Transgenic maize (corn) has been deliberately genetically modified to have agronomically desirable traits. Traits that have been engineered into corn are resistance to herbicides and incorporation of a gene that codes for the Bacillus thuringiensis (Bt) toxin, protecting plants from insect pests. Hybrids with both herbicide and pest resistance have also been produced. Transgenic maize is currently grown commercially in the United States.

Herbicide resistant corn

Corn varieties resistant to glufosinate (Liberty) herbicides and Roundup have been produced. There are also corn hybrids with tolerance to imidazoline herbicides marketed by Pioneer Hi-Bred under the trade mark Clearfield, but in these the herbicide tolerance trait was bred without the use of genetic engineering. Consequently the regulatory framework governing the approval, use, trade and consumption of transgenic crops does not apply for imidazoline tolerant corn.

Herbicide resistant GM corn is grown in the United States. A variation of herbicide resistant GM corn was approved for import into the European Union in 2004. Such imports remain highly controversial (The Independent, 2005).

Bt corn

Bt corn is a variant of maize, genetically altered to express the bacterial Bt toxin, which is poisonous to insect pests. In the case of corn, the pest is the European Corn Borer.

Expressing the toxin was achieved by inserting a gene from the lepidoptera pathogen microorganism Bacillus thuringiensis into the corn genome. This gene codes for a toxin that causes the formation of pores in the larval digestive tract. These pores allow naturally occurring enteric bacteria such as E. coli and Enterobacter to enter the hemocoel where they multiply and cause sepsis. (Broderick et al, PNAS 2006) This is contrary to the common misconception that Bt toxin kills the larvae by starvation.

In 2001, Bt176 varieties were voluntarily withdrawn from the list of approved varieties by the United States Environmental Protection Agency when it was found to have little or no Bt expression in the ears and was not found to be effective against second generation corn borers. (Current status of Bt Corn Hybrids, 2005)

Effects of Bt corn on nontarget insects

In May 1999, a laboratory at Cornell University published the results from a laboratory trial that appeared to indicate that the pollen of genetically modified Bt corn presented a threat to monarch caterpillars. Critics claimed that the popular media was wrong to report that monarch butterflies were threatened because this experiment did not duplicate natural conditions under which monarch caterpillars may come in contact with corn pollen. (Cornell News, 1999)

In 2001 the scientific journal the Proceedings of the National Academy of Sciences published six comprehensive studies that showed that Bt corn pollen does not pose a risk to monarch populations for the following reasons:

• The density of Bt corn pollen that overlay milkweed leaves in the environment rarely comes close to the levels needed to harm monarch butterflies. Both laboratory and field studies confirmed this.
• There is limited overlap between the period that Bt corn sheds pollen and when caterpillars are present.
• Only a portion of the monarch caterpillar population feeds on milkweeds in and near cornfields.

(Sears, et al., 2001)

Monarch populations in the USA during 1999 increased by 30%, despite Bt corn accounting for 30% of all corn grown in the USA that year. The beneficial effects of Bt corn on Monarch populations can be attributed to reduced pesticide use. (Trewavas and Leaver, 2001).

Numerous scientific studies continue to investigate the potential effects of Bt corn on a variety of nontarget invertebrates. A synthesis of data from many such field studies(Marvier et al. 2007) found that the measured effect depends on the standard of comparison. The overall abundance of nontarget invertebrates in Cry1Ab variety Bt corn fields is significantly higher compared to non-GM corn fields treated with insecticides, but significantly lower compared to insecticide-free non-GM corn fields. Abundance in fields of another variety, Cry3Bb corn, is not significantly different compared to non-GM corn fields either with or without insecticides.

Bees have been observed to forage on cracked corn kernels. This has not contributed to Colony Collapse Disorder.

Preventing Bt resistance in pests

By law, farmers in the United States who plant Bt corn must plant non-Bt corn nearby. These non-modified fields are to provide a location to harbor pests. The theory behind these refuges is to slow the evolution of the pests to the Bt pesticide. Doing so enables an area of the landscape where wild type pests will not be immediately killed.

It is anticipated that resistance to Bt will evolve in the form of a recessive allele in the pest. Because of this, a pest that gains resistance will have an incredibly higher fitness than the wild type pest in the Bt corn fields. If the resistant pest is feeding in the non-Bt corn nearby, the resistance is neutral and offers no advantage to the pest over any non resistant pest. Ensuring that there are at least some breeding pests nearby that are not resistant, increases the chance that resistant pests will choose to mate with a nonresistant one. Since the gene is recessive, all offspring will be heterozygous, and the offspring from that mating will not be resistant to Bt and therefore no longer a threat. Using this method scientists and farmers hope to keep the number of resistant genes very low, and utilize genetic drift to insure that any resistance that does emerge does not spread.

Cross pollination

The non-Bt pesticide status of the refuges is being compromised by wind-born pollen drifting into the non-Bt corn fields. Corn harvested from the supposed Bt-free zones has shown traces of Bt toxin. The levels found in the non-Bt corn decreases with distance from the Bt-corn fields indicating that the pollen is wind-borne rather than another method of transfer. The concentrations in the refuge fields were found to be low-to-moderate.

Possible solutions to the cross-pollination problem are to plant a wider refuge field or plant varieties of corn that bloom at different times than the Bt fields do. (Chilcutt & Tabashnik, 2004)

In sweet corn for human consumption

"Attribute" is the brand name for a line of Bt sweet corn. Seed is available only to large professional farmers who sign a stewardship agreement. The farmer must agree to not repackage or resell Attribute seed. Growers also must grow the corn exactly as directed. Herbicide resistant sweet corn has not yet been released for sale.

The StarLink corn controversy

StarLink is a variety of Bt corn patented by Aventis Crop Sciences (a subdivision of Aventis, acquired by Bayer AG in 2002), intended for use in animal feed.

U.S. regulatory authorities permitted the commercial sale of StarLink seed with the stipulation that crops produced must not be used for human consumption. This restriction was based on the possibility that a small number of people might develop an allergic reaction to the Bt protein used in StarLink that is less rapidly digested than the version used in other Bt varieties.

StarLink corn was subsequently found in food destined for consumption by humans. An episode involving Taco Bell taco shells was particularly well publicized . This led to a public relations disaster for Aventis and the biotechnology industry as a whole. Sales of StarLink seed were discontinued. The registration for Starlink varieties was voluntarily withdrawn by Aventis in October 2000.

28 people reported apparent allergic reactions related to eating corn products that may have contained the Starlink protein. However, the US Centers for Disease Control studied the blood of these individuals and concluded there was no evidence that the reactions people experienced were associated with hypersensitivity to the Starlink Bt protein .

Aid sent by the UN and the US to Central African nations also contained some StarLink corn. The nations involved refused to accept the aid.

The southern portion of the U.S. corn belt planted the greatest amount of StarLink corn. It is this portion of the U.S. where corn borer damage creates the greatest economic loss to farmers.

The US corn supply has been monitored for the presence of the Starlink Bt proteins since 2001. No positive samples have been found since 2004, showing that it was possible to withdraw this GM crop without leaving traces in the environment once it has been used in the field

Virus Resistant Corn

In 2007 researchers from South Africa anounced the production of transgenic maize resistant to Maize streak disease (MSD), caused by maize streak virus (MSV) . This maize is still in the research and development phase.

See also

• GM food

References

• "EU authorises Monsanto GMO maize for import". Reuters, .
• "EU deadlocked over latest GMO maize approval". Reuters, .
• "Food agency accused of Stalinist tactics over GM maize cover-up". The Independent, .
• Approval Status of Biotech Corn Hybrids. National Corn Growers Association (US). .
• "Engineered corn kills monarch butterflies". Cornell News, .
• Butterflies and Bt Corn. United States Department of Agriculture. .
• Mark K. Sears, Richard L. Hellmich, Diane E. Stanley-Horn, Karen S. Oberhauser, John M. Pleasants, Heather R. Mattila, Blair D. Siegfried, and Galen P. Dively (2001). "Impact of Bt corn pollen on monarch butterfly populations: A risk assessment". Proceedings of the National Academy of Sciences 98 (October 9): 11937–11942.
• Chilcutt, C.F., and B.E. Tabashnik (2004). "Contamination of refuges by Bacillus thuringiensis toxin genes from transgenic maize". Proceedings of the National Academy of Sciences 101 (May 18): 7526–7529.
• Biopesticides Registration Action Document: Preliminary Risks and Benefits Sections (Page 14). Environmental Protection Agency - Office of Science Coordination and Policy. .
• Current status of Bt Corn Hybrids. Kansas State University Research and Extension. .
• Bruce Chassy and Drew Kershen (2004). "Bt corn can reduce serious birth defects by limiting toxic mold". Western Farm Press (October 10):
• Marvier, M., C. McCreedy, J. Regetz, and P. Kareiva (2007). "A Meta-Analysis of Effects of Bt Cotton and Maize on Nontarget Invertebrates". Science 316 1475–1477.
• Broderick, N., K. Raffa, and J. Handelsman (2006). "Midgut bacteria required for Bacillus thuringiensis insecticidal activity". PNAS 103 15196–15199.

External links

• Bt Corn: Is it worth the RIsk? - a review from the Science Creative Quarterly
• GMO Safety - Biosafety research on genetically modified maize Projects and results
• Declaration of 500 European scientists: NO to a moratorium on the cultivation of GM maize
• Bees foraging on cracked corn kernels
• Cultivo de maíz OMG - BT en España
• Info on Attribute corn