Efficacy of a Bacillus sphaericus formulation as influenced by the quality of Culex quinquefasciatus breeding waters.

To understand the physico-chemical factors that influence the efficacy of B. sphaericus formulation in the breeding sites of Culex quinquefasciatus, a study was carried out in Mayiladuturai area of Tamil Nadu (India). The factors studied were hydrogen ion concentration (pH), acidity, alkalinity, chlorides, phosphates, total hardness, sulphates, total solids, dissolved solids, suspended solids, nitrate nitrogen, ammoniacal nitrogen, dissolved oxygen, biological oxygen demand (BOD) and chemical oxygen demand (COD). Efficacy of the formulation was assessed in terms of reduction in larval population in the treated habitats. pH of water in the treated sites was around neutral range (mean +/- SD 7.65 +/- 0.23). Phosphate content was low (2.27 +/- 1.34 ppm) whereas chlorides (326.1 +/- 55.8 ppm) and sulphates (38.9 +/- 23.8 ppm) were high. Total hardness ranged from 206 to 462.5 ppm with a mean of 312.1 +/- 80.5 ppm. The chlorides and sulphates, though present in considerable quantity, did not have any influence on the efficacy of B. sphaericus formulation. However, the proportion of insoluble chlorides and sulphates which contribute to total hardness seemed to influence the formulation adversely.

Efficacy of Bacillus sphaericus in different breeding habitats of Culex quinquefasciatus.

'Spherifix', an alginate based slow release formulation of Bacillus sphaericus was field tested in different types of breeding habitats of Culex quinquefasciatus at the dose of 15 kg ai/ha at bimonthly interval. The efficacy of the formulation was higher in most of the months except in rainy and post-rainy months. The mean percentage reduction +/-SD during the treatment phase of one year was 31.2 +/- 17.9, 50 +/- 29.4, 28.3 +/- 17.6, 30.3 +/- 21.1, 66 +/- 22.5 and 53 +/- 20.4 in larval density and 49 +/- 20.8, 65.1 +/- 26.1, 30.3 +/- 21.9, 59.8 +/- 22.6, 63.1 +/- 21.9 and 47.7 +/- 24.2 in pupal density respectively in cement tanks, cesspools, cesspits, disused wells, unlined drains and cement lined drains. The reduction in immature density was relatively higher in undisturbed, debris free and shallow habitats such as cesspools, unlined drains and cement lined drains. After withdrawal of treatment, the effect of the formulation could be seen for a period of four months.