Strains of the bacterium Bacillus thuringiensis produce over 200 different Bt toxins, each harmful to different insects. Most notably, Bt toxins are insecticidal to the larvae of moths and butterflies, beetles, cotton bollworms and flies but are harmless to other forms of life. The gene coding for Bt toxin has been inserted into cotton, causing it to produce this natural insecticide in its tissues. In many regions, the main pests in commercial cotton are lepidopteran larvae, which are killed by the Bt protein in the transgenic cotton they eat. This eliminates the need to use large amounts of broad-spectrum insecticides to kill lepidopteran pests (some of which have developed pyrethroid resistance). This spares natural insect predators in the farm ecology and further contributes to noninsecticide pest management.
However, Bt cotton is ineffective against many cotton pests such as plant bugs, stink bugs, and aphids; depending on circumstances it may be desirable to use insecticides in prevention. A 2006 study done by Cornell researchers, the Center for Chinese Agricultural Policy and the Chinese Academy of Science on Bt cotton farming in China found that after seven years these secondary pests that were normally controlled by pesticide had increased, necessitating the use of pesticides at similar levels to non-Bt cotton and causing less profit for farmers because of the extra expense of GM seeds.
Bt cotton was created through the addition of genes encoding toxin crystals in the Cry group of endotoxin. When insects attack and eat the cotton plant the Cry toxins are dissolved due to the high pH level of the insects stomach. The dissolved and activated Cry molecules bond to cadherin-like proteins on cells comprising the brush border molecules. The epithelium of the brush border membranes separates the body cavity from the gut whilst allowing access for nutrients. The Cry toxin molecules attach themselves to specific locations on the cadherin-like proteins present on the epithelial cells of the midge and ion channels are formed which allow the flow of potassium. Regulation of potassium concentration is essential and, if left unchecked, causes death of cells. Due to the formation of Cry ion channels sufficient regulation of potassium ions is lost and results in the death of epithelial cells. The death of such cells creates gaps in the brush border membrane.
The use of Bt cotton in India has grown exponentially since its introduction. Recently India has become the number one global exporter of cotton and the second largest cotton producer in the world. India has bred Bt-cotton varieties such as Bikaneri Nerma and hybrids such as NHH-44, setting up India to benefit now and well into the future.
Socio-economic surveys confirm that Bt cotton continues to deliver significant and multiple agronomic, economic, environmental and welfare benefits to Indian farmers and society including halved insecticide requirements and a doubling of yields.
However, India’s success has been subject to scrutiny. Monsanto's seeds are expensive and lose vigour after one generation, prompting The Indian Council of Agricultural Research to develop a cheaper Bt cotton variety with seeds that could be reused. The cotton incorporated the cry1Ac gene from the soil bacterium Bacillus thuringiensis (Bt), making the cotton toxic to bollworms. This variety showed poor yield, was removed within a year, and contained a DNA sequence owned by Monsanto, prompting an investigation. In parts of India cases of acquired resistance against Bt cotton have occurred. Monsanto has admitted that the pink bollworm is resistant to first generation transgenic Bt cotton that expresses the single Bt gene (Cry1Ac).
The state of Maharashtra has banned the sale and distribution of Bt cotton in 2012, to promote local Indian seeds, which demand less water, fertilizers and pesticide input.
The main selling points of Bt cotton are the reductions in pesticides to be sprayed on a crop and the ecological benefits which stem from that. China first planted Bt cotton in 1997 specifically in response to an outbreak of cotton bollworm, Helicoverpa armigera, that farmers were struggling to control with conventional pesticides. Similarly in India and the US, Bt cotton initially alleviated the issues with pests whilst increasing yields and delivering higher profits for farmers.
Studies showed that the lower levels of pesticide being sprayed on the cotton crops promoted biodiversity by allowing non-target species like ladybirds, lacewings and spiders to become more abundant. Likewise it was found that integrated pest management strategies (IPM) were becoming more effective due to the lower levels of pesticide encouraging the growth of natural enemy populations.
Following the introduction of Bt cotton in northern China, non-target pests such as mirid bugs (Heteroptera: Miridae) have been becoming more abundant due to the reduction in conventional pesticides being sprayed. Following from this, a second issue being seen across the world, is the development of resistance genes in pest populations limiting the effect of Bt crops.
The main drivers for this widespread resistance in China and India included the high proportion of Bt cotton being planted, 95% and 90% respectively in 2011. and the low implementation of refuge areas.
Refuge areas act to limit the development of resistance genes in a targeted pest population. The Environmental Protection Agency (EPA) in the US requires farmers to have refuge areas of 20%-50% non-Bt crops within 0.8 km of their Bt fields. Such requirements were not seen in China, where instead farmers relied on natural refuge areas to decrease resistance.
A novel solution to the resistance problem was trialed in Arizona, with sterile male pink bollworms (Pectinophora gossypiella) being released into populations of their wild Bt-resistant counterparts. The hypothesis being tested was whether sterile males mating with the few surviving females, who had developed resistance, would lead to a reduced density of pests in the following generation. The result was a dramatic reduction in pink bollworms, with only two pink bollworm larvae being found by the third year of the study.
In India, Bt cotton has been enveloped in controversies due to its supposed links with seed monopolies and farmer suicides. However, the link between the introduction of Bt cotton to India and a surge in farmer suicides has been refuted by other studies, with farmer suicides actually having fallen since the introduction of Bt cotton according to some studies. Bt cotton accounts for 93% of cotton grown in India.
Notes and references
- http://www.umt.edu/ethics/debating%20science%20program/odc/Biotechnology/Alternatives/Bt%20Cotton1/default.php, 2013.
- Susan Lang (July 25, 2006). "Seven-year glitch: Cornell warns that Chinese GM cotton farmers are losing money due to 'secondary' pests". Cornell University.
- University of Montana. 2013. Bt Cotton. [online] Available at: http://www.umt.edu/ethics/Debating%20Science%20Program/ODC/Biotechnology/Alternatives/Bt%20Cotton1/default.aspx [Accessed: 6 Nov 2013].
- "Maharashtra lifts ban on Mahyco’s Bt cotton seeds". Nagpur. Indian Express. May 6, 2013. Retrieved 2013-09-03.
- Choudhary, B. & Gaur, K. 2010. Bt Cotton in India: A Country Profile. ISAAA Series of Biotech Crop Profiles. ISAAA: Ithaca, NY
- James, Clive. 2009. Global Status of Commercialized Biotech/GM Crops: 2009. ISAAA Brief No.41. ISAAA: Ithaca, NY.
- Choudhary, B. & Gaur, K. 2010. Bt Cotton in India: A Country Profile. ISAAA Series of Biotech Crop Profiles. ISAAA: Ithaca, NY. James, Clive. 2009. Global Status of Commercialized Biotech/GM Crops: 2009. ISAAA Brief No.41. ISAAA: Ithaca, NY.
- "Maharashtra bans Bt cotton seeds", The Times of India, Aug 9, 2012 http://timesofindia.indiatimes.com/india/Maharashtra-bans-Bt-cotton-seeds/articleshow/15420778.cms
- Lu, Y; Wu, K; Jiang, Y; Guo, Y; Desneux, N (Jul 2012). "Widespread adoption of Bt cotton and insecticide decrease promotes biocontrol services". Nature 487 (7407): 362–5. doi:10.1038/nature11153.
- Kathage, J; Qaim, M (Jul 2012). "Economic impacts and impact dynamics of Bt (Bacillus thuringiensis) cotton in India". Proc Natl Acad Sci U S A 109 (29): 11652–6. doi:10.1073/pnas.1203647109.
- Morse, S; Bennett, RM; Ismael, Y (2005). "Genetically modified insect resistance in cotton: some farm level economic impacts in India". Crop Prot 24 (5): 433–40. doi:10.1016/j.cropro.2004.09.008.
- Wu, K-M; Lu, Y-H; Feng, H-Q; Jiang, Y-Y; Zhao, J-Z (Sep 2008). "Suppression of cotton bollworm in multiple crops in China in areas with Bt toxin-containing cotton". Science 321 (5896): 1676–8. doi:10.1126/science.1160550.
- Lu, Y; Wu, K; Jiang, Y; Xia, B; Li, P; Feng, H et al. (2010). "Mirid bug outbreaks in multiple crops correlated with wide-scale adoption of Bt cotton in China". Science 328 (5982): 1151–4. doi:10.1126/science.1187881.
- Tabashnik, BE; Brévault, T; Carrière (2013). "Insect resistance to Bt crops: lessons from the first billion acres". Nat Biotechnol 31: 510–21. doi:10.1038/nbt.2597.
- Jayaraman KS. India investigates Bt cotton claims. Nature. 2012
- Tabashnik, BE; Sisterson, MS; Ellsworth, PC; Dennehy, TJ; Antilla, L; Liesner, L et al. (Dec 2010). "Suppressing resistance to Bt cotton with sterile insect releases". Nat Biotechnol 28 (12): 1304–7. doi:10.1038/nbt.1704.
- Wan, P; Huang, Y; Wu, H; Huang, M; Cong, S; Tabashnik, BE et al. (2012). "Increased frequency of pink bollworm resistance to Bt toxin Cry1Ac in China. Smagghe G, editor". PLOS ONE 7 (1): e29975. doi:10.1371/journal.pone.0029975.
- Hutchison, WD; Burkness, EC; Mitchell, PD; Moon, RD; Leslie, TW; Fleischer, SJ et al. (Oct 2010). "Areawide suppression of European corn borer with Bt maize reaps savings to non-Bt maize growers". Science 330 (6001): 222–5. doi:10.1126/science.1190242.
- Vandana, Dr. Shiva; Afsar H. Jafri (Winter 2004). "Failure of GMOs in India". Research Foundation for Science, Technology and Ecology. Retrieved 27 September 2013.
- Gruere, Guillaume (2008). "Bt Cotton and farmer suicides in India: Reviewing the Evidence" (PDF). International Food Policy Research Institute. Retrieved 24 February 2014.
- Plewis, I (2014). "Indian farmer suicides: Is GM cotton to blame?". Significance 11 (1): 14–18. doi:10.1111/j.1740-9713.2014.00719.x.
- Jayaraman, K.S (14 February 2012). "India investigates Bt cotton claims". Nature - International weekly journal of science. Retrieved 27 September 2013.