Interactions of fertilizer, cultivar selection, and infestation by cowpea aphid (Aphididae) on growth and yield of cowpeas.
Field and greenhouse studies were conducted in western Kenya in 1990-91 to assess the effects of fertilizer application on the bionomics and population dynamics of Aphis craccivora and on the growth and yield of cowpea (Vigna unguiculata). Five fertilizer regimes were applied to seedlings of aphid-resistant (cv. ICV-12) and aphid-susceptible (cv. ICV-1) cowpeas. Treatments consisted of: NH4NO3 at 40 kg/ha, P2O5 at 20 and 60 kg/ha, NH4NO3 at 40 kg/ha plus P2O5 at 60 kg/ha, and no fertilizer. Test plants were initially infested with five adult apterae or were left uninfested. Treatments were maintained for 22 days. Response variables recorded included extended leaf heights (cm), aphid counts and crop growth rate (g/dm2 land area/day). At harvest, pods per plant and seeds per pod were also recorded. ANOVA indicated significant differences in crop growth rate between infested and uninfested ICV-1 plants, and also between infested plants of ICV-1 and ICV-12, except for the 20 and 60 kg/ha P treatments. Also, apart from the 60 kg/ha P, ICV-1 yields were significantly reduced and aphid counts were higher for all fertilizer regimes. Thus, aphid damage in cowpeas was mitigated by the deployment of the resistant cultivar (ICV-12). In the case of the susceptible cultivar (ICV-1), application of 20-60 kg/ha P also abated the adverse effects of A. craccivora infestations. Application of 40 kg/ha N did not diminish the deleterious effects of aphids on ICV-1 plants, but in some cases, actually aggravated plant growth deformities and yield reductions.