Armyworms are the larval, or caterpillar, stage of several species of nocturnal moths, not true worms. These insects are notorious agricultural pests because of the sheer volume of plant matter they consume and their tendency to move in large, dense groups. The term “armyworm” describes this behavior, as the caterpillars appear to “march” across fields, devouring vegetation in their path.
Key Armyworm Species and Their Range
The two most frequently encountered armyworm species in North America are the True Armyworm (Mythimna unipuncta) and the Fall Armyworm (Spodoptera frugiperda). The True Armyworm is found across the continent and typically becomes a pest during the cooler spring and early summer months. This species overwinters in the pupal stage in the southern United States and Canada, with moths migrating north as temperatures rise.
The Fall Armyworm is generally more destructive and cannot survive the winter north of the Gulf Coast states. Infestations result from adult moths migrating northward each year from tropical and subtropical regions. This species often causes damage later in the growing season, typically from mid-summer through the fall.
Primary Host Plants: Grasses and Cereals
Armyworms exhibit a strong initial preference for plants belonging to the grass family, Poaceae, which includes many valuable cereal crops. This preference is particularly pronounced in the True Armyworm, which heavily targets cool-season grasses. The larvae seek out dense, lush stands of plants like wheat, oats, barley, and rye for both feeding and shelter.
The Fall Armyworm also readily consumes grasses, including crops like corn, sorghum, and millet. Both species are devastating pests of turfgrass, such as bermudagrass, fescue, and bluegrass, often stripping lawns and pastures down to the soil. Destruction occurs rapidly because the final two larval stages consume over 90% of the total foliage eaten during the caterpillar’s development.
This heavy consumption of grasses is tied to the nutritional requirements of the growing larvae before pupating. Early instars feed on the most tender parts of the plant, while later, larger caterpillars consume entire leaves and stems. The availability and density of these preferred grasses make them the primary target for newly hatched armyworm populations.
Secondary and Opportunistic Feeding
While grasses are the preferred food source, armyworms have a broad diet and will feed opportunistically on more than 80 different plant species. This less selective feeding behavior occurs when a large population of larvae exhausts its primary grass hosts. Once a field is depleted, the caterpillars begin their characteristic migration in search of new food sources.
This mass migration can lead them into fields of broadleaf crops they would otherwise avoid. Plants susceptible to damage from migrating armyworm populations include:
- Alfalfa
- Cotton
- Soybeans
- Peanuts
- Tobacco
- Garden vegetables like beans, cabbage, carrots, and potatoes
The Fall Armyworm, in particular, is known to be highly polyphagous, meaning it can feed on a wide variety of plants, including many common weeds. This adaptability ensures population survival even when cereal crops are unavailable. This opportunistic feeding requires farmers and gardeners with non-grass crops to remain vigilant if a nearby grass field is infested.
Recognizing Specific Feeding Damage
The method by which armyworms consume plants leaves visual evidence that helps identify an infestation. Early-stage larvae cause damage known as “window-paning” or skeletonizing. They scrape the green tissue from one side of the leaf, leaving the transparent membrane on the opposite side intact. This damage usually occurs high up on the plant.
As the larvae grow larger, their chewing mouthparts create ragged, irregular holes, consuming the entire leaf blade. In corn, the Fall Armyworm often feeds deep within the whorl, leaving a characteristic pattern of holes as the leaves unfurl.
A different type of damage, often caused by the True Armyworm, is the severing or “clipping” of plant stems and heads in small grains like wheat and oats. The caterpillars chew through the stem just below the head, causing the grain head to drop to the ground. This clipping behavior results in the complete loss of the developing grain.