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Armyworm in small grains

Armyworm larvae
Figure 1: Armyworm larvae. Note the banding on the abdominal prolegs.

Armyworm is an occasional pest of Minnesota small grains (Figure 1). Scouting for armyworm is fairly straightforward and the larvae are easily controlled with insecticides.

Species

The true armyworms, referred to as Mythimna unipuncta and formerly known as Pseudaletia unipuncta (Haworth), are relatives of cutworms and are in the Noctuidae moth family.

Life cycle

Armyworms are native to eastern North America, but they can’t overwinter in Minnesota. 

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True armyworm
Figure 2: True armyworm. Note the head’s net-like pattern and the body’s stripe pattern. The larva has five pairs of prolegs. Color can vary from tan-olive to nearly black.

Identifying armyworms

Armyworm larvae can range from tan and olive to nearly black. Regardless of color, you can distinguish them by the series of lengthwise stripes on their body. Identifying characteristics include:

  • Two pale orange to pink stripes with white borders separated by a dark stripe on the side of the body (Figure 2).

  • Net-like pattern on the head.

  • A dark band at the base of the abdominal prolegs.

Hosts

Larvae feed in the area where they hatched until they pupate in the soil or run out of food. Armyworms, like some cutworms, tend to feed at night and hide throughout the day.

The true armyworm prefers to feed on grasses. When their food source depletes, they’ll migrate in groups to find a new food source.

These migrating swarms or “armies” eat and destroy crops as they move. They can easily cross a road and feed well into a field on the other side in a single night. Outbreaks tend to occur when moist, lush vegetation is available.

Natural enemies

Armyworm larvae have their share of problems. They’re often heavily parasitized by flies and wasps, and they can be infected by fungal and virus diseases. Eggs of fly parasites can sometimes be seen behind the heads of larvae, and the cocoons of parasites cover some infested larvae.

Scouting and management

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True armyworm lookalikes

Be aware there can be armyworm imposters lurking on field edges in Minnesota spring cereal crops.

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Bruce Potter, integrated pest management specialist, Southwest Research and Outreach Center and Ian MacRae, Extension entomologist

Reviewed in 2018

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