Baseline resistance to Cry1Ac toxin in cotton bollworm, Helicoverpa armigera (Hubner) in south Indian cotton ecosystem

摘要

Insecticidal protein genes coding for crystal (Cry) toxins of Cry1A group from the bacterium Bacillus thuringiensis (Bt) have been transferred to and expressed in a number of crops in order to confer resistance against lepidopteron insect pests. Several transgenic crop species incorporating cry1A genes have been commercialized and cultivated in a number of countries over the past few years. Bt transgenic cotton was cleared by the Department of Biotechnology, Government of India for commercial cultivation from the year 2002, after a long debate and discussion. The primary target pest of this technology in India and several other countries is the cotton bollworm, Helicoverpa armigera (Hubner). It is a polyphagous pest with a wide host range of 181 plant species including cotton, pigeonpea, tomato, chickpea, maize, sunflower and several vegetable crops which is currently estimated to cause economic losses up to about Rs 250 billion in India. Lately, the problems of pest management in cotton and other crops have been compounded by the development of resistance to insecticides in H. armigera. This led to excessive and indiscriminate use of insecticides by desperate farmers in many parts of the country. Outbreak of H. armigera in south Indian cotton and pigeonpea ecosystem usually leads to severe socioeconomical disturbances, including several cases of suicide by farmers. Introduction of insect-resistant transgenic crops, especially Bt transgenics, is expected to be useful in management and effective control of lepidopteron pests with a significant reduction in the overall use of insecticides. However, long-term exposure to Bt transgenic crops is likely to render lepidopteron pests resistant to the Cry toxins due to continuous selection pressure. Moreover, the introduction of transgenic plants expressing a Cry toxin under the influence of constitutive promoters, is likely to hasten this process. The development of resistance to Bt toxins can be quite distinct, depending upon the species, selection regime or geographical origin of the founder colony. Hence, initial survey to assess the susceptibility of test insect to the Cry toxins will establish a baseline that can be used in monitoring resistance development in future. We report the resistance of H. armigera to CrylAc toxin in 11 distinct geographic populations representing the entire south Indian cotton ecosystem.