Genetically modified crops have greatly reduced the use of chemical insecticides as they are made resistant to various pathogens through genetic engineering. 

Bt toxins 

1. These are toxic insecticidal proteins produced by a soil bacterium Bacillus thuringiensis 

2. It produces crystalline proteins (cry proteins).they form crystalline inclusions in the bacterial spores. 

3. These spores kill certain groups of insects such as lepidopterans (tobacco worm, bud worm,army worm), coleopterans (beetles) and dipterans (flies and mosquitoes) 

4. Cry proteins dissolve in the alkaline juices of the mid gut. 

5. They bind to receptors present in the brush border of mid gut epithelial cells of the insects. 

6. The brush border membrane develops pores. 

7. These pores allow influx of ions and water, which causes their swelling and eventual death. Bt- cotton: 

1. It is a transgenic plant produced by cloning BT (Bacillus thuringiensis) genes isolated from BaciUus thuriengiensis bacterium. 

2. It is resistant to cotton boll worms and stem borer which feed on them. 

3. Bt genes produce proteins (Crystal Protein-cry I AC, cry IIAB) that kills the insects. 

4. Hence cry-genes have been introduced in plants to produce crystal proteins as Protoxin (inactive toxin), which is converted to toxins in alkaline medium (i.e. in the gut of insects) and cause death of the insect larva.