Abstract:
The principal goal of this work was to identify specific conditions of river water in West Africa, that influence the efficacy of the microbial insecticide, Bacillus thuringiensis H-14 formulation (VectoBac®) against Simulium damnosum sensu lato larvae for onchocerciasis control. One hundred and thirty assays of a standard formulation using the mini-gutter system were carried out over a period of 15 months at a field station situated near Pra River in southern Ghana. To identify the river conditions influencing the performance of the VectoBac®, both univariate and multivariate statistical procedures were applied to the dataset of lethal concentrations (LC) values, river temperature, conductivity, turbidity and pH measured over the study period. River temperature, conductivity and turb.dity (in that order) were the factors identified as having direct effect on the efficacy of VectoBac®. Water temperature and conductivity were negatively correlated, whilst turbidity and pH were positively correlated to LC values Analysis of representative water samples from the field revealed that, total solids, sodium and potassium cations and chloride concentrations differed significantly between wet and dry season and these were thought to be responsible for the observed variations in turbidity and conductivity of the River Pra. A simple method for rearing S. damnosum was adopted tc study the effect of conductivity on potency of VectoBac® in the laboratory This showed that increasing the conductivity of either water or the insecticide, up to 3,000|iS both improved potency by about a maximum of 42% and 36.7% respectively. The implications of these findings for the operational use of VectoBac® in blackfly control in West Africa, for the development of laboratory based assay system and improved formulations were discussed. Furthermore, the study provides the basis of determining the real potency of products tested under vagary conditions in the field.
