Distribution
The armyworm (Mythimna unipuncta) is native to the Americas, but localized invasive populations have been observed in areas of Europe, Africa, the Middle East, 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 the 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
The adult armyworm moth feeds on plant nectar and is 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
“Armyworm” is an appropriate name describing the larvae’s behavior of congregating and moving in large groups when looking for new food sources. 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
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 of the forewing’s outside (away from the body when spread).
The hindwings are pale to dark gray with a light outer border.
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 1,800 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 of 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
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).
The larvae need to molt to larger exoskeletons as they grow and typically pass through six larval stages (instars). The first instar larvae are pale with dark heads. The first set of prolegs is shorter than the others during the first- and second-instar, causing the larva to have a “looping” gait, arching their back when walking.
A larva’s length and head capsule width can be used to estimate its age. Larval size can be highly variable based on temperature and available food. See the table below for average instar measurements and the level of damage caused by each instar.
Overall, late-instar caterpillars can range from tan and olive to nearly black. The pattern of a long dark band flanked by white-bordered pink to orange bands along the side is a distinguishing characteristic of the older larvae, as is the net-like pattern on the head and a dark band at the base of the abdominal prolegs. Full-grown larvae are approximately 1-3/8 inches long. The larvae are primarily active at night.
A Canadian study found that larvae were only able to complete development at temperatures above 50°F. Between 50 and 84°F, armyworms developed and matured faster as the temperature increased. However, larvae took longer to develop, and survival of older larvae decreased at 88°F, a temperature near the upper developmental threshold.
Larval development from egg hatch to pupa was completed in 16 to 87 days at 84°F and 55°F, respectively, with the length of the larval stage, averaging 26 days at 77°F. At cooler temperatures or when food is inadequate, larvae may undergo one or more additional instars, prolonging development.
Multiple or extended egg-laying and the varied microclimates within the field can lead to a wide range in larval development.
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.
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 cold temperatures or environmental conditions kill the insects, triggering a southward migration of adults.
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.
The larvae are voracious feeders. Most of the foliage is consumed by the last instar (see table above). The presence of the insect and its feeding often go unnoticed until large populations defoliate a field or pasture overnight. After a large armyworm infestation moves through a field of whorl-stage corn, only stalks and leaf midribs may remain. Young corn and small grains may be completely defoliated.
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 en masse to find new feeding areas.
Crops and forages are often damaged by larvae 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.
What makes a field high risk for armyworm damage?
Armyworm outbreaks are often worse in cool, wet conditions. This may reflect optimal temperatures for the insect, but those same environmental conditions also favor lush dense growth of small grains and cool-season forage grasses. It may also reflect multiple weather systems bringing moisture and insects from the south.
Early planted small grains tend to be more dense and taller. This creates a more attractive habitat for egg-laying moths. Dense stands of grasses provide protection for the larvae, as do lodged small grains or cool-season forage grasses. These are often areas where larvae congregate and eventually move from as they deplete their food.
Areas with dense grass weeds within the field may be attractive to egg-laying moths. Overwintered cereal rye cover crops also may harbor small armyworms. Hungry larvae can move to the crop when weeds or a cover crop are terminated.
Average armyworm larval measurements<sup>1</sup> and damage to crop
| Instar | Body length (mm) | Head capsule width (mm) | Avg days / life stage2 |
Corn foliage consumption (mg)3 |
Percent of total foliage consumption3 |
|---|---|---|---|---|---|
| 1 | 1.8 | 0.35 | 4.8 | 1 | 0.1 |
| 2 | 1.8 | 0.57 | 3.3 | 1.5 | 0.2 |
| 3 | 7 | 0.94 | 3.3 | 6 | 1.2 |
| 4 | 11 | 1.5 | 3.8 | 21 | 4.2 |
| 5 | 18-20 | 2.3 | 4.4 | 75 | 14.9 |
| 6 | 30-35 | 3.0-3.5 | 10.3 | 400 | 79.3 |
1After Breeland, 1958.
2After Guppy, J.C. 1951. Three-year average in an Ontario, Canada environment (1957-1959).
3Adapted from Mukerji, M.K. and J.C. Guppy (1970). Estimated individual instar values determined from measurement of the manuscript’s graphic data.
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.
Moths use environmental and visual cues to begin migration, orient in flight, and terminate migration. Armyworm moths can move short distances north on their own, but can take advantage of weather systems using low-level jet streams to cover long distances quickly.
In the North American plains, the Rocky Mountains to the west of the Great Plains and warm Gulf of Mexico waters provide the energy for these long-distance transport systems. Cool, dry, low pressure in the western plains interacts with warm, moist, high pressure systems in the eastern plains to create strong southerly winds that are especially strong at night.
Moths migrating from the overwintering areas fly upward at dusk. With cooperative weather systems, surface winds and updrafts in advance of thunderstorms pull the moths into the low-level jet stream. These winds are strongest at night, moving at 30 to 80 miles per hour, and can occur from about 330 to 3,000 feet in altitude.
The flight is mostly passive with moths carried along until they decide to “drop out,” encounter cold air, or rain out in thunderstorms, arriving in the north in excellent shape. A high-pressure center to the east, with a strong low-pressure system approaching from the west, produces strong, persistent southerly winds that can bring armyworm moths northward to Minnesota.
Conditions necessary for long-distance migration to Minnesota
For spring, long-distance migrant moths to arrive in Minnesota, the air reaching Minnesota must have passed through the overwintering areas where migrating adults are present.
The track of the low-pressure center is critical. If the low tracks too far south, migration is cut off south of Minnesota. If the low tracks through Minnesota or northern Iowa, we have the potential for moths to reach Minnesota.
These weather systems may stall with the frontal boundary cutting across Minnesota. In that case, moth deposition is often heavy south and east of the front. Moths often drop out on the edges of heavy rainfall. Several low-pressure systems may ripple across the moist air moving and compound the moth deposition in Minnesota.
Radar studies in the 1980s found that most evening migrating insects move at an altitude of 1,700 feet or so. Wind trajectories can be used to estimate where an immigration event might have originated.
Not all Minnesota armyworms are destined to freeze to death in the fall. Armyworm migration south in the late summer and fall can be assisted by southerly flows associated with cold fronts.
Armyworms are not tolerant of hot temperatures, and populations are low in southern states during the heat of summer. Migration allows the armyworm to avoid both high summer temperatures in the southern wintering areas and the freezing winter temperatures in the northern part of its range.
Hydrogen and carbon isotopes in armyworm wing chitin (a polysaccharide in insect exoskeletons) were used to trace moths captured in Texas, much further north into Ontario, Canada, in the spring and back south again in the fall.
The presence of a large migration flight of true armyworms into Minnesota can be detected with black light traps. These moth captures can predict if a problem is likely and when it will occur.
It’s more challenging to know where the problem will occur due to the possibility of immigrant moths re-migrating. Historically, if a blacklight trap captures near or above sixty moths in a single night, it has been an indicator that an armyworm problem will occur somewhere in Minnesota within a few weeks.
Like blacklight traps, some non-specific feeding attractants can capture both sexes of armyworm and other moths. However, pheromone traps for true armyworm are commercially available to detect moth flights. They are relatively inexpensive and easily deployed. The pheromone traps, for the most part, only capture unmated armyworm male moths. The numbers of moths captured tend to be small and the capture period short compared to blacklight traps. Qualitatively, they can indicate a moth flight has occurred. Quantitatively, it is unclear how pheromone captures relate to crop damage.
While both black light and pheromone traps can indicate when a moth flight has occurred and perhaps the relative size of the flight, scouting for larvae remains the best way to determine the risk of armyworm damage to a crop. In a year when armyworm larvae are abundant, finding the first economically threatening armyworm populations early can help encourage scouting efforts and prevent armyworm defoliation.
Natural enemies
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.
A range of fly, wasp, and nematode parasites have been isolated from armyworm eggs, larvae and pupae.
Viral, fungal and bacterial pathogens can also infect armyworm larvae, 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 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.
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, scouting for armyworms is straightforward, and they are 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.
Focus scouting 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 indicate that an insect was present. Still, the presence of live larvae indicates that 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 usually 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.
Crop-specific recommendations
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. 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 usually occurs.
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 per plant or 75% of plants have one larva or more. On tassel stage corn, focus on minimizing defoliation at or above the ear leaf.
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 senesced.
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 per 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.
Weeds and cover 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 seeded 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
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.
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., chlorantraniliprole and cyantraniliprole) may show increased control of armyworm.
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.
Fortunately, armyworms can be controlled effectively with a timely application of a rescue, foliar insecticide. Many post-plant insecticide products provide effective control of armyworm with products in the pyrethroid, organophosphate, carbamate and diamide insecticide groups.
Do not base treatment decisions solely on field-edge populations. The presence of live armyworm larvae should be confirmed before an insecticide is applied to a field.
Insecticide treatment of populations that are starting to pupate or are heavily parasitized is not likely to provide economic benefit.
Long insecticide residuals are generally not needed because of the short time a larval generation is damaging. Refer to the insecticide label for rates.
Spot treatments can be effective when combined with careful scouting.
Partial field or border insecticide treatments for armyworm are often sufficient when infestations are well identified by scouting early or when armyworm populations are migrating. Treat several boom widths ahead of the infestation.
Most insecticides have little activity on armyworm eggs. Moth migration occurring over a long period of time and a prolonged egg-laying period increases the difficulty of timing the insecticide.
Applying too late risks economic crop loss, and applying too early allows late-hatching larvae to escape.
During daylight hours, armyworms spend much of their time hiding low in the canopy and under lodged plants, so good insecticide coverage is important. Use appropriate water volumes and pressures matched with spray nozzles suited to insecticide application. Do not use reduced rates, particularly when large armyworms are the target.
Spraying in the evening may help control the problem, but could also expose insecticides to temperature inversions and drift, reducing effectiveness.
There have been reports of issues with the performance of pyrethroid insecticides against armyworm. However, pyrethroid insecticide resistance has not been documented. These issues might be related to coverage, environmental conditions, short insecticide residual, missing late-hatching larvae, and applications made too late to non-feeding prepupal larvae.
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.
True armyworm lookalikes in corn, cereal crops and forage grasses
Be aware that there can be an armyworm imposter lurking on field edges.
Grass sawfly larvae range from tan to green. They are in the order Hymenoptera (bees and wasps) rather than Lepidoptera (butterflies and moths).
The fleshy prolegs are a giveaway because they number more than five. In the Lepidoptera, the prolegs number 5 or less. Lepidoptera caterpillar prolegs have minute hooks (crochets) while those of sawflies do not.
Sawflies can clip small grain heads. Minnesota infestations heavy enough to require treating have not occurred in recent memory.
Grass sawfly larvae are mostly found in the grassy borders of fields.
The wheat head armyworm, Dargida diffusa (Walker, 1856), is another moth also in the Noctuid family.
The adults are similar in appearance to the true armyworm, but have a dark streak the length of the forewing. They are captured in Minnesota light traps and can be mistaken for true armyworms. They are believed to overwinter as pupae in the soil.
They are gray to green with a broad pale lateral stripe. They are slender and have a proportionally larger head than the true armyworm.
They feed on the heads and seeds of cereals and other grasses, but are rarely a pest of cereal crops in Minnesota.
Another look alike moth is the lesser wainscot moth Mythimna oxygala (Grote, 1881), whose larvae also feed on grasses. The larvae are similar to the armyworm but the prolegs lack the black band.
Some moths in the genus Leucania (Ochsenheimer, 1816) are also similar in appearance to armyworm and can cause confusion in interpreting trap captures.
Several species of cutworms may be found in corn and small grain crops. Anticipate increased populations of other migratory and overwintering cutworm species in weedy fields or fields with a cover crop.
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. Be sure that the area you wish to treat is listed on the label of the pesticide you intend to use. Remember, the label is the law.
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Reviewed in 2021