Forages: Fall 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. Action Thresholds
5. Management Strategies
6. Updates on Forages: Fall Armyworm
7. Related links...

Description

Full-grown fall armyworms are 4 cm (11/2 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 emerging from them. 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.

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

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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

Feeding in forages usually occurs in late summer, early fall. Fall armyworm is an occasional pest of grass crops such as corn and wheat but will also feed on mixed stands of alfalfa and grass. Fall armyworm feeding occurs in the daytime, unlike true armyworm feeding, which occurs at night. Initial leaf feeding appears as ragged-edged holes and moist, reddish-brown frass can be found nearby. Grazed fields will develop browned or burned spots and will appear thinned out due to the armyworm chewing off the new foliage. Where pastures or hayfields have substantial growth accumulation, armyworms can defoliate entire fields, leaving only short, dried-out stems. Newly fall-seeded alfalfa stands are most at risk.

Action Thresholds

If 50% of the plants are infested with unparasitized larvae smaller than 2.5 cm (1 in.), insecticide may be warranted.

Management Strategies

For insecticide recommendation, see OMAFRA Publication 812, Field Crop Protection Guide. Armyworms 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 greater than 2.5 cm or 1 in. 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. 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 promote the feeding larvae to migrate from the grassy weeds that are now dead to the crop itself. (Order OMAFRA Publication 812, Field Crop Protection Guide)

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Updates on Forages: Fall Armyworm

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

Related links...

• Insects
• Forages

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