Strategies to Enhance the Toxicity of Bacillus thuringiensis Against Spodoptera littoralis (Boisd.)

Abstract

Bacillus thuringiensis (Bt) is an aerobic, spore forming bacterium that produces crystal inclusions that contain Insecticidal Crystal Proteins (ICPs) used to control many major pests. ICPs, or Cry proteins, are in the form of protoxins in the crystal. ICPs are solubilized and processed to toxic peptides by gut proteases in insects. Because proteases are important to toxicity, research into interactions of proteases with Bt proteins may lead to improved toxin efficacy. Toxicity of two commercial formulations of Bt was evaluated against 2nd instar larvae of Spodoptera littoralis. Dipel DF was more effective than Dipel 2X (LC50 were 0.13 and 0.17, respectively). In addition, rice bran is an agricultural waste which contains protein, fiber and carbohydrate content. It was tested in the laboratory to determine its stimulatory effects on S. littoralis protease. Adding rice bran extract to commercial formulation of Bacillus thuringiensis var. kurstaki, (Dipel DF) and / or (Dipel 2X) revealed a potential action with Co-toxicity factor +34.33 and + 21.38, respectively indicating good potential for rice bran extract to enhance B. thuringiensis-based formulations. Sodium bicarbonate (NaHCO3), calcium carbonate (CaCO3) and sodium dodecyl sulphate (SDS) were also added to both Dipel DF and Dipel DX. Data showed that all tested bio agents and their mixture with different additives induced significant reduction in both pupation percentage and pupal weight (except in case of NaHCO3). The reduction effect was significantly high in Dipel DF with rice bran extract compared with the control. The pupal malformation ranged from 2.82 to 13.33%. Adult malformation resulted from treated 2nd instar larvae showed also varying degrees of deformities as a side effect of bioinsecticide alone and its combination with additives. The biochemical analysis revealed a significant increase in the proteolytic activity in all treated samples and a significant decrease in the activity of trehalase, invertase and amylase, as compared to control group.