True Armyworm

True armyworm (Mythimna unipuncta) is an occasional pest of corn, soybean, forages, and cover crops in Iowa. It is a migratory pest, making it unpredictable and sporadic. Careful scouting is required to determine whether management is required.

Identification

Adult: Adult true armyworm moths are approximately 1 inch long. Forewings are light brown in color with a white spot in the center of each wing (Photo 1). There is also a dark speckled line extending diagonally from the white spot to the wing tip. Some moths may appear grayer if they have many dark speckles (Photo 2). Wing markings are more distinct on newly emerged moths; moths may be difficult to identify as they age or become damaged.

Egg: Eggs are white to yellow in color and darken before hatching. They are laid in compact masses on concealed areas of grass, like leaf folds or between blades and stems. A single female may produce more than 1,800 eggs.

Larva: True armyworm larvae are caterpillars with three pairs of legs near the head and four pairs of fleshy prolegs on the abdomen. They have many stripes down their bodies and may be many colors, like green, tan, orange, or black (Photo 3). They are distinguished from similar caterpillars by the dull orange stripes on each side of the body (Photo 4), an orange head capsule with a network of black lines, and dark bands on each of the prolegs (Photo 5). Fully-grown larvae are approximately 1.5 inches long.

Pupa: Pupae are reddish-brown and occur in the top inch of the soil. 

Be aware of look-alikes! Both the moths and larvae can be confused with other species. The moths are not typically of interest except when using pheromone traps to monitor their arrival in the spring. Pinion moths (genus Lithophane) are also small and brown and attracted to the true armyworm pheromone (or are otherwise finding themselves in traps on a regular basis).

The larvae resemble other armyworms, like fall armyworm or yellow-striped armyworm (Spodoptera ornithogalli). There may be overlap in hosts and timing of infestations, but a close look at the head capsule, prologs, and general body characteristics should help with identification.

Biology

Distribution: True armyworm is native to the Americas, but it is known to occur in areas of Europe, Africa, and Asia. Adults and larvae are unable to survive the freezing temperatures common in Iowa, so they typically migrate northward each year from overwintering sites in the southern U.S. True armyworm is typically observed anywhere east of the Rocky Mountains, from the southern U.S. all the way into Canada.

Host Plants: The larvae feed primarily on grasses but may also feed on broadleaf species when their preferred food source has been eliminated. In Iowa, true armyworm may be found feeding on corn, soybean, alfalfa, small grains, forage grasses, turf grasses, and cover crop species.

Life Cycle: Since true armyworms do not overwinter in Iowa, we start to see adult moths migrating here in the spring to lay eggs on grasses and other green tissue. Eggs typically hatch within a week, depending on temperature (average hatch is four days at 77°F). Eggs are the most cold-tolerant life stage, but hatching will not occur below 41°F. Once eggs hatch, larvae pass through six instars in about 26 days, depending on temperature. It is common to see many larval stages present in a field due to multiple migrations or extended egg-laying. Larvae cease feeding to form a web-lined pupal cell in the soil. The pupal stage lasts one to three weeks before moths emerge. There may be multiple generations in Iowa before cold temperatures either kill the insects or trigger their southward migration.

Dispersion and Dispersal: Much like fall armyworm, true armyworm has the characteristic behavior of moving in large groups to find new areas to feed. They are highly mobile insects as adults and larvae, and the larvae readily move from plants into the soil or litter to escape sun exposure or hot temperatures.

Plant Injury

Larvae are the only damaging life stage of true armyworm. Like other caterpillars, they have chewing mouthparts and strip tissue from plants, causing defoliation. Sometimes, the defoliation is severe enough that entire plants or fields are stripped of photosynthetic tissue. Small, first instar larvae feed on the tips of younger leaves and may actively feed day or night. Older larvae chew holes and leave notches in leaf margins and may feed anywhere on the plant (Photo 6). It is uncommon to see them during the day unless it is cold or overcast. To avoid light and hot temperatures, they hide in the whorls of corn, in stubble, or under soil near the base of plants (Photo 7). True armyworm often goes unnoticed until significant defoliation has occurred in a short period of time (24 to 48 hours). Under severe infestations in corn, only stalks and midribs may remain. In small grains or forage grasses, larvae may clip seed heads. Most of the defoliation occurs during the final instar.

Risk Factors

In general, true armyworm outbreaks tend to occur in cool, wet conditions, likely an artifact of weather systems bringing rain and insects at the same time. Those conditions may also be more favorable for the growth of grasses that are targets for egg-laying when moths arrive in Iowa.

For corn and soybeans, the risk of injury is greatest when there is actively growing, dense grass in or near the field in the spring when moths arrive. This could be cover crops, grassy weeds, or dense fencerows or ditches. This is amplified when cover crops or weeds are not terminated well in advance of crop emergence. Once they have consumed the grass, hungry larvae move on to the crop.

Scouting

Trapping moths: Pheromone traps use a synthetic sex pheromone to attract unmated males arriving in an area. There are several types of traps that can be used, but a common style is the wing-style pheromone trap (Photo 8). Capturing true armyworm moths in pheromone traps can help determine when moths begin to arrive in Iowa to inform scouting. Unlike black cutworm, there is no established trapping threshold that can be used to aid in scouting. Capturing true armyworm moths does not necessarily mean there will be economic infestations in a particular location. Scouting fields is the only way to determine if true armyworms are present and whether management is warranted.

Scouting for larvae: Prioritize scouting in areas with dense, green grass in the spring that would attract adult moths for egg-laying. Check the crop, any grass that has not been terminated, and the soil or residue for larvae, defoliation, or green pellets (frass). If larvae are found, count them and use their size to determine the potential for future damage. Finding mostly large, final instar larvae indicates that feeding will end soon. Finding mostly small or mid-size larvae is a clue that there is great potential for further injury.

For corn seedlings (VE – V2), treatment is recommended when 10% or more of the plants are injured and larvae are still present and less than ¾ inch long. For corn that is V7-V8, treatment should be considered when larvae are less than ¾ inch long, there are more than eight larvae per plant, and 25% of the leaf area has been removed. On reproductive stage corn, focus on minimizing defoliation above the ear leaf.

In small grains, pastures, and forage grasses, treatment should be considered if there are four larvae per square foot.

In soybeans, lump true armyworms with other defoliators. If larvae are still present and less than ¾ inch long and soybeans are vegetative, consider treatment if field-wide defoliation exceeds 30%. If soybeans are reproductive, the threshold drops to 20%.

Scouting tips! When scouting during the day or when it is warm, check the soil surface near the base of plants, under residue, or just under the soil surface for larvae. 

Management

It is uncommon to have true armyworm infestations in Iowa that greatly reduce yield, so preventative insecticide treatments are not recommended. Scouting and applying rescue treatments as necessary is the most economical approach for true armyworm management.

Cultural: The most effective way to avoid a true armyworm infestation is to make sure the field does not have lush, green grass in the spring that attracts egg-laying moths. Maintain adequate early-season weed control and ensure that weeds and cover crops are terminated at least two weeks prior to planting the cash crop in the field. This will reduce the likelihood that larvae are still present when the crop emerges.

Biological: There are a number of natural enemies that regularly keep true armyworm populations under control. A suite of fly, wasp, and nematode parasites are known to feed on various life stages of true armyworm. One noteworthy parasite is the tachinid fly, which lays white eggs near the head of many caterpillar species, including true armyworm.

True armyworms may also be infected by several pathogens. Make note of any diseased larvae (they may be slow, shriveled, fuzzy, or dead) while scouting. Generalist insect predators may feed on true armyworm eggs, larvae, or pupae. Larger animals congregating in the field, especially birds, may be a sign that true armyworms are present.

Plant-incorporated: Bt corn hybrids containing the Vip3A protein (e.g., Viptera) may help control true armyworm. Consult the Handy Bt Trait Table to determine if your hybrid has this protein.

Chemical: Seed treatments, especially those that contain a diamide (Group 28) insecticide or high-rates of neonicotinoids (Group 4A), may offer some early-season protection from true armyworm. Foliar insecticide applications are typically effective against true armyworm if timed well. Ensure that larvae are still small enough (less than ¾ inch long) to be controlled and are not parasitized or infected with pathogens. Spot or perimeter treatments may be justified with careful scouting and if infestations are caught early. Because true armyworm larvae typically conceal themselves during the day, sufficient coverage is very important to ensure the insecticide comes into contact with larvae. Be aware of pre-harvest intervals later in the season, especially for small grains or forages.