Bt Cotton: Introduction, History, Mechanism, Production, Advantages, Disadvantages & Other Bt Crops

Table of Contents

• Introduction to Bt Cotton
• What is Bt cotton? 
• History and Development of Bt Cotton
• What is Bacillus thuringiensis?
• Advantages of Bt Cotton
• Disadvantages of Bt Cotton
• Other Bt Crops
• References

Introduction to Bt Cotton

• Cotton (Gossypium hirsutum) is one of the world's most important commercial crops, serving as a major source of natural fiber for the textile industry and seed oil for various industrial applications.
• Despite its economic significance, cotton production is heavily affected by a wide range of insect pests, which can cause severe yield losses and reduce crop quality.
• Traditionally, farmers control these pests by applying chemical insecticides and pesticides.
• Although effective against insects, excessive use of these chemicals has several drawbacks, including:
• Potential toxicity to humans and other non-target organisms.
• Environmental pollution due to poor biodegradability.
• Limited residual effectiveness, requiring repeated applications throughout the growing season.
• These limitations have created a need for safer, more sustainable, and long-lasting pest management strategies.
• One of the most successful solutions is Bt cotton, a genetically modified (transgenic) cotton variety engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt), providing built-in protection against major insect pests while reducing reliance on chemical insecticides.

What is Bt cotton? 

• Bt cotton is a genetically modified (GM) cotton plant developed to protect the crop against destructive insect pests, particularly cotton-eating caterpillars and bollworms.
• It was among the first commercially successful genetically modified crops introduced for agricultural pest control.
• Bt cotton is created by inserting specific cry (crystal) genes from the soil-dwelling bacterium Bacillus thuringiensis (Bt) into the cotton plant's genome.
• Bacillus thuringiensis naturally produces crystal insecticidal proteins (Cry proteins), a group of toxins that are highly effective against many insect larvae. More than 200 different Cry proteins have been identified, each targeting specific insect groups.
• After the cry genes are introduced into the cotton plant, its cells continuously produce Cry proteins, providing built-in protection against major cotton pests such as:
• Bollworms
• Budworms
• Caterpillars
• When susceptible insect larvae feed on Bt cotton, they ingest the Cry proteins, which damage the insect's digestive system, ultimately leading to its death.
• By producing its own insecticidal proteins, Bt cotton significantly reduces the need for chemical insecticides, helping improve crop protection and supporting more sustainable cotton production.

History and Development of Bt Cotton

• The history of Bt cotton began with the discovery of the soil bacterium Bacillus thuringiensis (Bt), whose insecticidal properties later revolutionized agricultural pest management.
• In 1901, Japanese scientist Shigetane Ishiwata isolated a bacterium responsible for the death of silkworm larvae and named it Bacillus sotto.
• In 1911, German microbiologist Ernst Berliner independently isolated a similar bacterium from diseased moth larvae found in a flour mill in Thuringia, Germany, and named it Bacillus thuringiensis.
• Subsequent research revealed that B. thuringiensis produces crystal insecticidal (Cry) proteins, which are highly toxic to specific insect larvae while remaining safe for most non-target organisms.
• Based on this discovery, the first Bt-based commercial insecticide, Sporine, was introduced in 1930.
• Although effective, early Bt insecticides had a major limitation: they degraded rapidly in the environment and required repeated applications throughout the growing season, increasing production costs for farmers.
• To overcome this challenge, researchers explored two main approaches:
• Developing more stable insecticidal proteins through protein engineering.
• Creating genetically modified crops capable of producing Bt toxins within their own tissues, eliminating the need for frequent insecticide applications.
• These efforts led to the development of Bt cotton, a transgenic cotton variety containing the Cry1Ac gene from B. thuringiensis.
• In 1996, Bt cotton was approved for commercial cultivation in the United States, marking a major milestone in agricultural biotechnology.
• In 1997, Bt cotton received commercial approval in China, and its cultivation subsequently expanded to many other countries.
• Today, Bt cotton is commercially grown in numerous cotton-producing nations, including:
• India
• China
• United States
• Australia
• Argentina
• Mexico
• South Africa
• Several other countries
• The global cultivation area of Bt cotton has expanded significantly since its introduction. By 2014, India had become the world's largest producer of Bt cotton, cultivating approximately 10.6 million hectares, followed by China and the United States.
• At present, Bt cotton seeds are produced and marketed by numerous agricultural biotechnology and seed companies worldwide, making Bt cotton one of the most widely adopted genetically modified crops.

What is Bacillus thuringiensis?

• Bacillus thuringiensis (Bt) is a Gram-positive, soil-dwelling bacterium widely used as a biological pesticide because of its ability to produce insecticidal proteins.
• It occurs naturally in a variety of environments, including soil, caterpillar intestines, leaf surfaces, animal feces, aquatic habitats, and flour mills.
• Bt is considered an environmentally friendly alternative to chemical insecticides due to its high specificity against target insect pests.

Crystalline Bodies (δ-Endotoxins)

• To protect itself from insect predation, B. thuringiensis produces intracellular crystalline inclusions during sporulation.
• These crystals contain δ-endotoxins, also known as Cry (crystal) proteins, which are toxic to susceptible insect larvae.
• Cry proteins are highly potent insecticides and are estimated to be approximately 800 times more toxic to target insects than many organophosphate insecticides, while remaining highly selective for specific insect groups.
• More than 200 Cry proteins have been identified, with different Cry proteins targeting different insect orders.

Types of Cry (δ-Endotoxin) Proteins

• CryI – Effective against Lepidoptera, including moths and butterflies (e.g., caterpillars and bollworms).
• CryII – Effective against both Lepidoptera and Diptera (flies and mosquitoes).
• CryIII – Effective against Coleoptera, including beetles.
• CryIV – Primarily effective against Dipteran larvae, such as mosquito and blackfly larvae.
• CryV – Effective against nematodes (roundworms).
• CryVI – Also targets nematodes (roundworms).

Mechanism of Action of Cry Proteins

• The Cry protein (δ-endotoxin) produced by B. thuringiensis is synthesized as an inactive precursor protein with a molecular weight of approximately 130–138 kDa.
• This inactive protein is insoluble under acidic conditions, making it harmless to humans and most other animals because their stomachs have an acidic pH.
• When susceptible insects feed on Bt bacteria or Bt crops, the alkaline environment (pH greater than 9) of the insect midgut dissolves the crystal proteins.
• Digestive proteases in the insect gut then cleave the inactive precursor into an active toxin of approximately 60 kDa.
• The activated toxin binds to specific receptors on the epithelial cells lining the insect's midgut.
• Binding of the toxin creates pores in the gut cell membranes, causing cell swelling, membrane disruption, and destruction of the intestinal lining.
• As a result, the insect stops feeding, becomes paralyzed, and eventually dies from starvation.
• Damage to the gut also allows Bt spores and other intestinal bacteria to invade the insect's body cavity, often leading to septicemia, which further contributes to insect death.
• Different insect species possess different gut receptors, explaining why individual Cry proteins are highly specific for particular groups of insects.

Production of Bt Cotton

• Bt cotton is produced using recombinant DNA technology, in which selected genes from Bacillus thuringiensis are inserted into the cotton genome to enable the plant to produce its own insecticidal proteins.
• A typical Bt cotton genetic construct consists of three major components: the Cry gene, a promoter, and a genetic marker.

Cry Gene

• The Cry gene encodes the insecticidal δ-endotoxin responsible for protecting the cotton plant against insect pests.
• Early Bt cotton varieties contained only the Cry1Ac gene.
• Modern Bt cotton varieties often contain stacked genes, combining multiple Cry genes or pairing Cry genes with additional traits such as glyphosate herbicide tolerance, thereby providing broader pest control and improved crop management.

Promoter

• The promoter regulates when, where, and how much of the Cry protein is produced within the plant.
• Some promoters restrict gene expression to specific tissues, whereas Bt cotton commonly uses promoters that allow Cry protein production throughout most of the plant, ensuring continuous protection against insect pests.

Genetic Marker

• A genetic marker enables researchers to identify plant cells that have successfully incorporated the inserted DNA.
• Herbicide-resistance genes are commonly used as selectable markers because transformed plants survive herbicide treatment, whereas non-transformed plants do not.
• These markers also assist plant breeders in developing stable Bt cotton varieties.

Transformation Methods

The foreign DNA can be introduced into cotton plants using either the direct (biolistic) method or the indirect (Agrobacterium-mediated) method.

Direct (Biolistic) Method

The biolistic or particle bombardment method delivers DNA-coated gold or tungsten particles into plant cells using high-pressure gas, allowing the foreign DNA to integrate into the plant genome.

Indirect (Agrobacterium-Mediated) Method

• This method utilizes Agrobacterium tumefaciens, a naturally occurring soil bacterium capable of transferring a modified Ti plasmid carrying the desired gene into plant cells.
• Because of its natural gene-transfer ability, A. tumefaciens is widely known as the natural genetic engineer of plants.

Chloroplast Transformation

• In some cases, the foreign gene is inserted into the chloroplast genome instead of the nuclear genome.
• Chloroplast transformation can increase gene expression and may reduce the likelihood of transgene spread through pollen in certain plant species.

Development of Stable Bt Cotton Lines

After successful transformation, the modified plants are propagated through four to five generations to obtain genetically stable lines that consistently express adequate levels of Cry proteins before they are released for cultivation.

Advantages of Bt Cotton

• Reduced use of chemical insecticides: Bt cotton produces its own insecticidal Cry proteins, significantly reducing the need for chemical insecticides. This lowers production costs, minimizes environmental pollution, and decreases the health risks associated with excessive pesticide use.
• Higher crop yield: By providing continuous protection against major insect pests, Bt cotton experiences less crop damage, resulting in improved plant health, increased productivity, and higher cotton yields compared with conventional cotton varieties.
• Selective action against target pests: Cry proteins specifically target susceptible insect pests, such as bollworms, while having little or no effect on most beneficial insects, pollinators, natural predators, humans, livestock, and other non-target organisms.
• Long-lasting pest protection: Unlike conventional insecticides that degrade over time or are washed away by rainfall, Bt cotton continuously produces Cry proteins throughout the growing season, providing sustained protection against susceptible insect pests.
• Lower environmental impact: Reduced dependence on chemical insecticides helps preserve biodiversity, decreases contamination of soil and water resources, and promotes more environmentally sustainable cotton farming.
• Economic benefits for farmers: Lower expenditure on insecticides, reduced labor for pesticide application, and higher crop yields can improve the overall profitability of cotton production.

   

Disadvantages of Bt Cotton

• Development of insect resistance: Continuous exposure of insect pests to Bt toxins can lead to the evolution of resistance over time. Resistance often develops through genetic mutations that alter the receptors in the insect's midgut, preventing Cry proteins from binding effectively. Several pests, including Helicoverpa armigera (cotton bollworm) and Pectinophora gossypiella (pink bollworm), have been reported to develop resistance to certain Cry proteins.
• Reduced effectiveness under environmental stress: Environmental factors such as high temperatures and low relative humidity can reduce the expression and activity of Cry proteins in Bt cotton. As a result, the plants may become more susceptible to damage from bollworms and other insect pests under unfavorable climatic conditions.
• Higher seed cost: Bt cotton seeds are genetically engineered and protected by patents, making them more expensive than conventional cotton seeds. The increased cost can be a financial burden, particularly for small-scale farmers.
• Dependence on biotechnology companies: Because Bt cotton seeds are patented, farmers generally need to purchase certified seeds from biotechnology or seed companies for each planting season rather than relying on farm-saved seeds. This increases dependence on commercial seed suppliers and raises economic, legal, and ethical concerns in some regions.
• Limited protection against all pests: Bt cotton is primarily effective against specific insect pests, particularly certain caterpillars and bollworms. It does not provide protection against sucking pests such as aphids, whiteflies, or mealybugs, which may still require additional pest management measures.
• Potential ecological concerns: The widespread cultivation of Bt cotton may influence insect populations and ecosystem dynamics over time. Continuous monitoring and resistance management strategies, such as refuge planting, are essential to maintain the long-term effectiveness of Bt technology.

Other Bt Crops

Besides Bt cotton, several other genetically modified (Bt) crops have been developed by introducing Cry genes from Bacillus thuringiensis. These crops are engineered to provide protection against specific insect pests while reducing the need for chemical insecticides.

• Bt Corn (Maize): Contains Cry1Ab, Cry2Ab, Cry1F, or Cry4Bb1 genes and provides protection against major pests such as the European corn borer and fall armyworm.
• Bt Potato: Contains the Cry3A gene, which protects the crop against the Colorado potato beetle, a major pest of potato plants.
• Bt Rice: Contains Cry1Ab or Cry2A genes and is engineered to resist the rice stem borer, helping to reduce yield losses.
• Bt Tomato: Contains Cry1Ac or Cry1Ab genes and provides protection against the tomato fruit borer, one of the most destructive pests affecting tomato production.

References

• Bt Cotton & Management of Tobacco Budworm-Bollworm Complex. (2001). United States Department of Agriculture. https://www.ars.usda.gov/ARSUserFiles/oc/np/btcotton/btcotton.pdf
• Bt Cotton—UT Crops. (n.d.). Https://Utcrops.Com/. Retrieved July 16, 2025, from https://utcrops.com/cotton/insects-and-mites/biological-control/bt-cotton/
• Genetically modified cotton: How has it changed India? (n.d.). Retrieved July 19, 2025, from https://researchoutreach.org/articles/genetically-modified-cotton-how-changed-india/
• Gothandaraman, R., Selvaraj, A., & Rajasekaran, R. (2023). Bacillus thuringiensis in Pest Management. Plant Health Archives, 1, 1–13.
• Ibrahim, M. A., Griko, N., Junker, M., & Bulla, L. A. (2010). Bacillus thuringiensis. Bioengineered Bugs, 1(1), 31–50. https://doi.org/10.4161/bbug.1.1.10519
• Nagaraj, S., Rajasekaran, R., Palaniappan, J., Rangasamy, S., Narayanasamy, C., & Narayanan, M. B. (2024). Emerging technological developments to address pest resistance in Bt cotton. Journal of Cotton Research, 7(1), 30. https://doi.org/10.1186/s42397-024-00192-z
• TNAU Agritech Portal: Bio Technology. (n.d.). Retrieved July 17, 2025, from https://agritech.tnau.ac.in/bio-tech/biotech_btcotton_env.html
• ZAFAR, M. M., RAZZAQ, A., FAROOQ, M. A., REHMAN, A., FIRDOUS, H., SHAKEEL, A., MO, H., & REN, M. (2020). Insect resistance management in Bacillus thuringiensis cotton by MGPS (multiple genes pyramiding and silencing). Journal of Cotton Research, 3(1), 33. https://doi.org/10.1186/s42397-020-00074-0