Multi-trophic interactions in cassava, Manihot esculenta, cropping systems in the subhumid tropics of West Africa

Abstract

The effect of fertilizer and mulch across trophic levels of the cassava ecosystem, i.e., the plant, the cassava mealybug (CM), the encyrtid wasp Apoanagyrus lopezi and its hyperparasitoids, was studied in two field experiments planted in southern Benin. Several cassava cultivars with different branching patterns and varying susceptibility to the cassava mealybug were planted on a sandy loamy and a sandy soil, respectively. Four to seven months after application of the soil treatments, differences in N, P, K in the first 50 cm of the soil had mostly disappeared, indicating that the soil nutrients had been taken up by the plant and/or had leached to deeper layers of the soil. The effect on leaf nutrient contents was still visible, however. Differences in response to soil nutrients and dry matter allocation pattern were found between late and early branching cassava. The former was more efficient in utilizing soil nutrients for root production whereas in the latter, harvest indices decreased with increasing soil fertility. With increasing soil fertility, the nutritional status of the plants increased. This led to larger mealybugs, which in turn resulted in a higher proportion of female Apoanagyrus lopezi. The overall effect was an enhancement of biological control activity leading to low CM population densities in plots with high soil fertility. Hyperparasitism had no effect on parasitism of A. lopezi, and was positively related to both CM and A. lopezi numbers. Generally, the infestations were too low to have an effect on growth of cassava leaves or roots. This study shows the beneficial effect that soil fertility can have on the efficiency of a biological control agent.