Long lasting persistence of Bacillus thuringiensis serovar. israelensis larvicidal activity in Aedes aegypti (Diptera: Culicidae) breeding places is associated to bacteria recycling

The performance of two experimental formulations of Bti, tablet and technical powder, was evaluated against Aedes aegypti larvae. The influence of larval density, and presence of active or inactivated spores upon Bti persistence and bacterial recycling was analyzed. The microbiological viability and toxicity of technical powder and irradiated-technical powder was analyzed under laboratory bioassays as well as under simulated field conditions (SFC). The SFC experiments were performed in plastic containers filled with 50 L of tap water, placed under shade and colonized with 50 L1 weekly or monthly. The initial efficacy of the product was estimated through mortality of L4 within 48 h after treatment. The persistence was defined as the period in which larval mortality was 80%, estimated by the number of live pupae. Water samples were collected from the containers throughout the experiments to verify spores concentration. Results showed that in SFC the tablet, technical powder and irradiated-technical powder eliminated 90-100% of larvae in 48 h, and this efficiency persisted for 6 months. There were no differences in the residual activity between technical powder or irradiated-technical powder, nor among containers colonized with different amount of larvae. Significant differences in bacteria concentration were observed along time between containers treated with technical powder or irradiated-technical powder, however, Bti concentration after a single treatment warranted high larvicidal activity for 6 months, under all experimental conditions, indicating the occurrence of bacterial recycling. (C) 2009 Elsevier Inc. All rights reserved.