Armyworm
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
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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)
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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
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Left to right: armyworm, fall armyworm and corn earworm. |
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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
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Injury to corn from armyworm in southern Iowa. |
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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
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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
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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
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Field Scouting
- Corn adjacent to small grain crops should be scouted along field borders for migrating armyworms as the cereal crop matures
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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