Bt toxin is produced by Bacillus thuringiensis in an inactive form (protoxin), which is transformed to its active form (delta-endotoxin) in the guts of certain insects. The active toxin binds to receptors in the gut, killing the insect. There are different forms of Bt toxin that are specifically active against certain groups of insects.
Spores and crystalline insecticidal proteins produced by B. thuringiensis have been used to control chewing insects since the 1920s based on the premise that they are harmless to humans. They are used in organic farming.
In transgenic crops, engineers isolate the active agent of Bt toxin, so it can be transferred to crop seeds. The subsequent plant then inherits the ability to produce the insect toxin on its own. A Belgian company, Plant Genetic Systems was the first company to develop genetically engineered tobacco plants with insect tolerance by expressing the cry genes from B. thuringiensis in 1985. Bt cotton, corn and potatoes were first planted in 1996; by 2006, Bt corn and cotton was planted in over 32 million ha. worldwide.
There is clear evidence from laboratory settings that Bt toxins can affect non-target organisms. Typically, exposure occurs through the consumption of plant parts such as pollen or plant debris, or through Bt ingested by their predatory food choices. In November 2009, Monsanto scientists found that the pink bollworm had become resistant to Bt cotton in parts of Gujarat, India.