Effect of Weed Removal on Insect Populations and Yield of Cowpea [Vigna Unguiculata (L) Walp] (Report‪)‬

Introduction In West Africa and many parts of the world cowpea [Vigna unguiculata (L) Walp] is an important grain legume. Total worldwide production of cowpea is estimated at 3.3 million tons (FAO, 2001) of dry grain of which 64% is produced in Africa. Conservative estimates suggest that 12.5 million ha are planted annually to cowpea around the world. Of this area, about 9.8 million ha are planted in West Africa, making it the region with the largest production and consumption of cowpea in the world (CGIAR, 2001). Cowpea production is regarded as an integral part of traditional cropping system throughout Africa (Isubikalu et al., 2000). The crop is very popular and unique in that it produces food for man and fodder for livestock. It has high potential to increase income of both farmers and traders (Owolade et al., 2004). However, the crop is attacked by spectrum of pest species (Isubikalu et al., 2000). It is considered too risky an investment by many growers, because of the numerous pest problems associated with it (Remison, 1997). Farmers obtained low average yield due to these field pests. For instance, average world yield of cowpea grain is quite low at less than 0.3 ton/ha. Within Africa, average cowpea yields vary dramatically from 0.05 to 0.55 ton/ha (Cisse et al., 1995). Alghali (1992) reported 200-300 kg ha-1 in Nigeria, 150-300 Kg [ha.sup.-1] in Uganda and more than 2000 Kg [ha.sup.-1] under research environment (Rusoke & Rubaihayo, 1994). These major pests of cowpea in the humid tropics are weeds (Ayeni, 1992) and insects (Jackai & Adalla, 1997). These pests, especially insect pests, damage cowpea from seedling emergence to storage (Karungi et al., 2000). Weeds constitute a major limiting factor to cowpea production in Nigeria (Okafor & Adegbite, 1991). Tijani-Eniola (2001) reported that weed could cause yield losses ranging from 50 to 80 %. Crop losses by weeds could be aggravated by delay in weeding or inability to weed throughout the entire crop growth period. However, studies of threshold levels of weeds have shown that complete weed elimination is not essential for high yields (Sangakkara, 1999), probably because the crop also competed strongly with weeds. In addition, to their repressive effects owing to competition, weeds also act as reservoirs or alternate hosts for insects, diseases and nematodes (Jackai & Adalla, 1997). Weeds and insects often coexist and reduce yields in agricultural systems. Weeds reduce yields by an estimated 12% annually, whereas insects account for a 13% annual reduction in yields in United States agricultural systems (Pimentel 1991). In addition to the individual effects that insects and weeds have on crops, these two types of pests and their management practices can interact and impact crop production. Weeds reduce crop yields and quality by competing for nutrients and water. They also may decrease the value and productivity of land, reduce harvesting and processing efficiency, increase cost and labor for control measures, and restrict flow of water to reservoirs, canals, and ditches (Smith and Hill 1990). Losses from insects include defoliation of root or leaf tissue, removal of fluid from phloem and xylem systems, mining of parenchyma tissue, formation of galls, or blemishing the harvested fruit or vegetable (Schoonhoven et al., 1998). Additional problems associated with insects are transmission of plant diseases, costs involved with insect management, and development of resistance to control measures (Paoletti and Pimentel 2000).