Effect of some materials on the potency of entomopathogenic bacteria against the cotton leaf worm, Spodoptera littoralis (Boisd.)

Abstract

The object of the present study is to investigate the effects of some natural and chemical additive materials, included mint oil, sesame oil, chamomile oil, green tea oil, ZnSO4, Na2CO3, MgSO4 and citric acid on the potency of the bio-insecticides; Spinosad and Bacillus thuringensis var. kuristaki represented as commercial products; Tracer and Bay-8, respectively applied against Spodoptera littoralis 2nd instar larvae. Toxicity, biological studies and some physico-chemical properties of the tested bio-insecticides either used only or combined with the candidate additives were evaluated. The obtained results cleared that all the tested additive materials were compatible with the tested bio-insecticides and increased the pH value of the resulting mixture with the exception of citric acid which reduced the pH values compared to those of the tested bio-insecticides only. Treatment with either Spinosad or B. thuringensis combined with the different additives greatly synergized their toxicity against the target pest. The LC50 values of the bio-insecticide-additive mixtures were reduced compared to those of the tested bio-insecticides when used only, except for the two bio- insecticides combination with green tea oil or Spinosad combined with Na2CO3, where their LC50 values increased than in the treatment implemented with the two bio-insecticides only. The LC50 values at 72h post treatment with the most efficient combinations Spinosad-mint oil and Spinosad-ZnSO4 mixtures recorded 8.068 and 12.805 ppm, respectively compared to treatment with Spinosad only (LC50 =20.482 ppm). B.t-mint oil and B.t-Na2CO3 recorded 3.549 and 3.972 ml/L, respectively compared to treatment with B.t only (LC50 =7.079 ml/L) against the target pest. The effect of the most efficient bio-insecticide-additive mixtures was studied on certain biochemical parameters including total protein content as well as the activity of some detoxification enzymes of the 2nd instar larvae.