HTIR-IPM for the West

A service of the Western Regional Work Group on Integrated Pest Management:
Consequences of Herbicide Tolerant and Insect Resistant Crops,
in cooperation with Colorado State University

Insect-Resistant Crops

• Introduction
• Cry gene classification
• Will insect pests become resistant to Bt toxins?
• Bt corn
• Bt cotton
• Acreage statistics for insect-resistant crops in the West

Introduction
Bt insect resistant corn and cotton represent a major portion of total transgenic crop acreage, accounting for 17% of the global transgenic area in 2002 (James, 2002). An additional 8% of the area was planted to corn and cotton varieties with "stacked" genes, combining both herbicide tolerance and Bt insect resistance.

"Bt" is short for Bacillus thuringiensis, a commonly occurring 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 receptors in the insect's intestinal lining, leading to the creation of pores, ion imbalance, paralysis of the digestive system, and after a few days, insect death.

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. Organic growers, in particular, value this biological method of insect control. 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 feeds on a leaf or bores into a stem of a Bt-containing plant, it ingests the toxin and will die within a few days.

The development of Bt and other insect resistant transgenic crops is reviewed by Babu et al. 2003. [Return to top]

Cry gene classification
Many different versions of the Cry genes, sometimes known as "Bt genes", have been identified. Originally, these were named and classified according to their effectiveness against certain orders of insect. For example, CryI genes encoded proteins that were toxic to Lepidoptera larvae (moths and butterflies), CryII proteins were toxic to both Lepidoptera and Diptera (flies), CryIII proteins were toxic to Coleoptera (beetles), etc. As more Cry genes were discovered and characterized, the system became increasingly inconsistent and difficult to implement. Therefore, in 1998 an international group of scientists proposed a revised system for Cry gene classification based on DNA sequence rather than functional similarity (Crickmore et al. 1998). In the new system, which has been widely adopted, Roman numerals are replaced with Arabic numerals in the primary ranking, followed by upper-case and lower-case letters, and another Arabic numeral. Some examples of old and new designations are listed in Table1.

Table 1. Original and revised nomenclature for selected Cry genes (Crickmore et al. 1998), crops in which they have been engineered, and examples of transgenic events that incorporate the genes. Event names are linked to the Ag Biosafety database where more details are available. Not all of these Bt crops are currently on the market.

* MON-15985 is a stacked event encoding both Cry1Ab and Cry1Ac proteins.

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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. The biochemistry and genetics of insect resistance to Bt are discussed by Ferre and Van Rie 2002. If pests do develop resistance to Bt, 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 has required 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.

However, research after seven seasons of commercial cultivation of Bt corn and cotton has indicated that little or no resistance to Bt has developed in insect populations (Fox 2003; Tabashnik et al. 2003a; Tabashnik et al. 2003b).

Combining Bt toxins that target the same pest but possess different modes of action has long been a theoretically attractive strategy for slowing the development of Bt resistance. Now there is experimental data that support this strategy (Gould 2003; Zhao et al. 2003). Broccoli plants with two distinct Cry genes resulted in significantly delayed development of resistance in diamondback moths compared to plants with single Cry genes.

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. Refuges for corn rootworm Bt hybrids are explained in Monsanto's Yieldgard Rootworm Insect Resistance Management Guide, http://www.monsanto.com/monsanto/us_ag/content /biotech_traits/yieldgardRootworm/irm.pdf. A comparison of refuge requirements for Bt hybrids targeted to corn rootworm and European corn borer is contained in Managing Corn Pests with Bt Corn: Some Questions and Answers.

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, 14% of U.S. Bt corn farmers still breaking the rules and More U.S. farmers are following the rules for Bt refuges.

Have Bt crops reduced the use of chemical pesticides?
Due to the difficulty of reaching stalk boring insects with insecticide sprays, most farmers do not apply chemical controls to their conventional corn fields. Therefore, Bt hybrids substituted for chemical pesticides on only about 20% of the total U.S. Bt-corn area (Ferber, 1999). For more information see a discussion of pesticide use on Bt crops.

Beyond Bt
Some scientists have begun looking to other sources of insecticidal proteins for use in crop genetic engineering. See Babu et al. 2003, Liu et al. 2003, and Moar 2003 for information on this topic.

Additional Information
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

Research Q&A: Bt Corn and Monarch Butterflies. USDA-Agricultural Research Service. http://www.ars.usda.gov/is/br/btcorn/

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/pubs-home.html#Gene

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

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Acreage statistics for insect-resistant crops in the West

Crop	Total acreage, 2006 (1,000 acres)	Estimated percent that is insect- resistant	Estimated acres that are insect- resistant
Alfalfa, new seeding	711	na	na
Canola	16	nc	nc
Corn	2,399	20	480
Cotton	890	26	231
Sorghum	374	>90	>337
Sugar beet	386	nc	nc
Sunflower	140	nc	nc
Wheat	13,581	11	1,494
Note . nc, not currently commercialized. Data from the National Agricultural Statistics Service, 2006, the National Center for Food and Agricultural Policy, and regional experts.

 

Insect-resistant crops in Western states
Bt corn to combat corn borer
State	Thousands of acres	Percent of crop acres	Year	Source
Arizona	17	75	2003	Clark
California
Colorado	298	35	2003	Peairs
Idaho
Montana
New Mexico	22	55	2003	Carpenter
Oregon
Utah
Washington
Wyoming
Bt cotton to combat corn rootworm
State	Thousands of acres	Percent of crop acres	Year	Source
Arizona
California
Colorado	3	<1	2003	Peairs
Idaho
Montana
New Mexico
Oregon
Utah
Washington
Wyoming
Bt cotton (Bollgard I)
State	Thousands of acres	Percent of crop acres	Year	Source
Arizona	168	77	2003	USDA
California	147	21	2003	USDA
Colorado
Idaho
Montana
New Mexico	22	35	2003	USDA
Oregon
Utah
Washington
Wyoming
Bt cotton (Bollgard II)
State	Thousands of acres	Percent of crop acres	Year	Source
Arizona	<1	<1	2003	USDA
California	0	0	2003	USDA
Colorado
Idaho
Montana
New Mexico	0	0	2003	USDA
Oregon
Utah
Washington
Wyoming
Data extracted from "Impact on US Agriculture of Biotechnology-Derived Crops Planted in 2003 - An Update of Eleven Case Studies", October 2004, S. Sankula and E. Blumenthal, National Center for Food and Agricultural Policy, http://www.ncfap.org.whatwedo/pdf/2004finalreport.pdf.

James, C. 2002. Global status of commercialized transgenic crops: 2002. ISAAA Briefs No. 27. ASAAA: Ithaca, NY. http://www.isaaa.org/home.htm .

Babu, R.M., A. Sajeena, K. Seetharaman, and M.S. Reddy. 2003. Advances in genetically engineered (transgenic) plants in pest management - an overview. Crop Protection 22:1071-1086.

Crickmore , N., D.R. Zeigler, J. Fertelson, E. Schnepf, J. van Rie, D. Lereclus, J. Baum, and D.H. Dean. 1998. Microbiology and Molecular Biology Reviews 62:807-813.

Munkvold, G.P., and Hellmich, R.L. 1999. Genetically modified, insect resistant corn: Implications for disease management. APSnet Feature: (www.scisoc.org/feature/BtCorn/)

Ferré, J. and J. Van Rie. 2002. Biochemistry and genetics of insect resistance to Bacillus thuringiensis. Annual Review of Entomology 47:501–533.

Fox, J.L. 2003. Resistance to Bt toxin surprisingly absent from pests. Nature Biotechnology 21:958-959.

Tabashnik , B.E., Y. Carrière, T.J. Dennehy, S. Morin, M.S. Sisterson, R.T. Roush, A.M. Shelton, and J.-Z. Zhao. 2003a. Insect resistance to transgenic Bt crops: Lessons from the laboratory and field. J. Econ. Entomol. 96:1031-1038.

Tabashnik , B.E., Y. Carrière, T.J. Dennehy, S. Morin, M.S. Sisterson, R.T. Roush, A.M. Shelton, and J.-Z. Zhao. 2003b. Insect resistance to Bt crops: Lessons from the first seven years. Information Systems for Biotechnology. Nov. 2003. http://www.isb.vt.edu/news/2003/news03.nov.html.

Gould, F. 2003. Bt-resistance management-theory meets data. Nature Biotechnology 21:1450-1451.

Zhao, J.Z., J. Cao, Y. Li, H.L. Collins, R.T. Roush, E.D. Earle, and A. M. Shelton. 2003. Transgenic plants expressing two Bacillus thuringiensis toxins delay insect resistance evolution. Nature Biotechnology 21:1493-1497.

Ferber, D. 1999. Risks and benefits: GM crops in the cross hairs. Science 286:1662-1666.

Liu, D., S. Burton, T. Glancy, Z.S. Li, R. Hampton, T. Meade, and D.J. Merlo. 2003. Insect resistance conferred by 283-kDa Photorhabdus luminescens protein TcdA in Arabidopsis thaliana. Nature Biotechnology 21:1222-1228.

Moar , W.J. 2003. Breathing new life into insect-resistant plants. Nature Biotechnology 21:1152-1153.