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Armyworm

Mythimna unipuncta (Haworth, 1809), (Lepidoptera: Noctuidae)

Distribution

The armyworm is native to the Americas, but localized invasive populations have been observed in areas of Europe, Africa, the Mideast, and Asia. It is a member of the moth family Noctuidae, a large group that includes most species of cutworms.

In North America, crop damaging infestations are most often observed east of the Rocky Mountains as far north as southern Canada. The highly migratory behavior of the armyworm adults allows them to exploit new geographic areas when weather is suitable. The armyworm cannot survive winters with persistent freezing winter temperatures. In Minnesota, annual infestations are the result of adult moths migrating from wintering areas in the south.

Host range

Armyworm adults feed on plant nectar and are not a threat to crops. The larvae, however, feed primarily on grasses and are an infrequent, but significant, pest of cereals including small grains, corn, rice, forage grasses, and turf grasses. The larvae can feed and complete their development on a wide variety of broadleaf species when large populations deplete their preferred food plants.

Description and life cycle

What is in a name?

The larvae’s behavior of congregating and moving in large groups when looking for new food sources is the basis for the name armyworm. The caterpillars of several other insect species display similar group behavior.

Most species that include armyworm in the common name belong to the Noctuidae family. One example is the fall armyworm (Spodoptera frugiperda), an infrequent late-season migrant visitor to Minnesota from the tropics.

Some Minnesotans also call the eastern forest tent caterpillars (Malacosoma americanum) that create communal webbed shelters (tents) and feed on the leaves of broadleaf trees and shrubs “armyworms.” However, these moths belong to the insect family Lasiocampidae and are not related to Mythimna unipuncta

To avoid confusion with other species that have armyworm as part of their approved common name or are regionally known as “armyworms,” Mythimna unipuncta is often referred to as “true armyworm.”

Adult

Armyworm moth
Figure 1. True armyworm moth. Note the tan color, diagonal line of dark spots and white dot on the forewing. Photo: B.D. Potter

The armyworm is a moderately sized moth with a wingspan of approximately 1½ inches.

On specimens in good shape, the forewing is tan to reddish-brown with a small white mark in the center. The species name “unipuncta” is based on this single white point.

A faint diagonal line of small black dots extends to the tip on the forewing’s outside (away from the body when spread).

The hindwings are pale to dark gray with a light outer border (Figure 1).

As moths age, or wing scales are otherwise damaged, these identifying marks are lost and identification becomes more difficult.

The armyworm moths are nocturnal and, like many insects, are attracted to light. Receptive females release a sex pheromone to attract mates.

Egg

The female moth seeks out areas of lush forage grasses, small grains, or grassy weeds. She deposits her eggs in compact masses of one to several rows that hold a few to more than 200 white to yellowish eggs, which darken prior to hatching. The eggs are concealed between the blades and stems of grasses and the folds of leaves. Over several days, a single female has been observed to produce over 1800 eggs.

The armyworm’s rate of development depends on temperature. The eggs have been reported to hatch in as few as three days at 84°F and an average four days at 77°F. The eggs are the most cold-tolerant immature life stage. Egg hatch is greatly slowed at temperatures below 55°F, with  no hatch occurring below 41°F.

Larva

armyworm larva
Figure 2. View of a newly molted, 4th instar armyworm larva. Note the dark band bordered by pink to orange bands, the net-like pattern on the head and black bands at base of the anal prolegs.

The caterpillar is the crop-damaging insect stage. Armyworms have three pairs of claw-like true legs near the head and five pairs of fleshy ‘prolegs’ along the abdomen (four pairs of abdominal prolegs and one pair of rear anal prolegs) (Figure 2).

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Several armyworm larval stages compared to nickel
Figure 3. An armyworm infestation can consist of larvae of multiple sizes. A fecal pellet (frass) can be seen in the lower right. Photo: B.D. Potter

Multiple or extended egg-laying and the varied microclimates within the field can lead to a wide range in larval development (Figure 3).

Armyworms do not have a fixed diapause (a life stage where development is suspended to survive adverse environments). Therefore, no life stage can survive in Minnesota or other states with persistent sub-freezing winter temperatures.

Further south, in latitudes such as Tennessee and Kentucky, mild winters allow the armyworm to overwinter as an immature larva. Adding instars allows the overwintering larva to delay maturity until conditions are more favorable. In frost-free areas such as Florida, the insect is active and breeds year-round.

Pupa

The larva stops feeding for a day or two and forms a web-lined pupal cell in the top inch of soil. There is no cocoon, and the moths emerge from the reddish-brown pupa in one to three weeks. The different developmental stages can be roughly predicted using a degree-day (DD) model with a base temperature of 50°F. It requires approximately 340 DD to reach the end of larval feeding and 575 DD to complete one life cycle from egg to adult. The average duration of each life stage is shown in Table 2.

In the northern part of its range, the armyworm life cycle from egg to adult takes 35–60 days. Multiple generations, usually two to three in Minnesota, are produced until the insects are killed by cold temperatures or environmental conditions trigger a southward migration of adults.

Table 2. Armyworm life table.

Life stage Avg days /
life stage1
Corn foliage
consumption (mg)2
Percent of total
foliage consumption2
ADULT (female)
Egg-laying 8.7
Total 17.2
EGG 7.5
Larva
1st instar 4.8 1 0.1
2nd instar 3.3 1.5 0.2
3rd instar 3.3 6 1.2
4th instar 3.8 21 4.2
5th instar 4.4 75 14.9
6th instar 10.3 400 79.3
Total larva 29.9
PUPA 18.3
TOTAL 72.9 504.5 100.0

1After Guppy, J.C. 1951. Three-year average in an Ontario, Canada environment (1957-1959).
2Adapted from Mukerji, M.K. and J.C. Guppy (1970) Estimated individual instar values determined from measurement of the manuscript's graphic data.

Crop damage

Armyworm outbreaks occur infrequently but can be very destructive when they do.

The first instar larvae are phototrophic and may feed during the day or the night. The larvae’s chewing mouthparts skeletonize plant tissues on the tips of younger leaves or beneath leaf sheaths. When resting they shelter in leaf sheaths or in stubble.  

The late second instar and older larvae chew holes and notches through the leaf from the edge of the leaf blade toward the midrib and may feed anywhere on the plant. Older larvae avoid light, hiding during the day in stubble, under leaves or soil clumps near the base of the plant, or in the whorls of larger corn. Large larvae are often found on the ground and under lodged small grains and grasses. From dusk to dawn, and on dreary, cloudy days, the larvae will move higher on the plant to feed. Third to sixth-instar larvae curl into a C-shape when disturbed.

corn defoliated by armyworm
Figure 4. Corn defoliated by armyworms after a rye cover was terminated. Photo: C. LaCanne

The larvae are voracious feeders. Most of the foliage is consumed by the last instar (Table 2). The presence of the insect and its feeding often goes unnoticed until large populations defoliate a field or a field or pasture overnight. After a large armyworm infestation moves through a field of whorl-stage corn, only stalks and leaf mid ribs may remain (Figure 4). Young corn and small grains may be completely defoliated (Figures 7-10).

The larvae may clip the seed heads of cereals and forage grasses in addition to feeding on leaves. Once a food source has been depleted, the larvae will move n masse to find new feeding areas (Figure 11).

Crops and forages are often damaged by larva moving from grasses within fields, field borders, and from adjacent crops. When hungry armyworms attack broadleaf crops, it is most often after grassy weeds or cover crops have been consumed or killed by herbicide.

In Minnesota, most of the damaging late-instar larvae populations occur in mid-June to mid-July. It typically takes 30-40 days from moth flight and egg-laying to the beginning of the sixth and most destructive larval instar.

Natural enemies

Tachanid fly eggs on armyworm
Figure 5. Tachinid fly eggs near the head of an armyworm larva. Photo: Robert J. Bauenfeind, Kansas State University, Bugwood.org

Most years, armyworm populations are kept in check  by natural enemies. The small, white eggs of a parasitic tachinid fly are often found attached near the head of caterpillars (Figure 5). A range of fly, wasp, and nematode parasites have been isolated from armyworm eggs, larvae, and pupae.

Armyworm larvae can also be infected by viral, fungal  and bacterial pathogens and the diseased larvae often appear less active or flaccid. Make note of any diseased or parasitized larvae while scouting. They could signal the start of a rapid armyworm population collapse.

Bird, mammal, and insect predators (notably ground beetles) also reduce larval populations. The presence of large numbers of birds (blackbirds in particular) early in the morning or in the evening can indicate an armyworm infestation in the field. Birds and bats prey on adults. Noctuid moths have evolved behaviors to avoid the high frequency echolocation of bats.

Armyworm moth migration

Lab experiments found no life stage of the armyworm able to survive two weeks of freezing temperatures. Historical observations reinforce that armyworms cannot overwinter in Minnesota and each year’s infestations originate in areas to our south.

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Scouting and economic thresholds

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 straightforward and they have been easily controlled with insecticides.

Lush grasses are preferred egg laying sites for the armyworm moth. Lodged areas of small grains, grasses or the grass borders of corn and small grain fields should receive special attention when scouting. When they have defoliated an area, larvae will move from field borders or between fields.

armyworm in wheat
Figure 7. Defoliated wheat seedlings, armyworms and frass. Photo:Ian MacRae.

Focus your efforts on high-risk areas first. Armyworms prefer to feed on the most tender foliage. Leaves with feeding damage and the presence of frass (insect fecal pellets) on plants and on the ground (Figure 7) indicate that an insect was present, but the presence of live larvae indicates the potential for future damage exists. Finding mostly large, last instar larvae, particularly if pupae are found beneath the soil surface, is an indication that the feeding is ending.

Armyworm larvae are most active at night and on cloudy days. During the heat and bright sunlight of day, larvae often hide under leaf litter or soil clods on the ground and scouting is 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.” Preliminary scouting for armyworms in small grains, field edges, forage grasses, 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. 

Corn

Grassy weeds are attractive to egg-laying moths. When scouting, pay close attention to field borders and within-field areas with current or past high grass weed pressure (Figure 8). If not killed before moths arrive, grass cover crops or winter rye may also be attractive egg laying sites.

Examine plants for feeding damage and larvae. On larger plants, the larvae can often be found in the whorl, where the nighttime feeding often occurs (Figure 9).  When there are large amounts of plant residue (cover crop, dead weeds, etc.) larvae may hide on the ground. Small corn (5 leaves or less) can typically recover from armyworm defoliation.

Economic threshold

Treat whorl stage corn when 25% of plants have two larvae/plant or 75% of plants have one larva or more. On tassel stage corn, focus on minimizing defoliation at or above the ear leaf.

 

corn damaged by armyworm
Figure 8. A corn plant damaged by armyworm feeding. Armyworms have a preference for the newer foliage. Photo: Bruce Potter.
A striped armyworm caterpillar wrapped around a young corn plant, eating the leaf whorl.
Figure 9. Armyworm feeding in a corn whorl. Photo: Claire LaCanne.

Wheat, barley and oats

armyworm feeding on wheat
Figure 10. Armyworm damage to seedling wheat. The larva is heading below ground. Photo: Ian MacRae.

Some studies indicate a difference in preference among the small grain species, but all are hosts.  When trying to detect larval populations, pay close attention to early planted, taller fields and to areas that are lodged, are near lodged grass borders, or have grassy weeds. When an economic armyworm infestation is suspected in a small grain field, populations per square foot should be estimated.

Head clipping is a behavioral change and usually occurs after leaves have been defoliated or senesce.

Economic threshold

Shake the plants and look for larvae on the ground in a square foot area. In small grains the treatment threshold is 4-5 larvae/square foot. Check under debris and soil clods. Do this in at least five locations within the field.

Pastures and forage grasses

The same scouting methods and economic thresholds described for small grains can be used for pastures, forage grasses and grass seed production fields.

What about other crops?

Despite their strong preference for grasses, armyworms may be forced to feed on less preferred hosts. Armyworms may clean out the weedy grasses while leaving a less desirable broadleaf crop alone but on occasion, starving armyworms will switch to the broadleaf crop when the grass food source has been eaten or killed. When terminating a grass cover crop in corn, sugarbeets, newly seeding alfalfa, or other crops, it is worth looking for armyworm larvae based on feeding or sweep net samples. The same applies to areas of dense grassy weeds. Removing their food source will force the armyworms to feed on the crop or move to a nearby field.

Treatment for armyworm

Bt hybrids

Only Bt corn hybrids containing the Vip3a protein (Viptera), alone or in stacks, are labeled for controlling armyworm. In addition to lush small grain and forage grasses, grassy spring weeds and cover crop growth, the Bt protein in the planted hybrid can help prioritize fields for scouting. The Handy Bt corn trait table shows which Bt proteins control armyworm and several other insect species.

Insecticides

High rates of neonicotinoid seed treatments (e.g., Poncho, Cruiser, Gaucho) are effective on many seed and seedling insects and may provide some early seedling protection against armyworm. Seed applied insecticides containing a diamide (e. g. chlorantraniliprole and cyantraniliprole) may show increased control of  armyworm.

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Important insecticide application notes

It is particularly important to check the pre-harvest interval of any small grain or forage pesticide. In corn, take precautions to protect pollinators, particularly if corn is nearing the tassel stage.

Scouting and rescue insecticide applications are the best defense against yield loss from armyworms. Before deciding to treat, make sure cutworms are still present and actively feeding. 

Cultural control

Maintaining good, early season weed control and avoiding late termination of grass cover crops can reduce the attractiveness of fields to egg-laying moths.

True armyworm lookalikes in corn, cereal crops and forage grasses

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

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CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

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

Reviewed in 2021

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