|
Bacillus thuringiensis (Bt) is a common soil bacterium that produces proteins that are toxic to specific classes of insects, including certain moths, beetles, and flies. It has been used as a biological control agent against these insects, which are agricultural pests, for at least 40 years. It is only effective against young insects (larvae). BT proteins are transformed into toxins by specific digestive enzymes present in susceptible insects; these toxins cause the eventual death of the insect within a few days. The digestive enzymes are present only in certain insect species. Thus, Bt proteins and Bt-treated crops are generally safe for humans, animals and most other beneficial insect species.
Use of Bt in Agriculture
Traditionally, spores (reproductive cells) of the Bt bacterium were used in spray formulations for insect control. Modern genetic engineering allows the genes for Bt proteins to be inserted directly into crops, so that the plants themselves produce the insecticidal proteins. This has led to the development of crops such as Bt corn and Bt cotton.
Advantages
Controlling insects reduces crop damage, and improves crop yields and quality. Bt insecticides are biodegradable and non-toxic, an improvement over strong chemical pesticides. These spores are approved for use in organic agriculture as natural pesticides.
Genetically engineered Bt crops have the increased advantages of:
· requiring less insecticide application with machines, leading to reduced labor and fuel costs;
· greater effectiveness in insect control because the crops continuously produce the Btproteins within their own tissues;
· control of molds and fungi that can infect holes in the crops left by burrowing insects.
Concerns
Many of the concerns about using Bt insecticides are similar to the concerns over using chemical insecticides. One concern is the development of resistant insects to the insecticide, in this case the Bt toxin. The industry uses a strategy known as “high dose refuge”, where some crop areas are not treated with Bt or planted with Bt plants. The insects within these areas are not exposed to the Bt toxin, and do not develop resistance. The resistant gene is not active in the offspring when resistant insects breed with the non-resistant ones, thus maintaining insect populations to which Bt toxins are fatal.
Although Bt proteins are fatal only to certain insects, there is some concern over the impact on organisms that are not the intended target of the insecticide, such as beneficial insects in the soil, or butterflies that might feed on plant material or pollen. A wide variety of Bt proteins exist that affect various types of insects differently. One way to deal with this concern could be to use genetic engineering to make forms of the Bt protein that are known to have no effect on organisms other than the targeted one.
A particular concern with Bt crops is that the Bt genes could be spread to other plants through open pollination, which is known as “gene flow”. Open pollination is when pollen is spread by the wind to any nearby plants, where insect resistance in these plants could then develop. The potential environmental impacts have yet to be fully explained, and research is ongoing. In the U.S., the Environmental Protection Agency requires a certain distance between Bt crops and areas with wild plants to limit gene flow.
Bt crops in Canada
The Canadian Food Inspection Agency and Health Canada work together to assess and regulate Bt crops in Canada. Bt crops that are approved in Canada include corn, cotton, potatoes and tomatoes. According to Health Canada, these products do not present health risks and do not require labeling. The Canadian General Standards Board has issued a national standard, stating that labeling of genetically modified foods is voluntary; the national standard contains guidelines for producers who wish to label their products to help consumers make informed choices.
Council for Biotechnology Information. (2007, August 12). Regulation of biotech crops and food in the United States and Canada. Biotech Brasil. Retrieved from, http://www.biotechbrasil.bio.br/
Macdonald, P., and Yarrow, S. (2003). Regulation of Bt crops in Canada. J Invertebr Pathol. 83(2):93-9.
Marvier, M., McCreedy, C., Regetz, J., Kareiva, P. (2007). A meta-analysis of effects of Bt cotton and maize on nontarget invertebrates. Science, 316(5830), 1475-1477. doi: 10.1126/science.1139208
Nester, E.W., Thomashow, L.S., Metz, M., and Gordon, M. (2002). 100 Years of Bacillus thuringiensis: A critical scientific assessment. AmericanAcademy of Microbiology, Washington, DC.
Sanchis, V., Bourget, D. (2008). Bacillus thuringiensis: applications in agriculture and insect resistance management. A review. Agron. Sustain. Dev., 28, 11-20. doi:10.1051/agro:2007054
Sears, M.K., Hellmich, R.L., Stanley-Horn, D.E., Oberhauser, K.S., Pleasants, J.M., Mattila, H.R., Siegfried, B.D., and Dively, G.P. (2001). Impact of Bt corn pollen on monarch butterfly populations: a risk assessment. Proc Natl Acad Sci USA. 98(21):11937-42.
|