Lablab (Lablab purpureus L.) is an important food and livestock feed legume that can also enhance soil fertility. However, its production is limited by insect pests, notably the black bean aphid (Aphis fabae). The present field study was conducted to determine the difference in the contribution of lablab genotypes and natural field margin vegetation (FMV) to the abundance and diversity of natural enemies and the damage, incidence, and abundance of bean aphids. Eighteen lablab genotypes were planted in the presence or absence of FMV in a randomized complete block design experiment replicated four times. Data on aphid abundance, incidence, and severity of damage were collected at four growth stages of the crop. Lablab genotypes significantly influenced aphid incidence, suggesting some level of tolerance to aphid colonization. Findings showed that lablab genotypes were a significant influence on natural enemy species richness with no statistical difference for abundance and natural enemy species diversity. However, the genotypes did not vary significantly in their influence on the number of aphid natural enemies. FMV was associated with low bean aphid damage. Overall, the presence or absence of FMV did not influence the number of natural enemies caught on the crop. This concurs with recent work that shows a similar number of natural enemies with field margin plants but may reflect the reduced number of pest insects. Cropping seasons influenced aphid abundance and damage severity, with the populations developing at the early stages of lablab development and decreasing as the crop advanced. This pattern was similar both in the presence or absence of FMV. The findings of this study highlight the important contribution of crop genotype together with the presence of field margin species in the regulation of aphids and their natural enemies in lablab.

Millet/cowpea intercropping is a promoted practice in Sub-Saharan agriculture. However, because cowpea is known as a host for plant-parasitic nematodes that may also infect millet, we examined whether intercropping may increase the risk of nematode-mediated millet damage, and if this risk may be controlled by organic amendments. In twelve Senegalese farmers' fields which had been either non-manured or regularly manured over the past 10 years at least, we assessed the effects of intercropping millet and cowpea on the abundance of free-living and plant-parasitic nematodes, ecological indices, and land equivalent ratios (LER). Six fields were manured, and six non-manured. Each field included four plots: millet and cowpea as pure stands, and two plots with millet intercropped with cowpea sown at two densities. Soil nematofauna was evaluated before sowing and at cowpea flowering. Soil nematofauna was dominated by plant-parasitic nematodes. Before manure application and crop sowing, regularly manured fields had higher structure indices of nematofauna than non-manured ones, and Pratylenchus was almost absent. At crop flowering, abundance of Pratylenchus increased and was drastically higher in pure cowpea (149 individuals 100 g(-1) dry soil) than in intercropping and pure millet (18 and 17 individuals 100 g(-1) dry soil, respectively), regardless of the manuring regime. Manuring had significant positive effects on various bacterivorous families, on fungivorous and plant-parasitic trophic groups, but not on Pratylenchus. Millet and cowpea yields of manured fields were the highest, regardless of cropping pattern. LER averaged 1.7 and 1.5 in manured and non-manured intercropping, respectively. Regardless of the treatments, ecological indices indicated that the soil food web was undisturbed, with moderate enrichment, and suppressive against crop pests. Intercropping millet with cowpea, even in poorly manured fields and with high cowpea density, constitutes an appropriate way to improve productivity without increasing Pratylenchus pressure in nutrient-poor soils of central Senegal.