Multi-trophic level interactions in a cassava–maize mixed cropping system in the humid tropics of West Africa

Multi-trophic level interactions in a mixed crop, involving cassava and maize, were studied in derived-savanna in Benin, West Africa. Two trials were planted, one during the short rainy season two months before onset of the dry season and one during the long rainy season in spring. Key pests under study on maize were the noctuidSesamia calamistisHampson and the pyralidsEldana saccharinaWalker andMussidia nigrivenellaRagonot, and on cassava, the exotic mealybug,Phenacoccus manihotiMatile-Ferrero and its encyrtid parasitoidApoanagyrus lopeziDe Santis. Both crops received insecticide treatments to assess the crop loss by a pest species. On maize, intercropping with cassava reduced egg and immature numbers ofS. calamistisby 67 and 83%, respectively, as a result of reduced host finding by the ovipositing adult moth and of higher egg parasitism byTelenomusspp. Both trials showed similar effects on maize yields: on insecticide-treated maize, intercropping with cassava reduced maize yields by 9–16%, while on untreated maize the net effect of reduced pest density and increased plant competition resulted in zero yield differences; yield losses were lower in inter- compared to monocropped maize. For cassava, cropping system had no effect on parasitism byA. lopezi. Yield differences between mono- and intercropped cassava depended on time of harvest: they were large at the beginning and zero at final harvest. Land equivalent ratios were mostly > 1.5 indicating that a maize/cassava mixed crop, protected or unprotected, considerably increased the productivity per unit area of land.