INTERNATIONAL INDOOR AND OUTDOOR EVALUATION OF BACILLUS-SPHAERICUS PRODUCTS - COMPLEXITY OF STANDARDIZING OUTDOOR PROTOCOLS

Only one Bacillus sphaericus strain, strain 2362, is currently used co mmercially to control Culex larval populations. A reliable methodology , easily used, was developed to identify new strains for field applica tion. Larvicidal activities of 3 highly mosquitocidal strains, strains C3-41, Mal, and LB24, previously selected in the laboratory, were com pared with that of strain 2362 in tropical and European countries. The following steps were performed: production and titration of acetonic powders from these 4 strains on local Culex species, survey of initial and residual activity under standardized indoor and outdoor condition s, and evaluation of the efficacy of liquid formulations of the 4 stra ins in natural breeding sites of Culex. In indoor conditions, strain C 3-41 showed the highest activity on both Culex pipiens and Culex quinq uefasciatus; strain Mal was the least active. The residual activity ca using 80% mortality differed from 20 to 90 days according to the strai ns and the country. Outdoor experiments with powders (0.02-1.6 mg/lite r) were performed and the initial toxicities were similar in all cases . Residual activities were very different, from 6 to 95 days posttreat ment. Liquid formulations were applied to larval habitats (from 0.1 to 10 g/m(2)). In tropical countries, larval recolonization in cesspits or ponds occurred after 10-35 days. In Europe, higher doses were neede d in polluted water than in clear water (from 3 to 10 liter/ha) for th e same control, and the time before 80% residual activity was reached was less than 9-12 days. However, in cesspits, residual activity could be observed for 12 days to 5 mo. A strain 3-5 times more active than the others in bioassays is not significantly detectable from those str ains in field trials.