Armyworms Found in Delaware County Cropfields; Scout Fields Now!



This week there have been a couple reports of corn fields in Delaware County with significant armyworm damage.  In one case the fields were later planted corn after sod.  A few fields have been sprayed for control.

Armyworms (also known as True Armyworms) are a pest of grass, corn and small grains.    Moths and caterpillars are both nocturnal. Caterpillars are smooth and marked with two orange, white-bordered strips on each side.  They vary in color from dark greenish-brown to black and are marked with two orange, white-bordered strips on each side. Caterpillars start out ⅛ inch and grow to approximately 1½ inches long, then pupate just below the soil surface. The caterpillar stage lasts about 3 weeks, but they are usually 10 to 14 days old before damage is noticed.

Damage in corn fields appears as ragged holes chewed from the leaf margins and pellet-like droppings (frass) in the whorls and scattered on the ground. The caterpillars will be found in the leaf whorls or at the surface of the soil. In grass hay fields, caterpillars will begin feeding during the night time on lower parts of the plant and spend the daylight hours in plant debris on the ground surface.  As caterpillars grow in size, they spend more time feeding during daylight hours and feeding on the upper parts of the plant.

Recommended economic spray thresholds for corn:

  • Seedling: 10 percent or more plants show damage and larvae are still present.
  • whorl-stage: apply an insecticide when there are three or more larvae per plant.
  • Tall corn seldom needs treatment unless the leaves above the ear are also damaged.

NOTE: When larvae are larger than 1.25”, control is not as effective, and most damage is already done.

Recommended economic thresholds for small grains:

  • Wheat/small grains – 5 or more larvae per linear ft of row, larvae less than 1.25 inches and not parasitized, watch for flag leaf reduction or if grain heads clipped off – yield losses, a spray before soft dough to save the remaining 3 upper leaves is generally beneficial since these tissues are still important to grain filling.

Recommended economic thresholds for grasses:

  • Grasses – no specific guidelines available, need for treatment based on the level of damage observed in relation to the expected value of grass harvest

REMEMBER… if you have a true armyworm infestation in a mixed alfalfa – grass stand, alfalfa and grass BOTH NEED to be on the LABEL!!!

For more on True Armyworm click here

True Armyworm larvae and feeding damage on corn.

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