Scouting for Fall Armyworm

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Pest Importance

Fall armyworm, Spodoptera frugiperda, is a minor pest of corn in the United States.
Economic damage is sporadic from year to year.
Fields at highest risk from injury are:

Fields planted to non-Bt hybrids without Lepidoptera control
Fields in the southern U.S. and Texas
Late-planted fields
Late-maturing hybrids

Photo: Early-stage fall armyworm larva — Early stage fall armyworm larva

Injury Symptoms in Whorl-Stage Corn

Early stage larval feeding:
- Causes “windowpane” and shot holes in leaves
Late-stage larval feeding:
- Causes elongate, ragged holes (see photo below)
- May cut leaves in half
- Injures developing tassel in VT stage
- Plugs whorl with wet, yellowish-brown frass

Photo: Whorl-stage corn injury from fall armyworm — Whorl-stage corn injury from fall armyworm

Scouting for Fall Armyworm

Only larvae in whorl-stage corn can be controlled with insecticides; larvae in corn ears are protected
Scout for larvae in whorl-stage corn:

Select 20 consecutive plants in a row.
Inspect plants for feeding injury.
Confirm identity of pest species as fall armyworm.
Repeat 20-plant sample at 4 additional locations.
Determine percent infestations based on 100 plants.
Consider insecticide application when 20% of whorl-stage plants are infested with live larvae.

Identification of Fall Armyworm

Dark spots (tubercles) on dorsal surface arranged in "trapezoid" or when near the tail, in a "square" pattern. Spots are darkened in photo for emphasis.

Fall armyworms can be distinguished from similar corn caterpillars by 2 physical characteristics:

Inverted “Y” pattern on the front of a dark head (see next 2 photos)
Dark spots (tubercles) on dorsal surface arranged in “square” or “trapezoid” pattern (see third photo)