Crops, Traits, and Acreage Herbicide Tolerance Bt Insect Resistant Crops Insect Resistance to Bt Toxins Papaya Global Status of Approved Genetically Modified Plants Transgenic Foods on the Supermarket Shelves Greenpeace's List of GM and Non-GM Foods Discontinued Transgenic Products

Crops, Traits, and AcreageThe most important transgenic crop in terms of acreage planted is soybean, followed by corn, cotton, and canola.

The number of acres for each crop are given in the graph below (Source: James, 2002, 2001a, 2001b, 1997). This graph is also available in hectares instead of acres.

Adoption of transgenic crops in the United States has been far greater than in many other countries. The following graph shows the acreage of transgenic crops in the United States from 1996 to 2003.

In 1999, the area planted to transgenic varieties was approximately half of the U.S. soybean crop and about 25% of the U.S. corn crop. The estimated worldwide area planted to transgenic varieties in 2000 increased 11% over the 1999 area (James, 2000b). Most of the transgenic crop varieties currently grown by farmers are either herbicide tolerant or insect pest-resistant. In addition to the crops listed below, minor acreages were planted to transgenic potato, squash, and papaya.

For information on transgenic crop acreage as a percentage of the total U.S. acreage in 2000, see the news update entitled Acreage for transgenic cotton and soybeans up, corn down.

Herbicide Tolerance
Weed control is one of the farmer's biggest challenges in crop production, because poorly controlled weeds drastically reduce crop yield and quality. Many herbicides on the market control only certain types of weeds, and are approved for use only on certain crops at specific growth stages. Residues of some herbicides remain in the soil for a year or more, so that farmers must pay close attention to the herbicide history of a field when planning what to plant there.

Herbicide tolerant crops resolve many of those problems because they include transgenes providing tolerance to the herbicides Roundup® (chemical name: glyphosate) or Liberty® (glufosinate). These herbicides are broad-spectrum, meaning that they kill nearly all kinds of plants except those that have the tolerance gene. Thus, a farmer can apply a single herbicide to his fields of herbicide tolerant crops, and he can use Roundup and Liberty effectively at most crop growth stages as needed. Another important benefit is that this class of herbicides breaks down quickly in the soil, eliminating residue carry-over problems and reducing environmental impact. Herbicide tolerant varieties are popular with farmers because they enable less complicated, more flexible weed control. These varieties are commonly marketed as Roundup Ready® or Liberty Link® varieties.

Weed-infested soybean plot (left) and Roundup Ready® soybeans after Roundup treatment. Source: Monsanto

For more information on herbicide tolerant transgenic crops see the article "Herbicide Tolerant Soybeans: Why Growers Are Adopting Roundup Ready Varieties", J. Carpenter & L. Gianessi, AgBioForum online journal, Vol. 2 No. 2, http://www.agbioforum.org/

Bt Insect-Resistant Crops

European corn borer (left) and cotton bollworm (right) are two pests controlled by Bt corn and cotton, respectively. Source: USDA.

"Bt" is short for Bacillus thuringiensis, a soil bacterium whose spores contain a crystalline (Cry) protein. In the insect gut, the protein breaks down to release a toxin, known as a delta-endotoxin. This toxin binds to and creates pores in the intestinal lining, resulting in ion imbalance, paralysis of the digestive system, and after a few days, insect death. Different versions of the Cry genes, also known as "Bt genes", have been identified. They are effective against different orders of insects, or affect the insect gut in slightly different ways. A few examples are shown in the table below.

The use of Bt to control insect pests is not new. Insecticides containing Bt and its toxins (e.g., Dipel, Thuricide, Vectobac) have been sold for many years. Bt-based insecticides are considered safe for mammals and birds, and safer for non-target insects than conventional products. What is new in Bt crops is that a modified version of the bacterial Cry gene has been incorporated into the plant's own DNA, so that the plant's cellular machinery produces the toxin. When the insect chomps on a leaf or bores into a stem of a Bt-containing plant, it ingests the toxin and will die within a few days.

Bt insect-resistant crops currently on the market include

• Corn: primarily for control of European corn borer, but also corn earworm and Southwestern corn borer. A list of approved Bt hybrids is available through the National Corn Growers Association web site (http://www.ncga.com/biotechnology/know_where/know_grow_approved.htm). Click on the "event" name to see the list of hybrids.

• Cotton: for control of tobacco budworm and cotton bollworm

• Potato: for control of Colorado potato beetle. Bt potato has been discontinued as a commercial product. See our Discontinued Products page for more information.

Corn hybrid with a Bt gene (left) and a hybrid susceptible to European corn borer (right). Source: Monsanto

Results of insect infestation on Bt (right) and non-Bt (left) cotton bolls. Source: USDA

Corn hybrids resistant to corn rootworm Corn rootworm (Diabrotica spp.) is a serious pest of corn in many U.S. growing areas. It damages roots of young corn seedlings, resulting in reduced growth and poor standability of the plant. This insect is responsible for the application of the largest amount of insecticide to U.S. corn fields. What's more, to control this pest the insecticide must be applied directly to the soil, where it may leave residues or leach into the ground water. By replacing these chemical insecticides, corn rootworm resistant hybrids may provide major benefits to environmental quality.

Corn rootworm feeding on a young maize root. Source: USDA

Range of damage due to corn rootworm feeding, from severe (left) to no damge (right). Source: USDA

Although rootworm-protected hybrids apparently offer pest management and environmental benefits, there are serious concerns about development of resistance to Bt in this adaptable insect. More information about rootworm-resistant Bt hybrids is available in articles by Byrne (2001), Moellenbeck et al. (2001), and Ostlie (2001). Michigan State University has a discussion of rootworm-protected corn at http://www.msue.msu.edu/ipm/CAT02_fld/FC4-11-02.htm#4. Rootworm-resistant corn was approved in 2003.

Have Bt crops reduced the use of chemical pesticides?
The use of Bt varieties has dramatically reduced the amount of chemical pesticides applied to cotton. According to a story in Science (Ferber, 1999a), US farmers used 450,000 kg less pesticides on Bt-cotton than they would have used on conventional varieties in 1998. Yields and profits also improved in Bt-cotton fields. The benefits from Bt-corn, however, were not as clear-cut. Due to the difficulty of effectively controlling corn borers with insecticides, most farmers do not apply chemical controls to their conventional corn fields. Thus, Bt hybrids substituted for chemical pesticides on only about 20% of the total US Bt-corn area. Profitability of Bt-corn is not as certain as for cotton; it will vary over years and locations, depending on the intensity of the corn borer population. See our discussion of pesticide use on Bt crops on this site.

Will insect pests become resistant to Bt toxins?
Although Bt genes have proven to be quite effective in the short term for protecting against crop insect damage, as well as reducing fungal contamination of corn [Munkvold and Heimlich, 1999, http://www.apsnet.org/online/feature/BtCorn/Top.html], there are concerns that widespread use of Bt varieties will accelerate development of resistance to Bt in the target pests. This could mean the loss of Bt as an effective, environmentally friendly insecticide. In response to these concerns, the U.S. Environmental Protection Agency has mandated measures to reduce the risk of resistance development. These measures depend on a combination of high dose of the Bt toxin and a planting of refuges. A refuge refers to an area planted to a non-Bt variety that is physically close to a field planted with a Bt variety, as shown in the diagram below.

Beginning in 2000, the EPA requires that farmers growing Bt corn must plant at least 20% of their total corn acreage to a non-Bt variety. The rationale is that the few Bt-resistant insects surviving in the Bt field would likely mate with susceptible individuals that have matured in the non-Bt refuge. Thus, the insect genes (alleles) for resistance to Bt would be swamped by the susceptible alleles. Whether this strategy will work or not remains to be seen. Some of the potential problems with the refuge strategy are:

• The frequency of Bt-resistant alleles in insect populations may be greater than assumed in refuge models.

• Resistance to Bt in European corn borer may be semi-dominant rather than recessive.

• Resistant insects surviving in the Bt field may mature several days later than susceptible insects in the refuge, thus preventing their mating.

For information on compliance with the refuge requirements, see the news updates entitled 29% of Bt corn farmers in U.S. broke the rules last year, 13% of Bt corn farmers in U.S. still breaking the rules, compliance improves, and 14% of U.S. Bt corn farmers still breaking the rules.

Ferre and Van Rie 2002 discuss the biochemistry and genetics of insect resistance to Bt.

A discussion of designs for refuges is available from the University of Illinois Extension Office at http://www.ag.uiuc.edu/cespubs/pest/articles/200203e.html. Pioneer Hybrid explains the rules for planting a refuge at http://www.pioneer.com/usa/agronomy/insects/yg_pug.pdf.

Additional Information
Because there are a number of web sites with extensive information on Bt crops, we refer you to them for additional information on the topic.

Managing Corn Pests with Bt Corn: Some Questions and Answers. F.B. Peairs, Colorado State University. http://www.colostate.edu/programs/lifesciences/TransgenicCrops/BtQnA.html

Bt Corn: Health and the Environment. F.B. Peairs, Colorado State University. http://www.ext.colostate.edu/pubs/crops/00707.html

Bt Corn & European Corn Borer: Long-Term Success Through Resistance Management. 1997. University of Minnesota Extension Service. http://www.extension.umn.edu/distribution/cropsystems/DC7055.html

Genetically modified, insect resistant corn: Implications for disease management. G.P. Munkvold, Iowa State University, and R.L. Hellmich, USDA-ARS. http://www.scisoc.org/feature/BtCorn/Top.html

The Environmental Protection Agency's White Paper on Bt Plant-Pesticide Resistance Management. 1998. http://www.epa.gov/fedrgstr/EPA-PEST/1998/January/Day-14/paper.htm

Monarchs and Bt corn: questions and answers. 1999. Marlin Rice, Iowa State University. http://www.ipm.iastate.edu/ipm/icm/1999/6-14-1999/monarchbt.html

Now or Never: Serious New Plans to Save a Natural Pest Control. Union of Concerned Scientists http://www.ucsusa.org/publications/report.cfm? publicationID=153

100 Years of Bacillus thuringiensis: A Critical Scientific Assessment. American Academy of Microbiology http://www.asmusa.org/acasrc/pdfs/Btreport.pdf

Papaya
Papaya is a tropical fruit rich in Vitamins A and C, but susceptible to a number of serious pests and diseases. The transgenic variety UH Rainbow, resistant to the papaya ringspot virus, is currently in production in Hawaii.

Papaya is an important source of vitamins in tropical areas. Source: USDA

For more information, refer to:
Transgenic virus resistant papaya: New hope for controlling papaya ringspot virus in Hawaii. http://www.apsnet.org/education/feature/papaya/Top.htm

Cornell University's page on virus resistant GM crops, including payapa. http://www.comm.cornell.edu/gmo/traits/virusres.html

Global Status of Approved Genetically Modified Plants
Agriculture and Biotechnology Strategies (Canada) Inc. maintains a database of trangenic plants that have been approved for environmental release, use in livestock feed, or use in human food. Information is organized by crop and by trait. The information can be accessed at http://www.agbios.com/_Synopsis.asp.

Transgenic Foods on the Supermarket Shelves
The cooperative extension office at Cornell University has assessed the likelihood that food products contain genetically engineered ingredients. Their assessment is available at http://www.comm.cornell.edu/gmo/crops/eating.html.

Greenpeace's List of GM and Non-GM Foods
Greenpeace, which campaigns against transgenic foods, maintains a list of food brands that they claim contain or do not contain transgenic ingredients. The list is available at http://www.truefoodnow.org/shopping_list.html.

Discontinued Transgenic Products
Several transgenic products that received approval for marketing have been discontinued for a variety of reasons. Some, such as the FlavrSavr tomato and NewLeaf potato, were available for years before they were discontinued. We have assembled a list of these products with links to more information about their history and the reason for their disappearance.