Evaluation of Bacillus sphaericus against Mansonia dyari larvae in phosphate lakes in Polk County, Florida.

Effective Mansonia larval control is difficult to achieve in Polk County, Florida, because the larvae attach to the root and stem mass of water lettuce (Pistia stratiotes) and water hyacinth (Eichhornia crassipes). This study evaluated 2 Bacillus sphaericus larvicides as possible control products in 3 phosphate lakes in 2 areas maintained by Polk County Mosquito Control. VectoLex WDG was applied at 56 kg/km2 and 170 kg/km2 and VectoLex CG at 560 kg/km2 and 1,100 kg/km2. One lake in each site was an untreated control. Larval collections were made by collecting water lettuce plants that were shaken, washed, and processed through a series of sieves to grade larvae as early (1st, 2nd) and late (3rd, 4th) instars. Pretreatment and posttreatment collections were made in each lake. More than 3,100 larvae were collected during the study. Approximately 65% of the larvae collected were Ist/2nd instars in both areas. The higher rate of VectoLex CG reduced the larval population 63% and the higher rate of VectoLex WDG by 44%. Against the Ist/2nd instar aggregate the higher rate of VectoLex CG was more effective (69%) than the VectoLex WDG. The VectoLex CG granules appeared to remain nearer the surface and the larvae than the WDG granules. Better effectiveness might have been attained if the entire lake had been treated. In this study, VectoLex CG appeared to be more effective than VectoLex WDG.