Efficacy of new formulations of the microbial larvicide Bacillus sphaericus against polluted water mosquitoes in Thailand.

Two new water dispersible granular (WDG) formulations of the microbial control agent Bacillus sphaericus (strain 2362) were extensively evaluated in polluted waters against Culex quinquefasciatus in Thailand. The studies were carried out in stagnant as well as flowing waters during August 1997 to July 1998. The trial period covered both rainy and dry seasons. The two WDG formulations of B. sphaericus tested were low potency (350 ITU/mg) and high potency (630 ITU/mg) products. Both formulations were used at various rates to determine initial efficacy and longevity. The high potency formulations provided excellent control (80 to 90%) of immature mosquitoes at the rates of 50 to 100 mg/ m2, while the less potent formulation yielded similar control at the rates of 89 to 250 mg/m2. Longevity of control was anywhere from one week to four weeks or longer depending on the dosage, habitat, and environmental conditions. Two treatments with low dosages of Bacillus thuringiensis ssp. israelensis WDG provided short-term control lasting for one week. Frequent episodes of heavy rains facilitated long-term suppression of immature mosquitoes in klongs by flushing out the larvae. In the absence of rain, the longevity of treatments in flowing waters was shorter than in the presence of rain. Rain, however, had some but not pronounced effect on longevity in the stagnant water habitats. Operation of floodgates controlling water flow from stagnant water habitats into the Chaophrya River had greater influence on the abundance of mosquito immatures in the ponded polluted waters under dwellings. Precipitation, adding large amounts of water, was probably responsible for diluting the control agents and also resulted in lower counts of immatures per unit volume of water. These variables could influence the efficacy (especially longevity) of treatments employing formulations of microbial agents in operational control programs. The WDG formulations are preferred over other formulations, such as granules, because the former contain larger quantities of toxins per unit mass than the latter and are easily applied as aqueous sprays. Potent formulations with greater content of active ingredients are less costly to ship and transport to distant areas.