The African armyworm (Spodoptera exempta) also called okalombo, 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. African armyworms 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, population densities are very low. Because outbreaks are never observed during the dry season, it is called the "off-season" by those who monitor African armyworms.
S. 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 African armyworms do not exhibit the true gregarious behavior of locusts. The "gregarious" forms of AAW cause outbreaks. Generally, African armyworms 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 African armyworm 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 African armyworm 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 nor Ugandan officials had sufficient supplies to combat the insect and no central government was present 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 African armyworms, 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 had 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 had 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)
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