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. 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. 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.
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. In the last instar, larvae burrow 2–3 cm into the ground to pupate. Adults emerge in 7 to 10 days. The moths migrate over tens, and probably over hundreds, of kilometers between their emergence sites and their oviposition sites. 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.
In mid-April 1999, an army worm infestation started in southern Ethiopia, spreading into the north the following month and into the Jubba valley of Somalia in early May. Similar outbreaks affected the Rift Valley Province of Kenya and parts of Uganda at the same time. While Ethiopian officials had stocks of pesticides to treat 350,000 hectares of affected land, neither Kenyan or Ugandan officials had sufficient supplies to combat the insect and there was no central government to respond to the emergency in Somalia.
STAR radio in Liberia reported in January 2009 that Zota District in Bong County had been invaded by army worms, which had consumed vegetation and polluted creeks and running water. They are moving to Guinea and Sierra Leone. On 28 January 2009, the President of Liberia declared a state of emergency to deal with the infestation of army worms in the country.
December 2009 saw an infestation of ten regions of Tanzania. The infested regions include three of the five main grain-producing regions. The other two major grain-producing regions are at risk of infestation. Tanzania has trained farmers in fighting armyworms since 2007, and responded to forecasts of the late 2009 infestation by sending out hundreds of liters of pesticides to rural farmers. The first infestation was reported on 22 December and quickly spread to surrounding regions. The previous growing season saw Tanzania produce 10.872 million tons of grain; after 10.337 million tons of domestic consumption the remaining 0.534 million tons were exported. As of 31 December, almost 1,400 acres (5.7 km2) of grain had been destroyed by armyworms.
- Common armyworm or true armyworm (Mythimna unipuncta) (North and South America)
- Fall armyworm (Spodoptera frugiperda) (North and South America)
- Northern armyworm, Oriental armyworm or Rice ear-cutting caterpillar (Mythimna separata) (Asia)
- Odiyo, P. O. 1984. A guide to seasonal changes in the distribution of armyworm infestations in East Africa. Insect Sci. Applic. 5: 107–119.
- Yarrow, J. G. Otindo, B. L., Gatehouse, A. G., and Lubega, M. C. 1981. Dwarf variety of coconut, Cocos nucifera (Palmae), a host plant for the African armyworm, Spodoptera exempta (Walk.) (Lepidoptera, Noctuidae). Insect Science and its Application 1: 361–362.
- Haggis, M. J. 1984. Distribution, frequency of attack and seasonal incidence of the African armyworm, Spodoptera exempta (Walk.) (Lepidoptera: Noctuidae), with particular reference to Africa and south-western Arabia. Tropical Development Research Institute, London, Report No. L69, pp. 116.
- Haggis, M. J. 1986. Distribution of the African armyworm, Spodoptera exempta (Walker) (Lepidoptera: Noctuidae), and the frequency of larval outbreaks in Africa and Arabia. Bulletin of Entomological Research 76: 151–170.
- Odiyo, P.O. 1981. Development of the first outbreaks of the African armyworm Spodoptera exempta (Walk.), between Kenya and Tanzania during the 'off-season' months of July to December. Insect Science and its Application 1: 305–318.
- Brown, E. S. 1972. Armyworm control. Pest Articles and News Summaries.
- Dewhurst, C. F. 1985. The African armyworm (Spodoptera exempta) — the East African story outlined from 1962. Antenna 9: 12–18.
- Riley, J. R., Reynolds, D. R. and Farmery, M. J. 1983. Observations of the flight behaviour of the armyworm moth Spodoptera exempta, at an emergence site using radar and infra-red optical techniques. Ecological Entomology 8: 395–418.
- Jahn, G.C. 1995. Environmental assessment for armyworm control in Ethiopia. USAID, Washington, DC. 
- Rose, D. J. W., Dewhurst, C.F. and Page, W. W. 2000. The African Armyworm Handbook (2nd Edn). NRI, Chatham, UK, 304 pp ISBN 978-0859545235
- Judith Achieng for Inter Press Service News Agency. May 25, 1999 Army worm Invasion Poses Threat To Main Crop
- STAR radio Liberia: Caterpillar invasion: Government declares Zota District emergency zone
- "Liberia worms trigger emergency". BBC. 27 January 2009. Archived from the original on 28 January 2009. Retrieved 2009-01-27.
- AOL.com: Caterpillars Trigger Emergency in Liberia
- "Armyworm Scourge Spreads to Ten Regions". allAfrica.com. Archived from the original on 2 January 2010. Retrieved 2009-12-31.