African armyworm

African armyworm
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Lepidoptera
Family:	Noctuidae
Genus:	Spodoptera
Species:	S. exempta
Binomial name
Spodoptera exempta (Walker, 1856)
Synonyms
Agrotis exempta Walker, 1856 Prodenia bipars Walker, 1857 Prodenia ingloria Walker, 1858 Laphygma exempta

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).

See also

• Army worm, a related and similarly destructive species

References

• Belda, J. and Guerrero, L. 1992. Evaluation of some insecticides against beet armyworm Spodoptera exigua in watermelon. Tests of Agrochemicals and Cultivars 13. Ann. appl. Biol. 120 (Supplement): 12–13.

• Blair, B. W., Rose, D. J. W., and Law, A. B. 1980. Synoptic weather associated with outbreaks of African armyworm Spodoptera exempta (Walker) (Lepidoptera, Noctuidae), in Zimbabwe in 1973 and 1976/77. Zimbabwe Journal of Agricultural Research 18: 95–110.

• Blaney, W. M., Simmonds, M. S. J., Evans, S. V., and Fellows, L. E. 1984. The role of the plant secondary compound 2,5-dihydroxymethyl-3,4-dihydroxypyrroline as a feeding inhibitor for insects. Entomol. Exp. Appl. 36: 209–216.

• Brown, E. S. 1972. Armyworm control. Pest Articles and News Summaries.

• Brown, E. S., Betts, E. and Rainey, R. C. 1969. Seasonal changes in distribution of the African armyworm, Spodoptera exempta (Wlk.) (Lep. Noctuidae) with special reference to Eastern Africa. Bull. ent. Res. 58: 671–684.

• Brown, E. S., Stower, W. J., Yeates, M. N. D. B. and Rainey, R. C. 1970. Armyworm control on rangeland. A further application of aerial drift spraying. E. African Agric. For. J. 35: 350–358.

• Brownbridge, M. 1991. Native Bacillus thuringiensis isolates for the management of Lepidopteran Cereal Pests. Insect Sci. Applic. 12 (1/2/3): 57–61.

• Chandler, L. D. 1993. Use of feeding stimulants to enhance insect growth regulator-induced mortality of fall armyworm (Lepidoptera: Noctuidae) larvae. Florida Entomologist 76 (2): 316–326.

• Commonwealth Institute of Entomology. 1972. Spodoptera exempta (Walk.). Distribution Maps of Insect Pests 53.

• Den Boer, M. H. 1978. Isozymes and migration in the African armyworm Spodoptera exempta (Lepidoptera, Noctuidae). Journal of Zoology, London, 185: 539–553.

• Dewhurst, C. F. 1985. The African armyworm (Spodoptera exempta) — the East African story outlined from 1962. Antenna 9: 12–18.

• Fellows, L. E., Kite, G. C., Nash, R. J., Simmonds, M. S. J. and Scofield, A. M. 1989. Castanospermine, swainsonine and related polyhydroxy alkaloids: Structure, distribution and biological activity, pp. 395–427, in J.E. Poulton, J.T. Romeo, and E.E. Conn (eds.). Plant Nitrogen Metabolism. Plenum Publishing, New York.

• Gatehouse, A. G. and Hackett, D. S. 1980. A technique for studying flight behaviour of tethered Spodoptera exempta moths. Physiological Entomology 5: 215–222.

• Gunn, A. and Gatehouse, A. G. 1993. The migration syndrome in the African armyworm moth, Spodoptera exempta: allocation of resources to flight and reproduction. Physiological Entomology 18: 149–159.

• Gunn, A., Gatehouse, A. G.and Woodrow, K. P. 1989. Trade off between fight and reproduction in the African armyworm moth, Spodoptera exempta. Physiological Entomology 14: 419–427.

• 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.

• Hamm, J. J. and Shapiro, M. 1992. Infectivity of fall armyworm (Lepidoptera: Noctuidae) nuclear polyhedrosis virus enhanced by a fluorescent brightener. J. Econ. Entomol. 85 (6): 2149–2152.

• Hewitt, A. J. and Meganasa, T. 1993. Droplet distribution densities of a pyrethroid insecticide within grass and maize canopies for control of Spodoptera exempta larvae. International Pesticide Application Research Centre, Imperial College, Ascot, Berkshire, UK. Butterworth-Heinemann Ltd. pp. 59–62.

• Jahn, G.C. 1995. Environmental assessment for armyworm control in Ethiopia. USAID, Washington, DC. [1]

• Janssen, J. 1992. Why do droughts often result in devastating insect epidemics? The African armyworm, Spodoptera exempta, as an example. Proc. 8th Int. Symp. Insect-Plant Relationships, Dordrecht: Kluwer Acad. Publ., S.B.J. Menken, J.H. Visser & P. Harrewijin (eds.), pp. 49–51.

• Janssen, J. 1993. Soil nutrient availability in a primary outbreak area of the African armyworm, Spodoptera exempta (Lepidoptera: Noctuidae), in relation to drought intensity and outbreak development in Kenya. Bulletin of Entomological Research 83: 579–593.

• Jarret, P. and Burges, H. D. 1986. Isolates of Bacillus thuringiensis active against Mamestra brassicae and some other species: alternatives to the present commercial isolate HD 1. Biol. Agric. Hort. 4: 39–45.

• Khasimuddin, S. 1977. On the occurrence of an aestivating/diapausing phenomenon in the African armyworm, Spodoptera exempta (Walk.) (Lep., Noctuidae). East African Agriculture and Forestry Journal, 42: 350.

• Lecadet, M. M. and Martouret, D. 1987. Host specificity of the Bacillus thuringiensis delta endotoxin toward Lepidopteran species: Spodoptera littoralis Bdv and Pieris brassicae. J. Invertebr. Pathol. 49: 37–48.

• Marcovitch, S. 1957. Forecasting armyworm outbreaks — a possibility. J. Econ. Entomol. 50: 112–113.

• Morgan, Donald P. 1982. Recognition and Management of Pesticide Poisonings. Third Edition; United States Environmental Protection Agency publication no. EPA-540/9-80-005; 120 pages.

• Odiyo, P. O. 1979. Forecasting infestations of a migrant pest: the African armyworm, Spodoptera exempta (Walk.). -pp. 403–413 in Gunn, D.L. & Rainey, R.C. (Orgs.). Strategy and tactics of control of migrant pests. –Phil. Trans. R. Soc.(B) 287: 245–488.

• 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.

• Odiyo, P. O. 1984. A guide to seasonal changes in the distribution of armyworm infestations in East Africa. Insect Sci. Applic. 5: 107–119.

• Pedgley, D. E., Page, W. W., Mushi, A., Odiyo, P., Amisi, J., Dewhurst, C. F., Dunstan, W. R., Fishpool, L. D. C., Harvey, A. W., Megenasa, T. and Rose, D. J. W. 1989. Onset and spread of an African armyworm upsurge. Ecological Entomology 14: 311–333.

• Perfecto, I. 1991. Ants (Hymenoptera: Formicidae) as natural control agents of pests in irrigated maize in Nicaragua. J. Econ. Entomol. 84 (1): 65–70.

• Rainey, R. C. 1979. Control of the armyworm Spodoptera exempta in eastern Africa and southern Arabia: report of a mission to formulate an inter-regional project. Food and Agriculture Organization of the United Nations, Rome, FAO AGPP: MISC/32.

• 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.

• Rose , D. J. W. 1979. The significance of low-density populations of the African armyworm Spodoptera exempta (Walk.). Philosophical Transactions of the Royal Society of London, B, 287: 393–402.

• Rose, D. J. W. and Dewhurst, C. F. 1979. The African armyworm, Spodoptera exempta — congregation of moths in trees before flight. Ent. exp. & appl. 26: 346–348.

• Rose, D. J. W., Page, W. W., Dewhurst, C. F., Riley, J. R., Reynolds, D. R., Pedgley, D. E., and Tucker, M. R. 1985. Downwind migration of the African armyworm moth, Spodoptera exempta, studied by mark and capture and by radar. Ecological Entomology 10: 299–313.

• Rose, D. J. W., Dewhurst, C.F. and Page, W. W. 2000. The African Armyworm Handbook (2nd Edn). NRI, Chatham, UK, 304 pp.

• Schulthess, F., Bosque, N. A., and Gounou, S. 1991. Sampling lepidopterous pests on maize in West Africa. Bulletin of Entomological Research 81: 297–301.

• Simmonds, M. S. J., Blaney, W. M. and Fellows, L. E. 1990. Behavioral and electrophysiological study of antifeedant mechanisms associated with polyhydroxy alkaloids. Journal of Chemical Ecology 16 (11): 3167–3196.

• Simmonds, M. S. J., Fellows, L. E. and Blaney, W. M. 1989. Wild plants as a source of novel anti-insect compounds: Alkaloidal glycosidase inhibitors, pp. 365–377, in G. Wickens, N. Haq, and P. Day (eds.). New Crops for Food and Industry. Croom Helm, England.

• Tucker, M. R. 1983. Light trap catches of African armyworm moths, Spodoptera exempta (Walker) (Lepidoptera: Noctuidae). Entomophaga 26: 179–190.

• Tucker, M. R., Mwandoto, S. and Pedgley, D. E. 1982. Further evidence for windborne movement of armyworm moths Spodoptera exempta, in East Africa. Ecological Entomology 7: 463–473.

• U.S. Agency for International Development (USAID) 1995. Environmental assessment for armyworm control in Ethiopia. USAID, Washington, DC.

• USAID 1995. Environmental assessment for armyworm control in Eritrea. USAID, Washington, DC.

• Wheatley, P. E. and Crowe, T. J. 1967. Pest Handbook. The Recognition and Control of the More Important Pests of Agriculture in Kenya. Government Printer, Nairobi, Kenya, 35 pp.

• 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.