Histopathological effects of Bacillus subtilis SPB1 biosurfactant in the midgut of Ephestia kuehniella (Lepidoptera: Pyralidae) and improvement of its insecticidal efficiency

The present investigation aimed to throw light on the effect of Bacillus subtilis SPB1 biosurfactant on the third larval instars of the Mediterranean flour moth, Ephestia kuehniella, under laboratory conditions. The toxicity of this compound was investigated with emphasis on histopathological effects in the midgut of larvae. The tested dose levels showed strong histopathological disturbances in the midgut of this pest. The most frequently observed effects were cell vacuolisation, microvilli damage and epithelium cell contents passing into the midgut lumen. The present study was also extended to improve the insecticidal activity of the SPB1 biosurfactant against E. kuehniella larvae through optimisation of the pH of the biosurfactant solution and the incubation temperature using Central Composite Design. The experimental results were fitted to a second-order polynomial model that yielded a determination coefficient of R-2 = 0.998. The optimal insecticidal activity conditions were found to be a temperature of 33.20 degrees C and pH of 6.66. The predicted and observed responses were 200.4 ng mg(-1) and 182.57 ng mg(-1), respectively. In comparison to the original LC50 level, a 30% decrease was obtained.