African army worm

Authors: J. van der Berg, A. Erasmus, M. van Rooyen. Text extracted with permission from the editors from: Prinsloo, G.L. & Uys, V.M (Eds) 2015.  Insects of Cultivated Plants and Natural Pastures in Southern Africa.  Entomological Society of Southern Africa. Buy the book.
African army worm, Spodoptera exempta. Mature larvae - A. Erasmus, ARC
African army worm, Spodoptera exempta. Mature larvae - A. Erasmus, ARC

Spodoptera exempta

Other common names: kommandowurm (A); lagarta-invasora (P)

Lesser army worm Spodoptera exigua

Other common names: kleinkommandowurm (A)

Origin and distribution

The African army worm is widespread throughout sub-Saharan Africa and also occurs on the Arabian Peninsula, in southeast Asia, Australasia and Oceania, including Hawaii. The lesser army worm is found worldwide except for South America.

Identification

Gregarious larvae of S. exempta (illustrated) grow to a length of approximately 25 mm, have a black appearance with off-white longitudinal stripes and a characteristic white V-shaped mark on the front of the head capsule. Under magnification, however, the predominant colour is dark olive. Larvae of the solitary phase, which are seldom seen, are cryptically coloured in shades of green, brown or pink.

First instar larvae of S. exigua are pale green to yellow, developing longitudinal strips on the sides of the body as they mature; final instar larvae are 25-30 mm long; colour variable, green to greenish-brown with longitudinal stripes (illustrated); when gregarious they can be almost black.

lesser army worm, Spodoptera exigua. Larva. - D. Visser. ARC
lesser army worm, Spodoptera exigua. Larva. - D. Visser. ARC

Host plants

S. exempta feeds exclusively on poaceous plants, including maize, sorghum, small grains, sugarcane and turf gasses, to name but a few. In contrast, Spodoptera exigua, in addition to feeding on poaceous plants, also causes severe damage to broad-leaf crops such as vegetables, as well as Amaranthus, cotton, groundnut, lucerne and tobacco.

Damage

Both species sporadically attack maize. Spodoptera exempta more commonly invades pastures and fodder crops, while S. exigua usually attacks various dicotyledonous vegetable crops. At very low infestation levels, leaf damage to young maize plants caused by larvae of S. exigua resembles that of stem borers. Larvae eat small rectangular “windows” on the young leaves. These symptoms result from larval feeding on the upper surface of the leaf, leaving the lower surface intact. Larger larvae consume large parts of leaves.

Spodoptera exempta larvae are known to migrate into maize fields from surrounding vegetation once that food source is depleted. Larvae consume whole leaves, leaving only leaf veins. If ears are present in plants, the silks and tips of ears are also damaged. Solitary army worm larvae often feed on maize seedlings and cause damage resembling that of the chilo borer. As a result, maize plantings are sometimes unnecessarily treated with insecticide, since the damage caused by solitary army worms is negligible.

Life history

Infestation of the African army worm characteristically occurs in cycles of approximately ten years, with an epidemic year often being followed by a year without infestations. Larvae occur in swarms, migrating together as food sources become depleted.

Outbreaks of the lesser army worm do not occur in a cyclic pattern, although outbreaks may coincide with those of the African army worm. Since the larvae occasionally also occurs in swarms, they may be confused with those of the African army worm. An important aspect of army worm ecology is the occurrence of the so-called phase phenomenon (commonly associated with locusts). During years when the army worms appear to be absent, the larvae assume a different appearance and live solitary, rather than in swarms. The life cycle is extremely short, with only ten days needed from hatching to pupation. The distance covered by one generation of migrating larvae of S. exempta seldom exceeds 1000 m, and new infestations originate from moth movement rather than from previous larval infestations. Larvae pupate in the soil and there are several generations in one season.

Management

Several contact insecticides are registered for the control of both army worm species on various crops. The insecticidal characteristic of genetically modified Bt maize provides effective control of S. exempta, and would most likely also be effective against S. exigua.