Injury to Corn

• Armyworm larvae feed only on the leaf margins; they do not tunnel into the stalk or feed in the ear
• Severe defoliation (below) removes the softer leaf tissue, leaving only the stalk and tougher leaf midribs
• Plants will recover from moderate levels of injury unless the growing point has been damaged
• Field-wide defoliation may occur if the population is large, which is most likely in fields with weedy grasses

Armyworm Identification

• Adult moth is pale gray to pale brown, with a white speck in the center of each front wing

• Larva
  • 1½ inches long when full grown with 2 “orange” stripes alongside and 2 dark stripes on back
  • 4 pair of abdominal legs often have a lateral dark bar (see arrow in photo below)
  • Head with 2 vertical bars on face (see photo below)

• Similar species
  • Fall armyworm has black “warts” on back
  • Corn earworm has black “warts” on back; lacks dark bars on legs and dark bars on face

Left to right: armyworm, fall armyworm and corn earworm.

Pest Status

• Armyworm is an uncommon pest in corn
• Economic damage is rare in conventionally-tilled fields
• Injury most commonly occurs in reduced-tillage fields with grassy weeds or a grass cover crop
• Moths lay eggs on grasses before corn is planted, then when a herbicide is used to kill the grass, the larvae move to the young corn plants
• Larvae also will migrate from adjacent cereal small grains when that crop approaches maturity

Armyworm Facts and Impact on Corn

• Scientific name: Mythimna unipuncta
  • Former name: Pseudaletia unipunct

• Common name: armyworm
  • Often incorrectly called “true” armyworm
  • Common name relates to behavior of larvae crawling in large numbers from one area to another, often defoliating plants as they migrate

• Importance
  • Larvae feed primarily on wild and domestic grasses, including corn, bluegrass, timothy, cereal small grains, and foxtail
  • Late-stage larvae may defoliate vegetative-stage corn, especially along field edges or in areas with grassy weeds

Injury to corn from armyworm in southern Iowa.

Armyworm Life History

• Pupae overwinter in soil in southern climates
• Adults migrate north in April and May and lay eggs in clusters on lower leaves of grasses or small grains
• Eggs hatch in 1-2 weeks depending on temperature
• Larvae feed mostly at night or on overcast days; during sunny days they may remain hidden in ground litter or behind leaf sheaths
• Because adults are migratory and flight is strongly dependent upon weather fronts, injury is sporadic in an area from 1 year to the next
• There are 2-3 generations per year, but rarely in the same fields

Integrated Pest Management of Armyworm

• Transgenic Bt corn
  • Pioneer® brand corn products with Optimum® Leptra® insect protection provide excellent control of 1st to 3rd stage larvae
  • Pioneer brand corn products with Optimum® Intrasect® insect protection provide very good control of 1st to 3rd stage larvae

• Weed Management
  • Weedy grasses should be burned down with a herbicide at least 7 days before corn emergence
  • Herbicides applied to weedy grasses after corn emerges should be tank mixed with an insecticide

• Field Scouting
  • Corn adjacent to small grain crops should be scouted along field borders for migrating armyworms as the cereal crop matures

• Insecticides
  • Liquid insecticides should be applied to corn if larvae are less than 1½ inches long and significant defoliation is occurring; larvae longer than 1½-inch are nearly done feeding and an insecticide will be of limited value

Author: Marlin E. Rice
Reviewer: Herb Eichenseer