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Fall armyworm (FAW) larvae can damage corn at various stages of development by feeding on leaf or ear tissues. FAW larvae are smooth-skinned, vary in color from light tan to dark green or black, with 3 yellow stripes and a dark stripe down the back. There is an equally wide, wavy, yellow stripe, splotched with red next to the dark stripe. Early instar larvae are dark green with black heads and usually found in groups on the plant. Larvae have 4 pairs of abdominal prolegs and a pair of legs at the end of the body. Full-grown larvae are about 1.5 inches long.
Larvae of FAW, true armyworm (TAW), corn earworm (CEW), western bean cutworm (WBC), European corn borer (ECB), and southwestern corn borer (SWCB) can be mistaken for each other (Figure 1). Correct identification can impact management decisions. To differentiate FAW larvae from other species, look at the head of the larva. The head of a FAW has a prominent white, inverted Y-shaped mark between the eyes. The FAW larvae vary from light tan or green to almost black. Conversely, TAW larvae have a gray or greenish-brown head covered with a network of lines.
CEW larvae usually have an orange head with no Y. CEW larvae vary in color ranging from light green or pink to dark brown or nearly black. Early instars are nearly translucent light green and solitary. Bodies have alternating light and dark stripes running the length of the body and double dark stripes down the center of the back. The body is covered by regularly occurring tubercles with two or three stiff black hairs.
WBC larvae are tan with a darker, faint diamond-shaped pattern on their back, and dark stripes immediately behind their head. SWCB larvae are dull white with a pattern of raised black spots on the body. ECB larvae have smooth, dirty white colored skin, often having a pinkish tinge, with numerous dark spots scattered over the sides and top of the body. The head is dark brown to black.
FAW moths migrate north during the growing season from overwintering sites in South Texas/Northern Mexico and South Florida. Adult moths lay masses of 50 to 150 spherical, gray eggs on leaves. Larvae hatch in 3 to 5 days and initially move into the whorl to feed. Typically, FAW require about 30 to 45 days to complete one generation. FAW has a wider range of host plants that includes corn, sorghum, small grains, soybeans, cotton, bermudagrass, and rice.
Young FAW larvae remove the top layer of the leaf and eat through leaves, causing small pin holes. Very early symptoms can resemble the small holes and window pane feeding injury from ECB.2 Larvae continue to feed in the whorl, causing leaves to have a ragged appearance as they unfurl from the whorl (Figure 2). FAW actively feed during the day, particularly early in the morning and late afternoon, consuming large amounts of leaf tissue. Larvae can be found deep in the whorl, often protected by yellowish brown frass.
FAW will also infest corn from the tasseling to dough stage of growth. Larvae feed on tassels, immature ears, ear shanks, and tunnel into stalks. Heavy infestations of larvae feeding on kernels may result in yield losses. Yield losses may also occur from ear drop and lodging caused by larval feeding damage in ear shanks and stalks.1
Comparing larval ear feeding damage:3
FAW feed by burrowing through the husk on the side of the ear, unlike corn earworm. Larvae also enter at the base of the ear, feeding along the sides and may tunnel into the cob. They usually emerge at the base of the ear, leaving round holes in the husks.
CEW enter the ear through the silk channel, unlike ECB and FAW, which enter through the husks or cob. As silks dry, CEW begin feeding on kernels (Figure 3). Larvae feed at the tip and along the sides of the ear near the tip. CEW are cannibalistic with typically only one larvae surviving per ear.
WBC enter ears through silk channels or by chewing through husks, causing injury to the tip, base, and sides of the ear. Larvae of the WBC are not cannibalistic, and several may infest an ear.
ECB feed on pollen and silks before entering the ear and by tunneling through the shank and cob. ECB feeding injury can occur at both ends and along all sides of the ear.
Scouting for FAW can be difficult. Early FAW damage appears as “window paning�? and shot-holes in leaves. Damage from larger larvae results in ragged leaves. As corn ears develop, FAW larvae migrate from the whorls to the ears and damage kernels. If whorl damage exists, scout 20 consecutive plants at 5 places in the field.2 Determine the percentage of plants damaged by FAW. Pull some whorls and unroll the leaves to make larval counts. Scout corn around tasseling and silking. Look for large larvae in emerging tassels and very small ears. Continue to check closely until silks dry.
If the corn crop doesn’t contain B.t. traits that offer control against FAW, an insecticide application may be considered. However, the use of insecticides should only be used once the economic threshold has been met. Consult your local Extension office for economic thresholds in your area. Insecticides should be applied before FAW larvae burrow deep into the whorl or enter ears of more mature plants. Insecticides may not be effective if the larvae are burrowed in the whorl because the frass can block the FAW feeding tunnel.