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

Abstract

Technical powder of DOR Bt-127, a local isolate of Bacillus thuringiensis (Bt) var. kurstaki, containing 105 μm particles was subjected to milling in a planetary ball mill. Dynamic light scattering of samples drawn at 30 min intervals revealed reduced particle sizes ranging 1171–210 nm. Alkali-soluble protein was higher at 153–175 mg/g in milled Bt powders over 140 mg/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 60 min milling containing 559 nm particles with a low polydispersity index of 0.338 gave a lower LC50 value of 1.35 mg/ml against third instar S. litura larvae in comparison to 2.04 mg/ml for unmilled Bt powder. LC50 value of the suspension concentrate (SC) formulation developed with 559 nm Bt particles was 2.84 μl/ml containing only 0.95 mg 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 3.0 ml/l, respectively