Manipulating canopy structure in cassava intercropped with cowpea and its effects on cowpea insect population densities.
Pest populations on cowpea [Vigna unguiculata] intercropped with cassava were studied in relation to changes in the cassava crop canopy structure in Ibadan, Nigeria. The flower thrips (Megalurothrips sjostedti), legume pod borer, (Maruca vitrata) and pod sucking bug complex [Heteroptera] populations were investigated. Four-month-old cassava was pruned during the second crop season of 1987 and 1988 as follows: no pruning, two-thirds debranching, total defoliation or cut-back to 60 cm above ground. There were significant differences between the pest density on V. unguiculata in the pruned cassava treatments and that of the V. unguiculata monoculture. In 1987, seasonal means of 408, 387 and 298 flower thrips per 20 flowers were recorded in the cut-back, the two-thirds de-branched and the defoliated cassava, respectively, compared to 1080 flower thrips per 20 flowers in the V. unguiculata monoculture. The same trend was observed in 1988. The micro-environment created by the intercrop reduced the populations of flower thrips, but increased those of the pod borer. Seasonal means of 22.0, 17.5, 21.4 and 16.7 pod borers per 20 flowers were recorded in the intercrops, compared to 15.8 in the monocultures. Pod-sucking bugs followed the same trend as that of flower thrips. It is concluded that intercropping cassava with V. unguiculata can be manipulated to contribute to the management of V. unguiculata pests.