Evaluation of a new tablet formulation based on Bacillus thuringiensis sorovar. israelensis for larvicidal control of Aedes aegypti

Abstract

The effect of sunlight on the efficacy and persistence of an experimental tablet formulation based on Bacillus thuringiensis sorovar. israelensis (C4P1) was evaluated against Aedes aegypti larvae under simulated field conditions. The initial mortality ranged from 93 to 100%, and the residual activity (≥ 70% mortality) recorded in containers exposed to sunlight or shade were, respectively, 13-35 days and 40-54 days. The results suggest that C4P1 can provide long-term larvicidal effect and operational advantages. Key words: Bacillus thuringiensis israelensis -Aedes aegypti -tablet formulationBacillus thuringiensis sorovar. israelensis (B.t.i) has been used since the 80's in several programs for controlling black fly and mosquito species worldwide. It is considered highly toxic to the target organisms and environmentally safe. Furthermore, no resistance has been detected on populations submitted to long-term exposure to this pathogen (Regis & Nielsen-LeRoux 2000). B.t.i formulations have been continuously improved to optimise their potency, stability (shelf life), ease of field application and residual activity (Couch 2000). Trials under simulated field conditions have been considered a reliable methodology to evaluate the efficacy and persistence of B.t.i based products because, under these conditions, many factors involved in residual activity can be better controlled and investigated than under real field conditions (Thiéry et al. 1999.In the present work, the effect of sunlight on the efficacy and residual activity of a new tablet formulation based on B.t.i, C4P1 against Aedes aegypti larvae (Recife-Lab strain), was investigated under simulated field conditions. C4P1 is an experimental slow release solid formulation, developed by Far-Manguinhos-Fiocruz, Rio de Janeiro, Brazil. Initially, the larvicidal activity of the technical powder was determined by bioassay according to the standard protocol (de Barjac & LargetThiéry 1984). The LC 50 and the LC 90 were titrated against the B.t.i reference powder (IPS82), from Institut Pasteur. The technical powder showed LC 50 = 0.30 ± 0.01 mg/l and LC 90 = 0.44 ± 0.01 mg/l, resulting in potency of 1,146 ± 52 ITU/mg (International Toxic Units).The tests under simulated conditions were carried out in March (1st set) and May (2nd set) 2000 to evaluate the residual activity of C4P1 related to the incidence of sunlight. The containers, placed in an outdoor area, were submitted to direct sunlight and shade. The tests were conducted in plastic containers (38 x 42 x 30 cm) filled with 20 l of tap water where the temperature ranged from 27 to 29°C in the shade and from 27 to 32°C, in direct sunlight. Each container was treated with 1 tablet/ 10 l. Fifty early fourth-instar larvae (L4) were introduced into each container and the mortality rate was recorded 48 h after, by counting the surviving larvae. This procedure was always used to determine the initial mortality. Thereafter, the evaluation of residual activity was carried out using 50 L4 or 300 first instar (L1) larvae per conta...

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