Armyworm in corn

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

The mere mention of armyworms can cause angst in those who have experienced outbreaks, and the news of armyworms in the area can trigger unnecessary insecticide applications.

Fortunately, other than taking some time, scouting for armyworms is fairly straightforward and the larvae are easily controlled with insecticides (Figure 1).

Species

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

Don’t confuse armyworms with the tent caterpillars that feed on broadleaf trees and shrubs. Some mistakenly call these armyworms.

Life cycle

Armyworms are native to eastern North America, but they cannot overwinter in Minnesota.

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 Early instar true armyworm
Figure 2. Early instar true armyworm. Note net-like pattern on head and the stripe pattern on the body. The larva has five pairs of prolegs. Color can vary from tan-olive to nearly black.

Identification

The larvae can range from tan and olive to nearly black in color. Regardless of color, they can be distinguished by a series of lengthwise stripes on the body. Two pale orange to pink stripes with white borders separated by a dark stripe on the side of the body are diagnostic (Figure 2).

The net-like pattern on the head and a dark band at the base of the abdominal prolegs are also identifying characteristics.

Hosts

The true armyworm prefers to feed on grasses. When their food source is depleted, they will 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.

In previous infestations, armyworms have cleaned out the weedy grasses in a sunflower or soybean field and ignored the broadleaf crop.

Occasionally, they’ve been reported as a pest on some broadleaf crops, but the larvae may have simply been migrating after their favored food was depleted.

Natural enemies

Armyworm larvae have their share of natural enemies. They are 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

Monitoring migration

The presence of a large migration flight of true armyworm into Minnesota can be detected with black light traps. The capture of moths can predict when a problem is likely and when it will occur but, because immigrant moths can re-migrate, not where the problem will occur. Pheromone traps for true armyworm are available; however, what the captures mean in relation to crop damage is unclear.

 Armyworm damage to corn
Figure 3. Armyworm damage to corn in 2013. Note the weedy grasses killed by herbicide. Photo: Tyson Kaldenberg

Crop damage

Chewing damage on crop leaves and the presence of frass (insect fecal pellets) on plants and on the ground indicate that an insect was present (Figure 3). When live larvae are present, there’s potential for future damage.

Where to look for them

Armyworm larvae, like some cutworms, tend to feed at night and hide throughout the day. They may also be active on cloudy days.

During the heat and bright sunlight, larvae often hide under leaf litter on the ground. Therefore, scouting and insecticide applications are often more effective near dawn and dusk and on cloudy days.

When disturbed, armyworms drop to the ground and curl into a C-shape to “play possum”.

How to scout

Preliminary scouting for armyworms in small grains, field edges and even grassy areas within row crops can be done with a sweep net. Once armyworms are found, switch to a crop specific scouting method.

Because grassy weeds are attractive to egg-laying moths, pay close attention to field borders and areas within the field that have or had high grass weed pressure. If not killed before moths arrive, grass cover crops, particularly winter rye, may also be attractive egg laying sites.

Examine plants for feeding damage and larvae. Larvae can often be found in the whorl, and the nighttime feeding often occurs in the whorl.

Treat whorl stage corn when 25 percent of plants have 2 larvae per plant or 75 percent of plants have one larva or more. On tassel stage corn, minimize defoliation at or above the ear leaf.

Managing armyworm in corn

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True armyworm lookalikes in corn & spring cereal crops

Be aware that there can be an armyworm imposter lurking on field edges

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Bruce Potter, integrated pest management specialist, Southwest Research and Outreach Center

Reviewed in 2018

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