author(s)
Aijaz Ahmad Sheikh, Muneeb Ahmad Wani, Parveena Bano, Sajad Un Nabi, Tariq Ahmad Bhat, Mohammad Amin Bhat and Mohd Saleem Dar

abstract
Bacillus thuringiensis (Bt) is a ubiquitous, rod-shaped and sporulating bacterium that produces a wide variety of insecticidal proteins active against larvae of very diverse insect orders. Once ingested by insects, these crystals are solubilized in the midgut, then proteolytically activated by midgut proteases and bound to specific receptors located in the insect cell membrane leading to cell disruption and insect death. The process of genetic transformation allows genes to be transferred from one organism/source to another, products developed through this procedure are known as genetically modified organisms (GMOs) or transgenic/biotech crops. Cry1A family is the most commonly used Bt toxins, especially Cry1Ac in transgenic Bt cotton and Cry1Ab in transgenic Bt corn. The genetically engineered insect-resistant crops (Bt crops) were first commercially grown in 1996 and adopted in different countries. The area of Bt crops planted each year continues to increase, with 181.48 million hectares grown in more than two dozen countries in 2015. The reason behind this widespread adoption of GM varieties is that it causes reduced purchases of costly inputs such as pesticide, while increases farm income along with the benefits to the environment. Genetic modification of plants has helped the agricultural system flourish; insects are beginning to evolve resistance to the Bt crops. Unfortunately, the field population of pests evolved resistance to different Bt toxins and the number of resistant species is going to increase, which is threatening to the continuous success of Bt crops. The number of resistant species has been increased worldwide, 13 cases of field-developed resistance to 5 Bt toxins in transgenic corn and cotton have been reported. Therefore, understanding of the molecular and genetic basis of resistance to Bt could help in designing a suitable management approach to delay the resistance development in the insect pests. To delay the onset of resistance, it is essential that farmers understand and implement Insect Resistance Management (IRM) practices. The tactics available for sustainable deployment of insect resistance genes in transgenic crops can be grouped into four strategies. These are not essentially mutually exclusive. Two or more strategies can be combined together by deploying one or several genes (Gene strategies), produced at high dose of the endotoxin (Dose strategies) and may be grown along with refuges, as mixtures or separate.