Efficacy of Bacillus thuringiensis sprays and cotton cultivars expressing Cry proteins in the control of Earias biplaga (Walker) (Lepidoptera: Noctuidae)

Earias biplaga (Walker) (Lepidoptera: Noctuidae) forms part of the bollworm pest complex of cotton in South Africa. Genetically modified Bt cotton that expresses Cry insecticidal proteins successfully controls the bollworm complex in South Africa. Although many insecticides are registered for use against Lepidoptera on this crop, Bt sprays are not registered for control of E. biplaga. The aim of this study was to determine the efficacy of Bacillus thuringiensis spray applications for the control of E. biplaga on non-Bt cotton and to compare the efficacy of sprays to that provided by Bt cotton. Bioassays were conducted in which cotton bolls of a non-Bt cotton cultivar (Delta Opal) were sprayed with different dosage rates of Dipel DF (R), which contains the following insecticidal proteins: Cry1Aa, Cry1Ab, Cry1Ac, Cry2Aa and Cry2Ab. Bollgard (R) and Bollgard II (R) cotton which expresses Cry1Ac and both Cry 1Ac and Cry2Ab were used in the study. Neonate larvae were inoculated onto cotton bolls and survival observed 4, 7 and 10 days later. The Bt spray dosage rate recommended for control of Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) on groundnut and sunflower in South Africa was used in this study and provided between 77 and 88% control of E. biplaga after 10 days. Bt cotton provided 100% control after 10 days. This study provides baseline data on the susceptibility of E. biplaga against Bt cotton which can also be used for future monitoring of resistance evolution.