INTERACTIONS OF FERTILIZER, CULTIVAR SELECTION, AND INFESTATION BY COWPEA APHID (APHIDIDAE) ON GROWTH AND YIELD OF COWPEAS

Field and greenhouse studies were conducted in 1990 and 1991, to asses s the effects of fertilizer application on the bionomics and populatio n dynamics of cowpea aphid, Alphis craccivora Koch, and on the growth and yield of cowpea, Vigna unguiculata (L.) Walp. Five fertilizer regi mes were applied to seedlings of aphid-resistant (cv. ICV-12) and aphi d-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 fiv e adult apterae or were left uninfested. Treatments were maintained fo r 22 days. Response variables recorded included extended leaf heights (cm), aphid counts and crop growth rate (g/dm(2) and area/day). At har vest, pods per plant and seeds per pod were also recorded. ANOVA indic ated 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 cou nts were higher for all fertilizer regimes. Thus, aphid damage in cowp eas was mitigated by the deployment of the resistant cultivar (ICV-12) . In the case of the susceptible cultivar (ICV-1), application of 20-6 0 kg/ha P also abated the adverse effects of A. craccivora infestation s. Application of 40 kg/ha N did not diminish the deleterious effects of aphids on ICV-I plants, but in some cases, actually aggravated plan t growth deformities and yield reductions.