Study of Deltamethrin Persistence in Soil Microcosm, By Simulating Arid, Semi-Arid and Soudano-Sahelian Malarial Zones

According to pyrethroids high toxicity for aquatic animals and non-target insects, their extensive use in sub-Saharan Africa to control malaria is of concern. Deltamethrin biodegradation investigation have shown that this pyrethroid is more degraded in soudano-sahelian and semi-arid zones with annual rainfall extending from 800 - 300 mm compared to arid zone (0 mm). In local sandy and clay soils amended with 40 mg insecticide kg-1 dry weight soil, the observed biodegradation rates after three months varied respectively from 59 to 84% and 53 to 78% according to rainfall. The microbial activity is relatively inhibited in clay soil where the insecticide is firmly adsorbed to organic matters and sediments. The observed microbial activity depends also to the soil insecticide rate. Indeed, in Sahel sandy soil zone, deltamethrin was degraded after three months at 59, 73 and > 80% for respectively 40, 20 and 10 mg/kg amended soil. At high concentration, deltamethrin inhibits the microbial activity but a preliminary one week soil incubation before insecticide application increased the biodegradation rate. It reached 98 and 92% respectively in amended 40 mg/kg sandy and clay soils. Hence, deltamethrin treatment during the rainy season will present less environmental risks.