Enhanced bioefficacy of Bacillus thuringiensis var. kurstaki against Spodoptera litura (Lepidoptera: Noctuidae) through particle size reduction and formulation as a suspension concentrate

Technical powder of DOR Bt-127, a local isolate of Bacillus thuringiensis (Bt) var. kurstaki, containing 105 mu m particles was subjected to milling in a planetary ball mill. Dynamic light scattering of samples drawn at 30min intervals revealed reduced particle sizes ranging 1171-210nm. Alkali-soluble protein was higher at 153-175mg/g in milled Bt powders over 140mg/g in unmilled Bt powder. All milled powders gave higher mortality of Spodoptera litura (Fabricius) larvae in comparison to the unmilled Bt powder. Bt powder from 60min milling containing 559nm particles with a low polydispersity index of 0.338 gave a lower LC50 value of 1.35mg/ml against third instar S. litura larvae in comparison to 2.04mg/ml for unmilled Bt powder. LC50 value of the suspension concentrate (SC) formulation developed with 559nm Bt particles was 2.84 mu l/ml containing only 0.95mg Bt. Field evaluation of the SC formulation against S. litura on castor revealed highest per cent larval reduction of 92.4 and 96.2 at concentrations of 2.5 and 3.0ml/l, respectively.