Background

Cowpea, Vigna unguiculata (L.) Walp., is a highly important grain legume crop grown in semi-arid and dry savannah agro-ecological zones of the tropics.  Cowpea grains contain around 22% protein and provide a cheap source of dietary protein for low-income urban and rural populations. Within West Africa, cowpea is grown mostly in subsistence farming systems and on small scale in the lowland dry Savanna and Sahelian regions. Traditionally, it is inter-cropped with sorghum, millet or maize but in recent years some regions there has been a move towards mono-cropping as the crop’s economic importance increases. Cowpea can also form a vital cattle forage crop in subsistence cereal-based farming systems.  Africa produces 75% of world cowpea production of which the majority comes from West Africa.

Maruca vitrata Fabricius (syn. M. testulalis) (Lepidoptera: Pyralidae), the legume podborer, is a key pest of cowpea as well as other legume crops in Africa and south Asia. The larvae attack flower buds, flowers and young pods and for cowpea yield losses due to M. vitrata have been reported in the range 20-80% (Singh et al., 1990). In West Africa M. vitrata forms one of a complex of damaging insect pests of cowpea, which can also include aphids Aphis craccivora, foliage beetles Ootheca mutabilis, several species of pod bugs and legume bud thrips, Megalurothrips sjostedti.

Conventional insecticides can control cowpea insect pests effectively and raise yields several-fold (Afun et al., 1991; Asante et al., 2001) but where no control is attempted yields are correspondingly low.  However, expense limits insecticide use by many poor farmers.  Use may be higher in areas in which cotton is grown; for example in Benin farmers may often use cotton insecticides (which are sold at subsidised prices), which are not recommended for cowpea. Resulting from this, health and environmental hazards are increasing. Resistance in M. vitrata to three classes of insecticides has also been reported from Nigeria (Ekesi, 1999a).

Recent research efforts have turned to a number of alternatives to conventional pesticides for pest control, particularly of M. vitrata, in cowpea. These have included attempts to develop resistant cowpea varieties and classical biological control agents, discovery of a viral pathogen against M. vitrata and investigation of the fungal entomopathogen Metarhizium anisopliae.  While some of these technologies have show promise none are likely to be ready for implementation against M. vitrata at farm level for some time. 

Attention has also focussed on botanical pesticides such as neem Azadirachta indica, A. Juss. (Meliaceae) and papaya Carica papaya L. (Caricaceae) extracts, which already form indigenous methods of control for some farmers.  During the later stages of our work with pheromone traps  we combined their use with botanical pesticides where possible.

Afun et al. (1991) demonstrated the effective use of action thresholds, based on cowpea flower infestation rates, to achieve control of M. vitrata with reduced conventional insecticide sprays. Potentially, catches in pheromone-baited traps for M. vitrata could be used by cowpea farmers in the same way, thus minimising insecticide inputs whilst maximising control. Such an approach has been developed for pests of other tropical crops such as rice (Kojima et al., 1996) and cotton (Reddy & Manjunatha, 2000).  This motivation provided the principal rationale for this project.

In addition to this there is a lack of information on the behaviour and activity of this pest in the field, particularly in relation to migration patterns and off-season occurrence.  The use of pheromone traps for monitoring the activity and movements of adult M. vitrata could assist development of strategies such as manipulation of planting dates to reduce M. vitrata damage (Ekesi et al., 1996).