Evaluation on activity and efficacy of <i>Bacillus thuringiensis</i> var. <i>israelensis</i> and <i>S-methoprene</i> against the Asian tiger mosquito, <i>Aedes albopictus</i> (Diptera: Culicidae), in discarded tires

The Asian tiger mosquito, Aedes albopictus Skuse (Diptera: Culicidae), is an important vector of various arboviruses. Effective control of mosquito vectors is essential to prevent the transmission of mosquito-borne diseases; however, sustainable larval control against this species has been notoriously difficult. To enhance effective larval control against Ae. albopictus, a laboratory initial test was conducted to determine the activity of microbial larvicide Bacillus thuringiensis var. israelensis (Bti) and a juvenile hormone analog S-methoprene. The technical grade and formulated product performed similarly in either Bti or S-methoprene. Compared with larvae from a laboratory strain, field-collected mosquitoes showed similar susceptibility to Bti, but significantly lower susceptibility to S-methoprene at IE30 and IE50. In semifield studies to evaluate efficacy of the formulations, as short as a 2-d efficacy was observed for Bti alone at 0.50 ppm, while longer efficacy of up to 3 and 4 wk was achieved by the S-methoprene treatments alone at 1 and 5 ppb, respectively, due to different modes of action and formulation technologies. The combination of Bti at 0.25 ppm and S-methoprene at 2.5 ppb exhibited an extended effect for up to 4 wk as in S-methoprene alone at 5 ppb. The efficacy of both insecticides was impacted by sunlight and dilution. Larvae from field collections were less susceptible to S-methoprene than those of laboratory colony in the semifield evaluation in discarded tires. The practicality of Bti and S-methoprene products for controlling Ae. albopictus in discarded tires was discussed in relation to the findings in current studies.