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Only one #Bacillus sphaericus$ strain, strain 2362, is currently used commercially to control #Culex$ larval populations. A reliable methodology, easily used, was developed to identify new strains for field application. Larvicidal activities of 3 highly mosquitocidal strains, strains C3-41, Mal, and LB24, previously selected in the laboratory, were compared 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 conditions, and evaluation of the efficacity of liquid formulations of the 4 strains in natural breeding sites of #Culex$. In indoor conditions, strain C3-41 showed the highest activity on both #Culex pipiens$ and #Culex quinquefasciatus$; strain Mal was the least active. The residual activity causing 80% mortality differed from 20 to 90 days according to the strains and the country. Outdoor experiments with powder (0,02-1,6 mg/liter) were performed and the initial toxicities were similar in all cases. Residual activities were different, from 6 to 95 days posttreatment. Liquid formulations were applied to larval habitats (from 0,1 to 10 g/m2). In tropical countries, larval recolonization in cesspits or ponds occured after 10-35 days. In Europe, higher doses were needed in polluted water than in clear water (from 3 to 10 liter/ha) for the 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 strains in field trials. (Résumé d'auteur) |
