Forages: True Armyworm

Excerpt from Agronomy Guide for Field Crops (Chapter 5)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

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

Description

True armyworms are 4 cm or 11/2 in. (full grown), dull green-to-brown larvae that can be easily confused with other caterpillars, including variegated cutworm and fall armyworm. Two distinguishing features that true armyworm possesses are white-bordered stripes running laterally along the body and dark diagonal bands at the top of each abdominal proleg. The head is yellow-brown with a network of dark brown lines creating a pattern.

Plate 18. True armyworm larvae have black bands at the top of each proleg and the lighter brown head.

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Life History

In early spring, the moths prefer to lay their eggs in grassy vegetation. Larvae hatch from the eggs and feed at night or on overcast days for approximately a month. There are two generations per year, but it is the first generation that tends to do the most damage in corn in Ontario.

Damage

True armyworm larvae feed at night. Most feeding damage is done on cereals during July. True armyworms do not feed on pure stands of alfalfa but will feed on alfalfa/grass mixtures. The larvae strip the leaf margins, moving up the plant to feed on the panicles and flowers.

Scouting Technique

The best time to scout for true armyworm is early evening, just before sunset. Examine 20 plants from five locations in the field (100 plants in total). During the day, you may find the larvae amongst the crop debris on the soil surface or under soil clods. Also, you may find brown frass, often mistaken for eggs, on the soil near the plant. Record the size and number of larvae. When scouting, check the backs of armyworms for eggs. These small, oval, yellowish eggs are usually located just behind the head of the larva. These are eggs of a parasitic fly. The eggs will hatch, and the maggots will kill the armyworm larvae.

Action Thresholds

If 50% of the plants are infested with larvae smaller than 2.5 cm (1 in.), insecticide may be warranted. Avoid treating with insecticides when large numbers of parasitized larvae are present.

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Management Strategies

If the larvae are almost full grown (about 4 cm or 11/2 in. long), there is no benefit in applying insecticide since most of the feeding damage has already been done. Insecticides will only be effective on larvae that are less than 4 cm or 11/2 in. Refer to OMAFRA Publication 812, Field Crop Protection Guide, for insecticide recommendations. Treatment may be confined to infested areas. The parasites and other beneficial organisms usually keep armyworms from reaching damaging levels. Avoid treating with insecticides when large numbers of parasitized larvae are present. (Order OMAFRA Publication 812, Field Crop Protection Guide)

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Updates on Forages: True Armyworm

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

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Related links...

• Insects
• Forages

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