Occurrence of phytophagous insects on wild Vigna sp. and cultivated cowpea: comparing the relative importance of host-plant resistance and millet intercropping.
The effects of host-plant resistance on cowpea phytophagous insects and their natural enemies under pure and mixed crop conditions were evaluated at Minjibir, Kano State, Nigeria, in the 1992-94 crop seasons. Cowpeas (Vigna unguiculata) cv. IT86D-715 (susceptible to insect pests) and a wild Vigna line V. vexillata TVnu 72 (resistant to most insect species) were planted alone and in mixtures with millet (Pennisetum glaucum) in plots of 25×25 m. Mixed cropping had limited effect on major insects and natural enemies. Colonies of Aphis craccivora were significantly smaller and there were more adults of Maruca vitrata in crop mixtures than in monocultures. However, flower and raceme infestation by larval M. vitrata, Megalurothrips sjostedti, and Sericothrips sp. were similar in crop mixtures and monocultures. Empoasca sp. populations and seedling infestation by Ophiomyia phaseoli were also similar in mixtures and monocultures as well as pod damage by M. vitrata and populations of Clavigralla tomentosicollis. Parasitization rates of M. vitrata, C. tomentosicollis and O. phaseoli and predator-prey ratios of spiders and Orius sp. were similar across cropping systems. Host-plant resistance in TVnu 72 drastically reduced insect populations and damage. Grain yield per hill was high in IT86D-715 and was not affected by intercropping with millet. Seed yield of TVnu 72 was poor and reflected the low yield potential of this accession. It is concluded that host-plant resistance is an effective means of controlling insect pest damage in cowpea and there is no evidence that high levels of resistance reduced natural biological control.