Control of the cowpea aphid, Aphis craccivora Koch (Homoptera: Aphididae), in cowpea, Vigna unguiculata (L.) Walp. THOMAS I. OFUYA Department of Crop Production, The Federal Univeristy of Technology, PMB 704, Akure, Nigeria Cowpea, Vigna unguiculata (L.) Walpers, is a major dietary staple in tropical Africa where it is most often cultivated. The production is, however, greatly hampered by severe infestation and damage by insect pests including the cowpea aphid, Aphis craccivora Koch. The damage and bionomics of the aphid are brie¯y introduced. Chemical, cultural, physical and biological methods, and use of plant resistance for A. craccivora control in cowpea are reviewed. Finally, prospects for integrated pest management are discussed. Keywords: Aphis craccivora; cowpea; control methods. Introduction Cowpea, Vigna unguiculata (L.) Walpers is cultivated mostly in tropical Africa and the edible seeds constitute a major source of protein in the human diet. The crop is grown extensively in about 15 African countries, of which Nigeria and Niger together produce half the world's output (Pandey and Westphal, 1989). Africa's total annual produc- tion of cowpea for 1990 was estimated at 3.36 million metric tons, of which 1.5 million metric tons were produced in Nigeria and 368,000 metric tons in Niger (Nwokolo and Ilechukwu, 1996). The crop is also cultivated in Asia, Latin America and parts of the USA. Despite the extensive production and importance of cowpea in Africa, its yield per hectare is relatively low. It is estimated that average yield for the crop grown in monoculture is about 1500 kg ha À1 in the United States, 650 kg ha À1 in South America and Asia, but often below 400 kg ha À1 in Nigeria and other African countries (Singh, 1980; Pandey and Westphal, 1989). A major biological constraint to the production of cowpea in Africa is severe infestation and damage by various insect pests in the ®eld and during storage (Jackai and Daoust, 1986). In unprotected monocrops, yield losses due to the major ®eld pests may be 20±100% (Youdeowei, 1989). The cowpea or groundnut aphid, Aphis craccivora Koch (syn. A. leguminosae Th., A. laburni Kalt), is considered to be an important ®eld pest of cowpea in Africa, Asia and Latin America (Singh and Jackai, 1985). Other aphid species including A. fabae Scopoli, A. gossypii Glover, Myzus persicae (Sulzer) and Picturaphis brasiliensis Moreira may also infest cowpea, but are of no economic importance (Singh et al., 1990). Legumes, including a wide range of cultivated species, are the main hosts of A. craccivora, but the insect is not restricted to this family (Kranz et al., 1977; Berg, 1984). Host preference studies show that cowpea promotes highest growth and reproduc- tion of the insect (Hamid et al., 1977; Highland and Roberts, 1984; Ofuya, 1988a; Srikanth and Lakkundi, 1988) and indicate the existence of biotypes in the species. As many as ®ve biotypes of A. craccivora have been recognized (Saxena and Barrion, 1987). In West Africa, during the last decade, aphid populations have continously increased, and major losses to A. craccivora in cowpea have been observed (Singh et al., 1990). The bionomics of A. craccivora is fairly well known (Waghray and Singh, 1965; Dorge et al., 1966; Bernado, 1969; Radke et al., 1972; Ansari, 1984; Messina et al., 1985). Hill (1983) and Wightman et al. (1990) provided a general description of the species. Adults are mostly black or dark brown, variable in size, being from 1.5 to 2 mm long, siphunculi and cauda black; antennae are about two- thirds as long as the body. Nymphs are wingless, dark or dusty brown and fairly rounded in body shape. A common pattern in the tropics is for A. craccivora to spend the dry or winter season on wild hosts and weedy species such as Medicago spp., Melilotus spp., Trifolium spp., Euphorbia spp., Boerhaavia spp., as well as on volunteer growth of legume crops (A'Brook, 1964; Hamid et al., 1977; Kranz et al., 1977; Kumar, 1984; Ofuya, 1988a, 1989a). The aphid (alatae) disperses soon after the rains start, and in Nigeria has been observed to have already colonized the cowpea crop as early as 3 weeks after germination (Ofuya, 1991a). Farrell (1976a), working in Malawi, found that the main migratory ¯ights of A. craccivora are 5±6 weeks after the emergence of the earliest groundnut crop. In Hyderabad, India, the ®rst colonies are found on groundnut 3±4 weeks after crop emergence in mid-to-late July, 4±6 weeks after the start of the rainy season (Wightman et al., 1990). The alate immigrants reproduce parthenogenetically. The insect passes through four nymphal instars before reaching adulthood. Under conditions of abundant food and favourable climate, parthenogenetic apterous adult females are successively produced. The life cycle is typically anholocyclic (Dixon, 1985). Presumably, a genetic change in this species has resulted in the failure to produce sexual forms. Alate adults are produced Integrated Pest Management Reviews 2, 199±207 (1997) 1353±5226 # 1997 Chapman & Hall