BT Corn
Introduction
With the ever-increasing human population comes the increased demand for food production in the limited farmable land that we have left. Due to various insects or plant pathogens, our main food crops require large amounts of pesticides to maintain as high a yield as possible. It is estimated that seven percent or 40 million tons of corn are unmarketable due to insect damage (Novartis, 2000). With the understanding of genetics and the availability of advanced technologies, scientists have found a way, via genetic modification, to maintain the high yields demanded of our food crops while avoiding insect damage and high pesticide use. These plants are commonly known as transgenic, or genetically modified (GM), plants.
Genetic Modification Procedure vs. Traditional plant breeding
To understand Bt corn, its purpose and the many debates surrounding it, one must first understand the simplified matter of the very complex issue of genetic modification. The first step is to detect and identify the gene desired. The desired gene, once separated and replicated, must have additions made to it. These additions include a promoter, the 'on-off' switch, which aids the desired gene in expressing its traits. A termination sequence is also added to identify the end of the modified gene sequence. Marker genes are also required; these 'identify' the modified gene once it has been incorporated into the plant. Markers used in GM corn include an herbicide-resistant marker and an antibiotic-resistant marker. Once the construction of the desired gene and its necessary parts is complete, the gene is then inserted into plant tissue. These plant tissues are grown on an artificial medium, containing either an herbicide or an antibiotic. Plant tissue successfully containing the marker gene will grow on this media, thereby 'identifying' itself as containing the modified gene. Those surviving plants are then reproduced to assure that the offspring also have the modified gene (CO State, 2000).
In the case of Bt corn, the desired gene comes from the biological insecticide, Bacillus thuringiensis, or Bt. Bt is a naturally occurring bacterial organism that has been known to man since the 19th century (Boucias & Pendland, 1998a). Bt represented more than 90 percent of the biopesticides used worldwide in the 1990's (Boucias & Pendland, 1998b). Bt is a spore-forming bacteria that, during spore formation, produces a crystalline structure. Within this crystal is a protein containing a toxin known as delta-endotoxin (Metcalf & Luckmann, 1994a). Bacillus thuringiensis alone contains more than 34 different subspecies, with each having a variety of 4 different crystal genes (Metcalf & Luckmann, 1994b). These crystals have been given different names, such as Cry 1Ab, due to the manner and number of protoxins (protein toxins) that are present (Metcalf & Luckmann, 1994c). The crystal, when ingested by the target insect, breaks down, releasing the delta-endotoxin. This results in interruption of feeding by the insect, as well as gut paralysis, destruction of midgut epithelium and resulting disruption of cell membrane permeability (Metcalf & Luckmann, 1994d). These effects can occur within an hour of ingestion (Boucias & Pendland, 1998c). Use of a specific subspecies of Bacillus thuringiensis and its corresponding Cry gene depend on which insect will be targeted. Most Bt corn developed contains Cry1Ab, which is found in Bt kurstaki Hd-1, Bt aizawai or Bt sotto (EPA, 1999a) (Boucias & Pendland, 1998d). These microbial agents are fairly host-specific.
European Corn Borer
In the case of Bt corn, a Cry gene specific to the order Lepidoptera is used for the target insect, the European Corn Borer, Ostrinia nubilalis. This moth, native to Europe, is considered to be one of the most destructive corn pests known (Metcalf & Metcalf, 1993a) with an estimated $1billion spent annually in the US due to crop loss and control measures (IAState, 1996a). The European Corn Borer, introduced to the US around 1917, spread throughout all the corn-growing regions of this country by 1952 (Metcalf & Metcalf, 1993b). This insect damages corn during its larval stage by boring into the stalks of corn and then feeding on the interior of the plant or on the ears or leaves. This moth is multivoltine (>1 generation/season) in the southern and eastern areas of its range, causing considerably more damage than the univoltine variety found in the more north central areas (Metcalf & Metcalf, 1993c). All in all, Ostrinia nubilalis is now found in all but the seven most western states of this country (IAState, 1996b).
With the European Corn Borer being such a destructive force on our country's corn crops, scientists entered into transgenic research resulting in the development of Bt corn.
Excel graph of the effect of corn borer on Bt and non-Bt corn Stalks
Fear of Biotechnology
Many questions and concerns have been raised in response to this plant and whether it will have positive or negative impacts on the various components of our ecosystem. The public in general questions the wisdom in tinkering with the genetics of this bacteria and plant. People must recall, though, that man has been interfering with the genes of countless plants for many years in his or her quest for bigger and better. Hybridization of plants has rarely, however, dealt with the introduction of a completely foreign gene into a plant though. The long-term affects of this remain to be seen. In the meantime, Bt corn appears to have many positive aspects.
Benefits
The first and foremost benefit will be the decreased use of insecticides, such as organophosphates and pyrethroids. EPA has reported that the use of insecticides for Bt sweet corn has dropped by about 90 percent annually, which equates to a savings of 700,000 pounds of insecticide per year (EPA, 1999b). Considering that the US has approximately 80 million acres devoted to corn and the prediction that greater than half of that will be planted with Bt corn within 5 years (UIUC, 1998), this means a massive reduction in insecticide use overall.
Decreased insecticide use means several things for our ecosystem. Beneficial and non-target insects will be saved from the very non-specific chemicals once used. Bt, although registered as an insecticide, should not have any affect on the majority of these insects due to its host-specificity (Metcalf & Luckmann, 1994e). Fewer insecticides will equal decreased chemical leaching into wells and waterways and decreased exposure to potentially harmful chemicals by humans and other animals. The possible impact on the monarch butterfly has been raised, due to its classification in the order Lepidoptera, but the majority of information claims that the Monarch won't be affected by pollen containing Bt (Farmsource, 2001a). The Washington Times reported that researchers actually felt that the Monarch larvae manage better in fields of Bt corn due to less stress from predators (Farmsource, 2001b).
Another supposed benefit of Bt corn is its lack of human health risks and its action in decreasing other diseases of corn. The EPA feels pretty certain that aggregate exposure to Cyr1Ab will be harmless, not only to adults, but also to children (EPA, 2001a). In toxicity tests, the LD50 for Cry1Ab showed no dose related deaths at an amount of 4000mg/kg (oral) so the EPA has exempted this protein from tolerance levels (EPA, 2001b). The 4000 mg/kg was the largest amount possible to subject a test subject to.
Negative Side Effects
It is of positive note that the protein is considered to have no mammalian toxicity, but the public's view of the absence of tolerance levels may not sit too well. With the concern over ingesting modified genetic products, I believe that people may be unhappy about the quantity of GM organism that they may be eating. This being said, the EPA also reports that Cry1Ab should cause no allergic reactions in people due to its rapid degradation in our acidic digestive systems (EPA, 2001c). Allergenicity is the primary reason for the huge mistake made in contamination of food corn with Starlink corn. The Cry protein (Cry 9C) in Starlink corn has not been sufficiently tested for potential allergic reactions and is considered unsafe for human consumption at this time. Another issue with the Starlink corn, which requires further investigation, is that it is not easily digestible in our systems and remains stable at high temperatures (EPA, 2001a). Aside from oral ingestion, the majority of the population will have little or no exposure to the modified gene products as they are contained within plant cells. The EPA also states that there are no acknowledged metabolites in the modified gene that could affect our endocrine functioning (EPA, 2001d).
In another human health-related field are the findings that Bt corn plants have a decreased capacity for containing mycotoxins. Mycotoxins are fungus-produced toxins found in peanut butter, corn and other grains. As the name suggests, these compounds are toxic and considered to be potential human carcinogens. One belief for the reduction in mycotoxins with GM corn is the reduced transmission by insects due to decreased plant -insect contact (VT, 1999).
On the negative side of genetically modified corn is the very substantial concern of insect resistance. We have witnessed time and time again the resistance acquired by various insects due to overuse or misuse of insecticides. Maintaining the efficacy of a very important biological agent such as Bt is key to the entire Bt corn issue and to our insect control as a whole. Two decades ago it was believed that resistance to Bt could not occur because only univoltine insects were treated with it and thereby did not have the opportunity to develop resistance (Boucias & Pendland, 1998e). The more times an insect is repeatedly exposed to an insecticide the greater the chances of it developing a resistant strain of offspring. This is highly applicable to the European Corn Borer, especially those strains that are multivoltine. The EPA, the USDA North Central Regional Research Committee (NC-205), The International Life Sciences Institute/Health and Environmental Sciences Institute and the Union of Concerned Scientists' have all developed insect resistance management (IRM) plans to address and decrease the occurrence of resistance to Bt (EPA, 1999c). The IRM strategy arrived at includes high-dose plants and planting a sanctuary of non-Bt corn where unexposed European Corn Borers can feed. These moths will supply unexposed genes to the European Corn Borer gene pool in each area of Bt corn (UMN, 1998a). This concept is highly critical as European Corn Borers do not travel long distances within their habitat and so practice 'local non-random mating' or inbreeding (UMN, 1998b). If this inbreeding were to occur among Bt-exposed moths, resistance would be rapidly bred into the population, thereby rendering Bt corn, and Bt in general, completely ineffective.
Safety Methods & Procedures
All the above researchers have concluded that the following refuge is necessary: 20 percent of the corn crop planted should be non-Bt corn. This acreage should also not be treated with other insecticides. If insecticide use is predicted, the refuge amount should increase to 40 percent. The refuge corn should be planted within every 320-acre plot that has Bt corn (UMN, 1998c). A secondary, but equally important, key to this plan is the distance between Bt corn and refuge corn, as once again, the moths do not travel far in seeking a mate. Farmers are thus required to plant the refuge crop within ½ mile of the Bt crop, although within ¼ mile distance is the preferred. The ¼ mile distance becomes required if the refuge corn may be treated with insecticide (EPA, 1999d). The high-dose attack portion of the IRM strategy involves exposure of the Bt corn-feeding moths to a very high amount of Bt endotoxin. This will allow highly exposed moths to mate with highly unexposed moths nearby, producing non-resistant offspring. In areas where corn and cotton are produced in close proximity there lies an exception to the above plan. Because corn and cotton share a pest in the Corn Earworm, the refuge size increases to 50 percent with the same distance requirements for Bt corn containing the Cry1Ab gene. Bt corn containing the Cry 1Ac gene cannot be grown in cotton-growing areas. The production of these two crops in the same area would increase the risk of the Corn Earworm's development of resistance due to its potential contiguous exposure to Bt (EPA, 1999e).
In relation to the IRM plan developed is the education of all Bt corn growers. If resistance is to be avoided, the people growing and working with this plant every day need a clear and definite understanding of the entire IRM plan. Aside from potential public opposition of Bt corn, I believe that resistance management could be the largest downfall for this technology. Any other individuals involved with the marketing, sale or distribution should be aware of the plan so that they too can act as advisors to the growers. One creator of Bt corn conducted a survey of growers in the corn-producing states to determine the understanding and actual implementation of the IRM plan. Those included in the survey were active farmers with a minimum of 200 corn acres. The participants were not related to or employed by chemical companies, seed companies or distributors of either product. All total, 71 percent of the growers followed the IRM plan in terms of refuge size and distance away. Those not consistent with the plan were either uneducated to the requirements or were incorrectly assuming that their refuge size was adequate (Monsanto, 2000).
Original PowerPoint Picture
USENET Newsgroups
Most Newsgroups on the net are either completely against the use of Bt corn, most of those being organic farmers, and others who believe that it is a far better option than to lose all their crops, those being Biotechnologists and some Farmers.
Some Newsgroups are:
http://www.starchefs.com/boardTopics/12/messages/53.html Anti-Bt Corn
http://iubio.bio.indiana.edu/R68661-75332-/news/bionet/women-in-bio/9905.newsm Bt Corn Pollen's effect on Monarch Butterflies
http://www.ibiblio.org/ecolandtech/agriculture-list-archives-1999-2001/market-farming/Dec2000/msg00078.html Pro-Bt Corn
Related Sites:
Monsanto Co.
Plant Pathology Online
Novartis
USDA
Authors: Dioscaris Garcia and Janet Weilburg
Next Page