Formulations containing the entomopathogenic Bacillus thuringiensis serovar israelensis strain Bacillus thuringiensis is a Gram-positive, spore-forming bacterium which produces a parasporal body during sporulation. The proteins contained in the parasporal crystal (delta-endotoxins) are specific insect gut toxins which have no significant effect on non-target organisms (Siegel 2001). The mode of action after ingestion involves solubilization of the protein in the alkaline conditions of the insect midgut, followed by activation of the protoxin into toxin through cleavage of specific regions by gut proteases. Then, the so called toxin binds to specific receptors in the brush border membrane vesicles of the midgut columnar cells. The binding is a two-step process, the second part being the insertion of the toxin into the cell membrane leading to pore formation, osmotic imbalance, disruption of the cell membrane and finally, death of the insect (Van Rie et al. 1990, Knowles 1994.The use of bioinsecticides based on B. thuringiensis to control insects from the orders Diptera, Coleoptera, and Lepidoptera and more recently against Hymenoptera, Orthoptera, and Mallophaga, has stimulated the search for other natural pathogens (Schenpf et al. 1998 Between 1995 to 1998, an isolation program was set up to obtain entomopathogenic Bacillus from Simulium larvae and adults in the Southeast region of Brazil. Eighteen strains of B. thuringiensis and one of B. sphaericus were isolated (Cavados et al. 2001). Preliminary bioassays were performed and one of the B. thuringiensis strains (LFB-FIOCRUZ 1035) showed promising activity against Aedes aegypti and Culex quinquefasciatus (Cavados et al. 2001).In the present study, the toxicity of the autoagglutinating strain LFB-FIOCRUZ 1035 was evaluated against two Dipteran species, S. pertinax and Ae. aegypti larvae, and compared with a standard powder formulation of the reference B. thuringiensis serovar israelensis strain IPS-82, which is used in commercially available bioinsecticides.
MATERIALS AND METHODSBacterial strains -Two strains of B. thuringiensis were used, one of them IPS-82 (LFB-FIOCRUZ 584) serotype H-14, isolated from the standard powder prepared by the Pasteur Institute of Paris, and the other LFB-FIOCRUZ 1035, an autoagglutinating strain isolated from S. pertinax larvae collected in a natural breeding-site in Growth started with a pre-inoculum to reduce the duration of the lag phase of bacterial growth. After inoculation in 125 ml Erlenmeyer flasks containing 50 ml of the medium, the flasks were incubated in a New Brunswick Scientific agitator series 25D, at 175 opm and 30°C for 6 h. Subsequently, 3 ml were transferred to 500 ml Erlenmeyer flasks containing 150 ml of the soy flour and inorganic salts medium and incubated as previously described for a further 72 h period.Once sporulation had reached 95% of free spores and crystals, each culture was centrifuged (6000 g, 10°C), the supernatant removed and the remaining material was kept in an amber container with the pH adjus...