Common names: Fall armyworm, Alfalfa worm, Buckworm, Budworm, Corn budworm, Cotton leaf worm, Daggy’s corn worm, Grass caterpillar, Grass worm, Maize budworm, Overflow worm, Rice caterpillar, Southern armyworm, Southern grassworm, Wheat cutworm, Whorlworm, Corn leafworm, Gusano cogollero (Spanish)

Life cycle and description

• Adult stage: Adult moths are 20 to 25mm long with a wingspan of 30 to 40mm. Forewings are shaded grey to brown, often mottled with a conspicuous white spot on the extreme tip. Hindwings are silvery white with a narrow dark border. Adults are nocturnal and most active during warm, humid evenings. Females lay eggs in clusters of fifty to a few hundred and can lay up to 2000 eggs in a lifetime. The average adult lifespan is estimated to be 10 days.
• Egg stage: Eggs are white, pinkish or light green in color and spherical in shape. Clusters of eggs are frequently covered in moth scales or bristles giving a fuzzy appearance. Eggs are usually laid on the underside of leaves.
• Larval stage: Larvae generally emerge simultaneously 3 to 5 days following oviposition and migrate to the whorl. Mortality rate following emergence is extremely high due to climatic factors, predators, and parasites. There are six larval instar stages. In the 2nd and 3rd instar stages larvae are often cannibalistic, resulting in only one larva in the whorl. Mature larvae are 30 to 40mm in length and vary in color from light tan to green to black. Larvae are characterized by several subdorsal and lateral stripes running along the body. Dark, elevated spots (tubercles) bearing spines occur dorsally along the body. Larvae of fall armyworm can be distinguished from larvae of armyworm and corn ear worm by a distinct white inverted Y-shaped mark on the front of the head. They have four large spots on the upper surface of the last segment. Larvae mature in 14 to 21 days after which they drop to the ground to pupate.
• Pupal stage: Pupation occurs a few centimeters (2 to 8cm) below the soil surface. Cocoons are generally oval and 20 to 30mm in length. Pupae are reddish brown and measure 13 to 17mm in length. Pupation usually takes 9 to 13 days, following which adults emerge.
• In optimum conditions the entire lifecycle can be completed in 30 days. Maize crops can normally support two generations.
• Optimum temperature for larval development is 28۫ C, although the egg stage and pupal stage require slightly lower temperatures.
• Protracted periods of extreme cold will result in death of most growth stages. The fall armyworm has no diapause mechanism and therefore is only able to overwinter in mild climates and recolonize in cooler climates in the summer.

Figure 1. Life cycle of the fall armyworm

Confirmation

The adult moth is similar to Spodoptera exempta and S. littoralis, but can be distinguished by its distinct, dark veins on the hind wing and pale orbicular stigma present on the male forewing. Larvae are characterized by the distinct white Y-shaped mark on the top of the head. Unlike armyworm larvae, fall armyworm larvae feed during both the day and night.

Signs of feeding and crop damage are similar to other armyworm species (Spodoptera), earworms and maize borer damage. Individual larvae need to be inspected to confirm identity.

Presence of host species (preferably maize) and favorable climatic conditions dictate the prevalence and severity of fall armyworm. Grassy fields attract adult females for oviposition.

The fall armyworm has a wide range of hosts including maize, rice, sorghum, sugarcane, cotton, alfalfa, peanuts, tobacco, and soybean, in addition to various wild grasses. However, gramineous plants are preferred.

Fall armyworm is one of the most important pests of maize in the Americas. Distribution of the pest ranges from northern Argentina to southern Canada and includes the Caribbean. Closely related species are found in Africa and Asia.

• Mechanism of damage: Damage is caused by loss of photosynthetic area due to foliar feeding, structural damage due to feeding in the whorl, lodging due to cut stems, and direct damage to grains due to larvae feeding.
• When damage is important: Severe infestations are uncommon and most plants recover from partial foliar feeding. Under severe infestation complete defoliation of the maize plant is possible. Damage is most severe when worms cause direct damage to the ear. Under severe infestation larvae are frequently observed migrating in large numbers to new fields similar to the true armyworm. Late planted maize and advanced growth stages are more vulnerable to fall armyworm damage.
• Economic damage: Under severe infestation yield loss ranging from 25 to 50% has been documented.

Management options

Monitoring

• Regularly monitor leaves and whorls for presence of larvae and signs of crop damage.
• Look for masses of larvae migrating between fields.
• Pheromone traps can be used to determine incidence of adult moths and disrupt mating during the whorl stages.

Cultural control

• Plant early to avoid periods of heavy infestation later in the season.
• Plant early maturing varieties.
• Rotate maize with a non-host.
• Reduced tillage methods often result in an increase of natural predators and parasitoids. However, in areas where fall armyworm infestation is high, disking or plowing can effectively reduce the survival rate of pupae in the soil.

Biological control

• Numerous parasitic wasps, natural predators, and pathogens help to control the population of fall armyworms.
• The egg parasitoid Telenomus remus is frequently introduced to effectively control fall armyworm and other Spodoptera species.

Insecticides

• Insecticide application should be considered when eggs are present on 5% of seedlings or when 25% of plants show signs of feeding damage. In order to be effective, insecticide application should commence before larvae burrow into the whorls or ears and insecticide spray should penetrate the crop canopy.
• Insecticides recommended for control of Spodoptera species include various pyrethroids, carbamates and organophosphates. However, insecticide resistance has been widely reported.

Host resistance

• Certain varieties of Bt maize provide adequate control of fall armyworm.
• Certain maize varieties with a thick epidermis are also resistant to fall armyworm attack.

Risk of spread

International spread of this pest is a risk. The fall armyworm is often found in airfreighted vegetables and fruit destined for Europe and is also occasionally detected on exported herbaceous ornamentals.

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Contributors: Gabrielle Turner, David Bergvinson, and Biswanath Das