In a bioassay of the toxicity of four local isolates and a reference (HD-1) strain of Bacillus thuringiensis subsp. kurstaki (Bt kurstaki) to larvae of the spotted bollworm (Erias vittella), different concentrations of spore-crystal mixture were offered to third-instar larvae of the spotted bollworm (Erias vitella) utilizing okra seeds as diet. The number of dead larvae and the time of death were recorded, and corrected mortality was computed using SunShepard formula. Most isolates exhibited LT50 values far lower than the reference strain after 24 and 48 h of incubation. One such isolate (BO5) required just 22.54 h to kill 50% of E. vittella larvae when used at 2.4x10 9 spore-crystals/mL. Similarly, most isolates exhibited LC50 values lower than the reference strain at 24 and 48 h. However, despite the unsatisfactory performance by the reference strain at 24 and 48 h, it showed much improved performance at 72h, where only one isolate (K21) showed relative potency (RP) value higher than the reference strain, while another (KO9) was almost as potent as the reference strain. It is concluded that continued search for and evaluation of Bt isolates obtained from different environments could uncover great capabilities in these isolates.
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