African Armyworm: Identification, Damage & Control

The African armyworm, Spodoptera exempta, is a migratory pest found across sub-Saharan Africa, as well as in parts of Asia, Australia, and the Arabian Peninsula. It primarily targets grasses and cereal crops, which are foundational to food security and economic stability for many communities in these regions. The pest can generate sudden and widespread outbreaks, with its larval stage consuming vast amounts of foliage in a short period, leading to agricultural damage and crop yield reductions.

The pest’s rapid life cycle and migratory behavior allow it to spread quickly across large territories. During outbreak years, the damage can be severe, affecting both staple food crops and the pasturelands for livestock. Adult moths are responsible for the dispersal of the species, while the larvae inflict the direct damage.

Identifying the African Armyworm

The African armyworm undergoes a complete metamorphosis with four distinct life stages: egg, larva, pupa, and adult moth. A female moth can lay between 400 and 1,300 eggs in her lifetime, placing them in clusters on the undersides of leaves and covering them with protective scales from her abdomen. These eggs hatch within two to five days, giving rise to the larval or caterpillar stage, which lasts approximately two to three weeks.

The appearance of the larva changes depending on population density. In low-density, solitary phases, the caterpillars are green or brown and remain well-hidden. During high-density outbreaks, they enter a gregarious phase, developing a velvety-black upper body with pale lines along the sides and a distinctive inverted V-shaped or Y-shaped mark on their shiny black head. These hairless caterpillars can grow up to 25-30 mm long.

After the feeding stage, the larva burrows 2-3 cm into soft soil to form a pupa inside a loose silk cocoon. This pupal stage lasts about seven to twelve days, after which the adult moth emerges to restart the cycle. The adult moth has a wingspan of 29-32 mm, with greyish-brown forewings and whitish hindwings with dark veins.

Destructive Impact on Agriculture

The most significant damage is inflicted by the larval, or caterpillar, stage. These larvae have a voracious appetite, and during high-density outbreaks, they consume nearly all vegetation in their path. This can lead to the complete defoliation of plants, with caterpillars eating leaves, growing points, and young stems, sometimes leaving only the midrib of the leaves behind.

The pest targets a wide range of crops, with a preference for those in the grass family. Affected agricultural plants include maize, sorghum, millet, rice, wheat, teff, and sugarcane. Pasture grasses are also heavily consumed, which has secondary effects on livestock that depend on this forage. The damage occurs with remarkable speed; a field can be destroyed in a matter of days during a severe infestation.

The economic consequences of these outbreaks are substantial for smallholder farmers. Yield losses in maize can range from under 10% to complete crop failure, depending on the plant’s growth stage. In some outbreak years, losses of 30% have been recorded in affected cereal production areas, threatening food security and straining national budgets.

Outbreak Dynamics and Migration

The name “armyworm” originates from the collective marching behavior of the caterpillars during high-density outbreaks. When food sources in one area are depleted, massive groups of larvae move together across the ground in search of fresh vegetation, resembling an army advancing across the landscape. This behavior is a hallmark of the gregarious phase, triggered when populations reach a high density. In contrast, at low densities, the larvae are solitary and less active.

The emergence of large-scale outbreaks is closely linked to specific weather patterns, often following the onset of rainy seasons that come after a prolonged dry period. These conditions promote the synchronized mass emergence of adult moths from their pupal stage in the soil. These initial emergences occur in specific locales known as primary outbreak areas, which then serve as the source for subsequent infestations.

The adult moths are strong, long-distance flyers, capable of migrating hundreds of kilometers. They are carried by prevailing wind systems, such as the Inter-Tropical Convergence Zone (ITCZ) winds in Africa. This wind-assisted travel allows them to rapidly expand their territory, and a single generation’s migration can establish new populations across national borders.

Control and Management Approaches

Managing African armyworm infestations requires a combination of monitoring and varied control tactics. Early detection is important, as control measures are most successful when applied to young, early-instar larvae. Key management approaches include:

• Monitoring: This relies on networks of pheromone traps to capture male moths. The traps provide an early warning of moth presence and migration patterns, allowing authorities to alert farmers and prepare for potential outbreaks.
• Chemical Control: This involves the application of approved insecticides like synthetic pyrethroids. Low-dose methods such as Ultra Low Volume (ULV) spraying can cover large areas quickly, but treatments must be applied as soon as infestations are detected to be effective.
• Biological Control: This approach utilizes the pest’s natural enemies, including parasitic wasps, flies, birds, and ants. A species-specific nuclear polyhedrosis virus (SpexNPV) has also been developed as a biopesticide that can cause high mortality, especially during cool and wet weather.
• Integrated Pest Management (IPM): This strategy combines the above tactics for sustainable control. IPM emphasizes monitoring, biological methods, and the judicious use of chemical pesticides. Cultural methods, like digging trenches to trap marching larvae, and community-based forecasting are also important for timely and coordinated efforts.