Many fields in southern Illinois were replanted well into June, and the potential yield doesn’t look quite as encouraging. Because many of these fields will be pollinating in late July and early August, during what is typically the most stressful point of the summer, the potential payback of applying rescue treatments for fall armyworm is a very tough call. Provided are some facts about fall armyworms that may be of some assistance in making control decisions.

  • Fall armyworms are migratory moths that overwinter in more southern states.
  • Fall armyworms are unable to survive winters in locations where the ground freezes hard.
  • Each fall armyworm moth can lay upwards of 1,000 eggs.
  • Eggs are laid in "hairy" masses, each mass containing approximately 150 eggs.
  • Unlike true armyworms, fall armyworms do not leave plants during the daylight hours.
  • Fall armyworms typically reach lengths of 1-1/2 inches.
  • Usually only 1 generation occurs in northern states, in southern regions, 5 to 10 generations may occur within a single year.

Fall armyworms are very similiar in appearance to true armyworms. Larvae range in color from light tan or green to almost black. Also very noticeable are the presence of many stripes running the length of the body. Fall armyworms have a very striking inverted Y on the front of their head capsule (Figure 2). True armyworms do not display this feature.

Figure 2. Head capsule of fall armyworm; note the inverted Y.

Mike Gray (, Extension Entomology, (217)333-6652