Residual activity of Bacillus thuringiensis serovars medellin and jegathesan on Culex pipiens and Aedes aegypti larvae

Bacillus thuringiensis serovar medellin strain 163-131 and Bacillus thuring iensis serovar jegathesan (B.t.jeg.) strain 367 are very toxic to mosquito larvae. However, they are 10 times less toxic than Bacillus thuringiensis v ar. israelensis (B.t.i.) to mosquito larvae under laboratory conditions. Ly ophilized powders were produced from these strains and their toxicities wer e compared to that of powder produced from the B.t.i. strain. Larvicidal ac tivity was titrated using Aedes aegypti (Bora-Bora strain) larvae, with IPS 82 powder as the standard. The efficacy of these powders in the field was d etermined using Culex pipiens (Montpellier strain) in Paris, France, and Ae . aegypti larvae (French Guiana strain) in Cayenne, French Guiana, in stand ardized conditions. Residual activity was also assessed in the laboratory, using Cx, pipiens (SLAB strain), in Montpellier, France. Any negative effec t of direct sunlight, soil, or polluted water on the residual activity of t he 3 powders was recorded. Increasing bacterial concentration by a factor o f 8 had little effect on the duration of larvicidal activity, except in the presence of polluted water and when substrate was added. All powders had s imilar initial efficacies against both types of mosquito larvae, in all con ditions except water rich in organic matter. Bacillus thuringiensis serovar medellin had the lowest residual activity, both in the laboratory and in t he field, whereas B.t.jeg. remained toxic for as long as B.t.i.