African Armyworm

The African armyworm (AAW), Spodoptera exempta (Walker) (Lepidoptera: Noctuidae), also called okalombo or Kommandowurm or nutgrass armyworm, is an African moth. It is a very deleterious pest, capable of destroying entire crops in a matter of weeks. The larvae feed on all types of grasses, early stages of cereal crops (e.g., corn, rice, wheat, millet, sorghum), sugar cane, and occasionally on coconut (Odiyo 1984; Yarrow et al. 1981). The armyworm gets its name from its habit of "marching" in large numbers from grasslands into crops. AAWs tend to occur at very high densities during the rainy season, especially after periods of prolonged drought (Haggis 1984, 1986). During the long dry season in eastern Africa AAW population densities are very low. Because outbreaks are never observed during the dry season, it is called the "off-season" by those that monitor AAWs (Odiyo 1981).

Spodoptera exempta moths live about 10 days. The female can lay a maximum of about 1000 eggs in her lifetime. The ivory-coloured eggs of the African armyworm are laid in clusters on leaves. Eggs hatch in 2–5 days. Six larval (caterpillar) instars are completed in 2–3 weeks. Caterpillars occur in two morphologically distinct forms: a "gregarious" form, which is black with yellow stripes, and a solitary form, which is green or brown. The morphological form is determined by density — becoming "gregarious" at higher densities. However, the AAWs do not exhibit the true gregarious behavior of locusts. It is the "gregarious" forms of AAW that cause outbreaks. Generally, AAWs are not noticed by farmers until the caterpillars are 10 days old and change from green to black (Brown 1972). In the last instar, larvae burrow 2–3 cm into the ground to pupate. Adults emerge in 7 to 10 days (Dewhurst 1985). The moths migrate over tens, and probably over hundreds, of kilometers between their emergence sites and their oviposition sites (Riley et al. 1983). The observation that AAW outbreaks can suddenly occur in areas that were free of the pests for several months has led to the hypothesis that the moths migrate hundreds of kilometres. Evidence for this hypothesis includes:

1. outbreaks in a geographical progression at one generation intervals (Rainey 1979);
2. outbreaks are often preceded by high catches of moths and associated with wind convergence (Blair et al. 1980);
3. results of isozyme analysis indicate that AAW populations in Kenya, Tanzania, and Zimbabwe regularly interbreed (Den Boer 1978);
4. laboratory tests show that the moths can remain in flight for several hours (Gatehouse and Hackett 1980);
5. results of mark and capture studies, and radar tracking indicate long distance, downwind migration of AAW moths (Rose et al. 1985).

For additional information see Jahn (1995).

The species also occurs in Yemen, some Pacific islands, and parts of Australia (Rose et al. 2000).