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Background on Bt Cotton

What does B.t. stand for, and what is Bt cotton?
Bt stands for Bacillus thuringiensis which is a soil bacterium. There are numerous different strains of Bacillus thuringiensis. Some strains produce protein molecules that are toxic to caterpillars (order: Lepidoptera), some make proteins toxic to beetles (order: Coleoptera), and some to mosquitoes (which are from the order Diptera). (If you want to learn more about the different orders of insects, check out the Web site for the Tree of Life at http://phylogeny.arizona.edu/tree/.)

However, in all cases the toxins are detrimental to a limited range of pest species and are essentially non-toxic to beneficial organisms and humans.

Bt toxins are proteins, they are encoded by genes on the Bacillus thuringiensis genome. With the development of biotechnology, it is now possible to take the DNA sequences of these genes and insert them into plants, such as cotton or corn—hence, the cotton that has this foreign DNA sequence is called Bt cotton (promoter sequences are added to allow the genes to be expressed and possibly some other sequences are also introduced so scientists can readily tell whether the plant has the Bt gene). Organisms that have DNA from other species inserted into their genomes are called "transgenic" or "genetically engineered" since their genetic makeup has been altered by insertion of genes (DNA sequences) from either another species or constructed by humans. The plant typically grows like a normal plant except that, in addition to its normal proteins, it also produces the Bt protein that is toxic to some insects. The first steps of creating a transgenic organism is done in a laboratory.

The transgenic plant looks and grows like a normal plant except that it produces the Bt protein that is toxic to caterpillars. This plant is then bred and progeny plants that make the Bt protein are chosen and grown to make seeds for farmers to plant. For technical details.

The Bt cotton currently registered for use in Arizona and elsewhere in the USA contains the Bt toxin Cry1A (i.e., the delta endotoxin) gene of the kurstaki variant, or Btk. Details on the registration can be found at the U.S. EPA site Fact Sheets at http://www.epa.gov/pesticides/biopesticides/ingredients/

Why should I care about B.t. and insects resistance to the B.t. toxin?
B.t. has been an extremely safe insecticide for farm and home use for approximately 50 years. Many other insecticides have greater potential for causing harm to humans and/or the environment. Insects cause much damage to our agricultural crops and in controlling them, other things equal, it is better to use insecticides with less potential for causing harm to humans or the environment.

Why be concerned about B.t. now?

With the introduction in 1996 of Bt cotton, corn, and potatoes for commercial use, some insects are now being exposed to the Bt toxins much more intensively than they have in the past. Instead of limiting the application of the insecticides to only those times when insects reach damaging densities, the transgenic plants are always making the toxin so the insects are always getting exposed to the toxin. One big advantage of this strategy is that insects that develop inside a plant, and are therefore protected from insecticide sprays, can also be killed. The key disadvantage is that the insects are continually exposed to the B.t. toxin and so are substantially more likely to develop 'resistance' — meaning they are likely to change genetically so they are no longer affected by the B.t. toxin. If resistance develops and spreads through an insect population, we could lose the use of an extremely safe insecticide. Developing strategies that will delay the emergence of resistance is therefore beneficial for preserving this new technology.

Some relevant books and publications can be found under Publications.

For discussion of Pros and Cons of Bt use, see Pros and Cons

Learning more

Link to pages:

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Cotton Mainpage | Bt Cotton Mainpage | Background | Why Bt?
Resistance Management | Regulations | Contact Info | Publications | Site Map

document located at: http://cals.arizona.edu/crops/cotton/btcotton/
The Department of Entomology
College of Agriculture and Life Sciences
The University of Arizona
Contact: BtCotton@ag.arizona.edu or Al Fournier (fournier@ag.arizona.edu)
All contents copyright ©2002 by Elizabeth Willott. All rights reserved.
Last modified March 5, 2004