Bt Cotton and Insect Resistance

Bt cotton is genetically modified cotton that makes insecticidal proteins to protect itself against certain insect pests. It contains selected genes from the bacterium Bacillus thuringiensis (Bt), enabling its tissues to produce Cry proteins that kill susceptible insects after they feed on the plant.

How cotton acquires insect protection

Scientists introduce selected Bt genes into cotton so that the plant itself produces the corresponding insecticidal proteins. Protection therefore depends on the insect ingesting these proteins, rather than on the plant repelling the insect.

• The genes cry1Ac and cry2Ab encode proteins used to protect cotton against susceptible bollworms.
• Different Cry proteins act against different groups of insects, so the choice of gene determines which pests the modified plant can target.
• Bt cotton is not resistant to every insect pest; its protection is limited to pests susceptible to the proteins it produces.

What happens inside a susceptible insect

The decisive action occurs in the insect's midgut, where suitable chemical conditions and specific binding sites allow the toxin to damage the gut lining.

• A susceptible insect ingests the Bt protein while feeding on cotton tissues.
• Bt bacteria naturally produce their toxin as an inactive protoxin within protein crystals; the alkaline conditions of a susceptible insect's gut help dissolve these crystals and permit toxin activation.
• Gut enzymes process the protein into its active form.
• The activated toxin binds to specific receptors on the surface of midgut epithelial cells.
• The toxin forms pores in the cell membrane, disrupting the cells' normal regulation of water and ions.
• The damaged cells swell and break down, injuring the gut lining and ultimately causing the insect's death.

Why susceptibility matters

The presence of a Bt protein alone does not guarantee insect death. Effective protection requires suitable gut conditions, toxin activation and receptor binding.

• Specificity explains why a Cry protein can kill one insect species but be ineffective against another.
• Resistance can evolve when heritable traits reduce susceptibility, for example by altering toxin activation or receptor binding.
• Repeated exposure can select resistant insects, so resistance-management measures such as non-Bt refuges help preserve the effectiveness of Bt crops.