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Agronomic Crops > Crops Blog > Posts > Watch for Armyworm this Spring on Corn, Small Grains, and Grass Forages in Alabama

‚ÄčEntomologists can usually predict outbreaks of armyworm, Mythimna unipuncta, in Alabama.  That is because cool, wet springs favor the growth of large populations of this insect pest.  It has certainly been wet enough, although I am not sure it has been cool enough this spring.  Still, I think it will be worthwhile to keep an eye on grassy crops such as corn, sorghum, small grains, and grass forages for the next month or so. 

 

Armyworm is often called true armyworm so not to confuse it with the fall armyworm.  Armyworms vary in colorations but usually have several dark stripes.  The head of an armyworm caterpillar is light colored, with a mottled or netted pattern.  In profile, the head is rounded. 

armyworm.jpg



This insect overwinters as a partially grown caterpillar.  In early spring, the insect pupates then transforms into a moth.  Moths lay eggs in clusters of 2-5 rows on sheltered places in grass foliage.  Larvae hatch in a few days and begin to feed on the foliage.  Hungry caterpillars will march to find new food sources, as shown by the flattened corpses on the roadside in the image below.

 armyowrms on road.jpg

It is important to catch these insects when they are small, before they have time to do too much damage.  Approximately 80% of the damage from this pest is done by the largest sized caterpillar, in its last few days of feeding.  Small larvae skeletonize the foliage, but by the third instar begin to eat holes in leaves.  When they molt into larger sizes they can consume entire leaves.    Symptoms of this pest on maturing small grains include heads where the awns have been eaten, and heads that have been clipped off.  Caterpillars tend to do most of their feeding at night, and may hide in crop debris during the day.

 

If you find armyworms, here are some guidelines on whether there are enough insects to treat with an insecticide. 

Crop

Treatment Threshold

Corn

Treat if 25% of the plants have feeding damage

Small Grains

Treat when larval numbers exceed 4 larvae/sq ft before pollen shed and 8 larvae/sq ft after pollen shed.

Grass forages

Treat if there are more than 2-3 per square foot

 

Kathy Flanders

flandkl@auburn.edu


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