Fall Armyworm

The fall armyworm (Spodoptera frugiperda) is native to the tropical and subtropical regions of the Americas but has become one of the most globally significant agricultural pests since its accidental introduction to Africa in 2016, from which it has since spread across much of Africa, Asia, and Australia. It is the larval stage of a strong-flying noctuid moth capable of migrating hundreds of kilometers in a single season.

This caterpillar is a generalist feeder but shows a strong preference for corn and other grasses, and its rapid reproductive rate combined with long-distance migratory flight of the adult moths allows it to build damaging populations quickly across wide geographic areas. Outbreaks can develop and spread rapidly, particularly in warm climates without hard winter frosts to limit overwintering populations.

Despite its status as a major crop pest, the fall armyworm is an important part of agricultural food webs, providing prey for birds, predatory insects, and numerous species of parasitic wasps and flies that attack its eggs and larvae.

• Body color ranges from light tan or green to dark brown or nearly black, often with subtle longitudinal stripes
• Distinctive pale, inverted "Y"-shaped marking on the front of the head capsule
• Four dark spots arranged in a square pattern on the top of each abdominal segment near the rear
• Grows to about 3-4 cm at full size
• Lookalikes: true armyworm and corn earworm, but the inverted Y on the head and the square arrangement of dark spots near the tail are diagnostic of the fall armyworm

Fall armyworms are native to the Americas, ranging from the southern United States through Central and South America, and have spread as an invasive species to sub-Saharan Africa, South and Southeast Asia, and parts of Australia and Oceania since 2016. They are most abundant in corn fields but also infest pastures, rice, sorghum, and other grass crops. In warm, frost-free regions the species breeds continuously; in temperate zones it migrates northward each year but cannot overwinter where hard freezes occur.

Fall armyworm caterpillars feed on the leaves, whorls, and ears of corn and other grasses, often causing extensive defoliation when population densities are high. Older larvae are aggressively cannibalistic when confined together, which limits crowding within individual plants but does not prevent mass movement between fields when food is depleted, giving rise to the "armyworm" behavior after which the group is named. Adult moths are powerful fliers capable of long-distance dispersal on prevailing winds, contributing to the rapid spread of the species into new regions. As both larvae and adults, fall armyworms are preyed upon by birds, predatory insects, and a range of parasitic wasps and flies that provide natural population control.

Adult female moths lay egg masses of 100-200 eggs covered with protective scales, usually on the underside of leaves. Eggs hatch within two to four days, and caterpillars pass through six instars over about two to three weeks, feeding heavily and moving into the whorl or ear of corn plants as they mature. Mature larvae burrow into the soil to pupate, emerging as adult moths after one to two weeks in warm conditions. Adults are grayish-brown moths with a wingspan of roughly 3-4 cm, the males bearing more distinctly patterned forewings than the plainer females. In tropical and subtropical regions, the fall armyworm can complete multiple generations year-round.