Efficacy of Bt proteins and the effect of temperature on the development of spiny bollworm in South Africa
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Genetically modified cotton expressing Bacillus thuringiensis (Bt) proteins has been cultivated in South Africa since 1998 for control of the bollworm complex. Spiny bollworms, Earias biplaga (Walker) (Lepidoptera: Noctuidae) and Earias insulana (Boisduval) (Lepidoptera: Noctuidae) belong to this complex. Exposure to Bt crops could contribute to resistance development to the insecticidal proteins expressed in these crops. The aim of this study was to determine the efficacy of Bt proteins for control of and the effect of temperature on development of E. biplaga in South Africa. There is currently no resistance of E. biplaga to Bollgard® and Bollgard II® cotton in South Africa. The use of Bt spray applications for control of E. biplaga on cotton was also evaluated, although it is currently not registered for control of this pest on cotton in South Africa. Half the dosage rate registered for bollworm control on cotton, was too low for effective control. The recommended dosage rate controlled the larvae as effective as Bollgard® and Bollgard II®, but 100% mortality was not achieved. Environmental factors such as UV light and rain may reduce the efficacy of Bt sprays. The final instar larvae might not be controlled by Bt sprays. Effective coverage with the spray application is essential for successful control. The effect of temperature on the development of E. biplaga was studied at four different temperature regimes, namely 18, 20, 25 and 30 ± 1°C. Development time for all life stages was inversely related to temperatures from 18 to 30 °C. The relationship between temperature and developmental rate of E. biplaga was linear between 18 and 30 °C and more rapid development was observed with increasing temperatures. The total development period was 68.9 to 22.5 days at 18 and 30 days, respectively. The thermal thresholds for E. biplaga were 15.2, 11.3, 12.8 and 12.2°C and the thermal constants were 34.3, 195.1, 156.45 and 369.6 °D, for the completion of the egg, larval, pupal and egg-to-adult stages, respectively.