African armyworm
Encyclopedia
The African armyworm 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 grass
es, 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 egg
s 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 locust
s. 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 pupa
te. 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:
For additional information see Jahn (1995).
The species also occurs in Yemen
, some Pacific islands, and parts of Australia
(Rose et al. 2000).
Moth
A moth is an insect closely related to the butterfly, both being of the order Lepidoptera. Moths form the majority of this order; there are thought to be 150,000 to 250,000 different species of moth , with thousands of species yet to be described...
. It is a very deleterious pest, capable of destroying entire crops in a matter of weeks. The larvae feed on all types of grass
Grass
Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...
es, early stages of cereal crops (e.g., corn
Maize
Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...
, rice
Rice
Rice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...
, wheat
Wheat
Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...
, millet
Millet
The millets are a group of small-seeded species of cereal crops or grains, widely grown around the world for food and fodder. They do not form a taxonomic group, but rather a functional or agronomic one. Their essential similarities are that they are small-seeded grasses grown in difficult...
, sorghum
Sorghum
Sorghum is a genus of numerous species of grasses, one of which is raised for grain and many of which are used as fodder plants either cultivated or as part of pasture. The plants are cultivated in warmer climates worldwide. Species are native to tropical and subtropical regions of all continents...
), sugar cane, and occasionally on coconut
Coconut
The coconut palm, Cocos nucifera, is a member of the family Arecaceae . It is the only accepted species in the genus Cocos. The term coconut can refer to the entire coconut palm, the seed, or the fruit, which is not a botanical nut. The spelling cocoanut is an old-fashioned form of the word...
(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 egg
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
s 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)
Caterpillar
Caterpillars are the larval form of members of the order Lepidoptera . They are mostly herbivorous in food habit, although some species are insectivorous. Caterpillars are voracious feeders and many of them are considered to be pests in agriculture...
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 locust
Locust
Locusts are the swarming phase of short-horned grasshoppers of the family Acrididae. These are species that can breed rapidly under suitable conditions and subsequently become gregarious and migratory...
s. 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 pupa
Pupa
A pupa is the life stage of some insects undergoing transformation. The pupal stage is found only in holometabolous insects, those that undergo a complete metamorphosis, going through four life stages; embryo, larva, pupa and imago...
te. 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
Oviposition
Oviposition is the process of laying eggs by oviparous animals.Some arthropods, for example, lay their eggs with an organ called the ovipositor.Fish , amphibians, reptiles, birds and monetremata also lay eggs....
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:
- outbreaks in a geographical progression at one generation intervals (Rainey 1979);
- outbreaks are often preceded by high catches of moths and associated with wind convergence (Blair et al. 1980);
- results of isozymeIsozymeIsozymes are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. These enzymes usually display different kinetic parameters Isozymes (also known as isoenzymes) are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. These enzymes...
analysis indicate that AAW populations in KenyaKenyaKenya , officially known as the Republic of Kenya, is a country in East Africa that lies on the equator, with the Indian Ocean to its south-east...
, TanzaniaTanzaniaThe United Republic of Tanzania is a country in East Africa bordered by Kenya and Uganda to the north, Rwanda, Burundi, and the Democratic Republic of the Congo to the west, and Zambia, Malawi, and Mozambique to the south. The country's eastern borders lie on the Indian Ocean.Tanzania is a state...
, and ZimbabweZimbabweZimbabwe is a landlocked country located in the southern part of the African continent, between the Zambezi and Limpopo rivers. It is bordered by South Africa to the south, Botswana to the southwest, Zambia and a tip of Namibia to the northwest and Mozambique to the east. Zimbabwe has three...
regularly interbreed (Den Boer 1978); - laboratory tests show that the moths can remain in flight for several hours (Gatehouse and Hackett 1980);
- 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
Yemen
The Republic of Yemen , commonly known as Yemen , is a country located in the Middle East, occupying the southwestern to southern end of the Arabian Peninsula. It is bordered by Saudi Arabia to the north, the Red Sea to the west, and Oman to the east....
, some Pacific islands, and parts of Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
(Rose et al. 2000).