Corn: Fall Armyworm

Excerpt from Agronomy Guide for Field Crops (Chapter 3)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Description
2. Life History
3. Damage
4. Scouting Technique
5. Action Threshold
6. Management Strategies
7. Updates on Corn: Fall Armyworm
8. Related links...

Description

Full-grown fall armyworms are 4 cm (1 1/2 in.) long, varying in colour from light tan or green to near-black. Three white, thin stripes run down the back with one thicker, yellow band with red spots running along the side, just above the legs of the larvae. Scattered along the body are black spots called tubercles that have spines coming from them.

Plate 19. Fall armyworm larva damage on corn ear. Larvae leave entry holes on the exterior of ear. Note head is black wih a white, inverted "Y" on the front.

The fall armyworm larvae can be distinguished from the true armyworm by a white, inverted "Y" on the front of the head of the fall armyworm. The fall armyworm head is dark brown to black.

The adults are dark grey moths with a mottled pattern on their wings. A white spot can be found on the tip of the wing.

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Life History

Fall armyworm adult moths migrate from the southern states and show up later in the season when the corn is fully grown. This insect cannot overwinter in areas where the ground freezes.

Damage to the ear is similar to that of corn earworm, however, the fall armyworm entry hole can easily be seen coming from the side of the ear, and feeding is throughout the ear. Corn earworm enter the ear through the silk channels, and feeding is typically concentrated close to the tip of the ear.

Damage

Fall armyworm is an occasional pest of grass crops such as corn and wheat. The larvae feed on the whorl leaves and ears predominately from late July to September. Fall armyworm feeding occurs in the daytime, unlike true armyworm feeding, which occurs at night. Initial leaf feeding appears as ragged-edged holes; moist, reddish brown frass can be found nearby.

Scouting Technique

Examine 20 plants from five locations in your field to determine the level of infestation. Record the size and number of larvae. When scouting, check the backs of armyworms for parasite eggs. These small, oval, yellowish eggs are usually located just behind the head of the larva. These are eggs of a parasitic fly whose maggots will kill the armyworm larvae.

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Action Threshold

If 50% of the plants are infested with larvae smaller than 2.5 cm (1 in.), insecticide treatment may be warranted, but damage is usually not economical unless infestations are high and feeding is concentrated on the undeveloped tassels.

Management Strategies

For insecticide recommendations, see OMAFRA Publication 812, Field Crop Protection Guide. Armyworm tend to move in large numbers from one field to another. When this happens, it is advisable to spray the border of the invaded field. Chemical control is not effective on larvae larger than 2.5 cm (1 in.) or on those concentrated in the ear. Parasites and other beneficial organisms usually keep armyworms from reaching damaging levels. Avoid treating with insecticides when large numbers of parasitized larvae are present. Late-planted corn is most susceptible to leaf and whorl feeding. Grassy areas in and along the field borders are attractive to the egg-laying moths, therefore, eliminating grasses and weeds from the corn field is recommended. Late-season grass control may not be a good option since this will cause the feeding larva to migrate from the grassy weeds that are now dead to the crop itself. (Order OMAFRA Publication 812)

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Updates on Corn: Fall Armyworm

• Field Crop Protection Guide - Publication 812
  • Corn: Armyworm (True and Fall)

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Related links...

• Insects
• Corn

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