Bioactivity, Synergism and Potentiation of the Botanical Extracts, Nimbecidine and Bacillus thuringiensis Against the African Boll-worm Helicoverpa armigera Hübner

Abstract

African bollworm Helicoverpa armigera Hübner is a worldwide-spread spe-cies that shows a high degree of polyphagia and it is considered as one of the main agricultural pests in the world. Laboratory experiments were conducted in the Research Laboratory, College of Agricultural Studies, Sudan University of Science and Technology to investigate, through laboratory screening, the insecticidal activity of the ethanolic extracts of Cyperus rotundus, Datura stramonium, Cassia occidentalis and Ricinus communis and their co-toxicity as well as the synergistic effect of sesame oil against the second larval instar of H. armigera. Furthermore, the joint action of Bacillus thuringiensis var. kurstaki (Btk), Nimbecidine® and the ethanolic extracts of the tested plants were also studied. Experiments were also conducted to evaluate the efficacy of aqueous extracts of Balanites eagyptiaca, Solenostemma argel, Eucalyptus camaldulensis and Acacia nilotica on the mortality of the same larval instar. In each experiment five concentrations were used as followed 4%, 6%, 8%, 10% and 12% for ethanolic extracts, 1.8%, 3.75%, 7.5%, 15% and 30% for aqueous extracts, 0.62, 1.25, 2.5, 5 and 10 mg/ml for Bt and 1.25, 2.5, 5, 10 and 20 ml/L for Nimbecidine®. The experiments were conducted in a Com-pletely Randomized Design and the mortality percentages were recorded after 24, 48, 72 and 96 hrs of application. The study findings clearly showed that all tested plant extracts, Bt and Nimbecidine® generated a significantly higher mortality percentage than that of the control. It was also noted that the mortality percentage increases with the increase of both concentration and exposure period. Probit analysis of the mortality data showed that the median lethal concentrations (LC50) of the ex-tracts vary from one plant to another whether plant extract used alone or mixed with sesame oil. Tubers ethanolic extract of C. rotundus scored the lowest LC50 value (4.2 %) followed by R. communis (6.4%), D. stramonium (7.1 %) and C. occidentalis(7.1 %). However, when sesame oil was added to each concentration of the ethnolic extract it exhibited a synergistic effect and significantly reduced their LC50 values. Upon testing the joint action of the tested ethanoic extracts, the results showed that all combinations (plant/plant or plant Bt) gave a higher mortality percentage either through potentiation or additive effect. The binary mixture of R. communis and D.stramonium caused a potentiation effect throughout the experimental period. R. communis and C. occidentalis mixture gave an additive effect however, a potentiation effect was generated after 48 hrs of application. However, the binary mixture of C. rotundus and all other plant extracts(R. communis, D.stramonium and C. occidentalis) induced an additive effect throughout the experimental period. The obtained data proved that the binary mixture of Bt and R. communis as well as Bt and Nimbecidine® induced a potentaition effect throughout the experimental period. On the otherhand the binary mixture of Bt and C. rotundus, Bt and D.stramonium and Bt and C. occidentalis induced an additive effect. The results also showed that the aqueous extract of Balanites eagyptiaca was significantly more toxic than its counterparts of tested plants. In fact, after 96 hrs of exposure the LC50 values were 12.3%, 15.4%, 24.5% 27.9% for Bala-nites eagyptiaca, Solenostemma argel, Eucalyptus camaldulensis and Acacia nilotica respectively.