Armyworms are the caterpillar stage of several moth species, named for their tendency to move together across fields in large numbers, consuming vegetation in their path. These pests cause significant damage to turfgrass, corn, and other agricultural crops, making their identification an urgent matter for gardeners and farmers. Recognizing the specific visual characteristics of the armyworm, particularly the larval stage, is the first step toward effective management.
The Larva Stage: Size and Shape
The larva stage is responsible for nearly all feeding damage. Newly hatched larvae are tiny, less than one-tenth of an inch, and typically appear light-colored with a large, dark head. As they mature through several stages, called instars, they grow, reaching a final length of approximately 1.4 to 1.5 inches before pupating.
The body of the mature larva is generally smooth, plump, and cylindrical, though its texture appears granular upon close inspection. Larval coloration is highly variable, ranging from pale green or yellowish to dark brown or nearly black. This variation depends on the armyworm species and its diet.
Specific Identification Markings
Specific markings on the mature larvae distinguish armyworms from other common caterpillars. Many species, particularly the Fall Armyworm, exhibit a distinct, pale, inverted “Y” or “V” shape on the front of the head capsule between the eyes. This head marking is a primary feature for confirmation.
The body features prominent longitudinal stripes running down its length. Typically, light-colored bands run along the sides, often with a darker band between them. A key identifying feature is the presence of four large black spots, or tubercles, arranged in a distinct square pattern on the dorsal side of the second-to-last body segment.
The Adult Moth and Other Forms
The adult armyworm is a noctuid moth. These moths typically have a wingspan of about 1.25 to 1.5 inches and are dull gray or brownish in color. The forewings are often mottled, but a defining feature for males is a white or light-colored triangular spot near the tip and another in the center of the forewing.
Females lay pale yellow, dome-shaped eggs in masses of 100 to 200, often stacking them in layers on vegetation. The eggs are usually covered with grayish scales, giving the mass a distinctive furry or moldy appearance. The pupa is reddish-brown and is usually found buried a few inches deep in the soil within a loose cocoon of silk and soil particles.
Evidence of Infestation
The first clue to an armyworm problem is often the visual damage they inflict on plants. Newly hatched larvae cause “window pane” damage, appearing as transparent spots where they have chewed the underside of the leaf while leaving the top layer intact. As they grow, the feeding becomes more aggressive, leading to ragged holes and completely stripped leaves, especially in turfgrass and corn.
In severe infestations, the sheer number of larvae can cause grass to look suddenly brown and flat, as if a steamroller has passed over it. Another strong sign is the presence of frass, which is the caterpillar excrement, appearing as sawdust-like material accumulating in plant whorls or on leaves. This is a telltale sign of active feeding.