Scientific name: Mythimna unipuncta (formerly Pseudaletia unipuncta)
Order: Lepidoptera
Family: Noctuidae

Info

Biological Description

• Moths: Sand-colored with definitive white dots in the center of each forewing and dark markings on the hind wings
• Larvae: Brownish-green, hairless, and have alternate dark and light stripes down their backs. By final instar, larvae are about 2 inches long. Head is pale brown with dark markings.
• Pupae: Dark brown and approximately 3/4 inch in length. They are sharply tapered at the tail end with a much more rounded head end.
• Eggs: Greenish white and laid in rows or clusters on leaves
• Confused with: Variegated cutworm and other related species

Economic Importance

• True armyworm can be a serious pest of field corn.
• Outbreaks:
  • More severe following cold, wet, spring weather
  • Damage is sporadic and dependent upon migrations from the south
• Susceptible crops:
  • All grasses, particularly wheat, oats, corn, barley and rye
  • Some legumes
  • When under stress, armyworms will attack neighboring vegetable crops and seedling alfalfa
  • May be a problem in corn no-tilled into alfalfa or grass sod, or in fields with heavy weed pressure
  • The presence of grass weeds in fields will attract moths for egg laying.

Life Cycle

• It is not known if overwinters in Wisconsin
• Adults appear in Wisconsin in late April/early May. Mating occurs and eggs are laid.
  • Preferred hosts are grasses and small grains. Blades often folded and sealed to protect eggs.
• Larvae emerge 7-10 days after eggs laid. Larvae feed for 3-4 weeks before pupating.
• Pupation lasts 2 weeks. True armyworms are then adult moths.
• Generations:
  • There are three generations per season
  • Each generation lasts 5-6 weeks
  • First generation is small, causing little damage
  • Second generation appears in July and is the largest, most damaging generation in Wisconsin
  • Third generation is typically not injurious

Damage/Symptoms

• Damage is a result of larval feeding
• Larvae feed at night or on cloudy days and hide in the soil or under foliage during the day
• Two types of infestations can occur
  • Throughout a corn field in July if grassy weeds are present in the field
  • Field edges can be damaged as a result of armyworm migrations from pastures, oats, or grassy pea or alfalfa fields
• Damage usually greatest along field edges and in grassy spots

Scouting Procedure and Economic Threshold

• If you find signs of armyworm feeding, check 5 sets of 20 plants at random, recording the number of damaged plants and the number of larvae per plant
• Repeat in several locations within the field since infestations may be restricted to certain areas
• Damage usually begins at field edges and moves inwards as larvae migrate
• Spot treat if possible when there are two or more armyworms at 3/4 inch or shorter on 25% of the plants OR if there is one armyworm per plant on 75% of the plants
• When armyworms migrate from adjacent areas, treat only the border rows

Integrated Control

• Natural control: Several braconid wasps and tachinid flies help keep armyworm numbers down, as do birds, toads, skunks, and some domestic fowl.
• Cultural control: Keep grassy weeds controlled to avoid attracting egg-laying adults. Avoid planting susceptible crops in low wet areas or in rotations following sod. If this is unavoidable, plow in the fall of the previous season to decrease early spring egg-laying sites. Killing grass with an herbicide or tillage may drive armyworms to the susceptible vegetables.
• Biological Control: The red-tailed tachinid fly (Winthemia quadripustulata) lays eggs on the armyworm's back and the tachinid larvae bore into the larval armyworm's back to feed. Additionally, several ground beetles and parasitic hymenoptera prey upon the armyworm. There is also an egg parasite (Telenomus minimus) that is effective in preventing egg hatch and subsequent larval feeding damage.
• Chemical Control: For current Wisconsin recommendations, consult University of Wisconsin-Extension Bulletin #A3646, Pest Management in Wisconsin Field Crops.

Excerpts from Integrated Pest Management Program – University of Wisconsin-Extension, Cooperative Extension Service. Field Crop, Crop Scouting Manual. 2001

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Site Links

Insect Images

• Corn pest images

Extension Publications

• Moth Identification Guide for Blacklight Trap Catch in Wisconsin

Wisconsin Crop Manager Articles

• Advanced armyworm and outbreak pockets in western Wisconsin. Wisconsin Crop Manager, Vol.12 No.21: 171-172. Jul. 28, 2005. Cullen, E.

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