What the heck is Bt? The Agricultural and Ecological Credentials of a Bacterial Bug Beater

By Tara L. Moreau

Introduction
Bacillus thuringiensis (pronounced as bah-sill-us thur-in-jee-en'-sis) is a living bacterium that has been used by organic growers, non-organic growers and in genetically modified crops to control a plethora of insect pests. Bt is a naturally occurring biological organism that is considered to be remarkably safe for both humans and the environment, due to its specificity against target insects and limited persistence in the environment. Bt products have had widespread, versatile and long-term use and are commonly promoted as an environmentally safe pesticide alternative.

History
Bt was initially discovery in Japan in 1901 from the larvae of diseased silkworm. Bt products were first marketed and available in France during the 1930's, however, it was not until 1954 that the insecticidal activity of Bt was understood. Researchers discovered that the insecticidal action of Bt against Lepidoteran (butterflies and moths) insects was due to the presence of crystal-like proteins.
Bt was used commercially in 1958 and was registered by EPA (Environmental Protection Agency) as a pesticide in 1961. In 1983, the first genetic modification of plants was reported and in 1987 researchers studying Bt successfully isolated and cloned the Bt crystal protein gene. Nine years later, in 1996, large-scale corn crops containing Bt transgenic genes were planted. Bt transgenic crop production rapidly increased to more than 14 million hectares grown worldwide in 2002. The cumulative area of Bt crops grown from 1996 to 2002 was estimated to be 62 million hectares (James 2002).

Mode of Action
The insecticidal nature of Bt is attributed to crystal-like proteins that are produced by Bt cells during the spore-forming stage of its life cycle. The crystal protein in Bt kills insects by binding to and disrupting the midgut (stomach) membranes. Susceptible insects will stop feeding within hours of ingesting Bt and will generally die within 2-5 days. The specificity of Bt lies in the fact that particular stomach enzymes (present only in certain insects) must activate the crystal proteins before they are considered toxic. Under normal conditions the crystal protein is insoluble (not active) and is therefore safe to humans, mammals and the majority of insects. The Bt toxin is most effective against young insect larvae, which are more susceptible because they are relatively small. Once in the environment, the insecticidal protein of Bt degrades quickly (1-4 days) following exposure to sunlight and microorganisms.

There are numerous strains of Bt, which may exhibit different toxicity to various organisms. In Canada, three subspecies are registered for use. Bt subspecies Kurstaki (BTK) is effective against a wide range of Lepidopteran species (see Table #1). Bt subspecies Israelensis (BTI) is used for controlling the larvae of mosquitoes and blackflies and Bt subspecies Tenebrionis (BTT) is registered for use against Coleoptera (beetles) like the Colorado potato beetle.

Two Main Types of BT
Bt is used in two main ways, either as a spray formulation or it is genetically modified into crops. The sprayable forms of Bt can be applied directly to plants (as a liquid, powder, dust or granule), or it can be applied to surface waters, furthermore, it can be used in the watering lines of greenhouses. For many years, both organic and non-organic farms throughout the world have used Bt sprays to control target pests. Bt suitability for organic production is currently being reviewed because some products are prohibited by organic certifying agencies because they contain inert ingredients, which are prohibited (Boiteau 2004).

Genetic modification is a relatively new tool used by scientists to add desirable traits to a plant or crop. The process involves physically removing the DNA from one organism and transferring the genes into another organism. For Bt-transgenic crops, a modified version of the Bt insecticidal gene is incorporated into the plants DNA. Bt crops are currently engineered to produce a single activated insecticidal crystal protein throughout all parts of the plant and they expose insect populations to Bt toxins throughout all stages of the growing season, thereby creating a much higher probability that target insects will develop resistance.

Health Canada has classified genetically modified foods as novel foods (Health Canada 2004). Novel foods are products that have never previously been used as a food; foods that result from a process that has not previously been used for food; or, foods that have been modified by genetic manipulation. There are currently over 60 genetically modified food items that have been approved for sale in Canada. Canadian approved novel foods that contain Bt include corn (BTK), potatoes (BTT), tomatoes (BTT) and cottonseed oil from cotton lines (BTK).

Bt Products Registered in Canada
The Pest Management Regulatory Agency (PMRA) is the government body that is in charge of approving all pest control products prior to their use in Canada. Of the 34 Bt products currently registered by the PMRA, 3 are marketed for domestic use, 7 for formulation use, 11 for commercial use and 14 for restricted use (PMRA 2004). Label directions should be followed closely to ensure proper application techniques and rates.

Can insects become resistant to BT?
Although researchers initially believed that insects would not develop resistance to biological insecticides, they soon discovered that biological sprays were not exempt. Insect resistance to Bt-sprays has been reported in both laboratory and field research. Field-evolved resistance to transgenic Bt crops has yet to be documented, however, evidence from Bt-spray research and the widespread use of transgenic Bt crops has raised concerns about future resistance developments (Tabashnik and Carriere 2004). There are currently no published reports of field-evolved resistance to Bt crops (Tabashnik 2004).

Is Bt safe for humans and the environment?
Bt is considered to be a remarkably safe alternative pesticide due to its specificity against target insects and limited persistence in the environment. This is demonstrated by the fact that Bt is the only insecticide for which there are no mandated residue limits on foods, however, the unprecedented adoption of transgenic crops has raised several concerns among the public. Concern over genetically engineered foods has tended to focus on the unknown health and environmental effects. Some apprehensions include potential ecological consequences of gene flow to non-engineered crops and wild relatives, possible effects on non-target organisms, and concerns of putting new things into the human diet that have not been eaten before. To alleviate public concerns, further research is required for both the short and long-term effects of genetically modified organisms in our environment and our food.

The specific nature of Bt proteins limits its toxic effects to only certain insects. Unfortunately, some beneficial insects can be affected. A controversial article published in Nature in 1999, indicated increased monarch butterfly mortality following the ingestion of pollen from Bt-transgenic corn crops. This report prompted further studies with different Bt varieties of transgenic corn and indicated that the actual risk to monarch butterflies was negligible. Researchers found that only one variety of Bt was especially damaging to the monarchs and has since been withdrawn from the U.S. market. Furthermore, studies indicated that the amount and distribution of corn pollen on the monarch's host plant, milkweed, was very low. Ongoing Bt research seeks to discover new, more specific varieties that do not hurt beneficial insects.

Although concerns have been raised about Bt-transgenic crops and more recently about the use of Bt for organic farms, the ecological impacts of Bt are generally less than chemical pest controls. Furthermore, farm workers have benefited from using Bt sprays in place of hazardous insecticides. Yet Bt like its synthetic competitors, Bt is still an insecticide and should be used with care, and only after trying cultural and physical insect control methods.

Table #1 - Bt target insect pests, Canadian registered domestic and commercial products and allowable organic Bt products.

References

Boiteau, G. personal communication (2004). Current debates on suitability of Bt for organic production. Date of communication: March 2004.

Health Canada. (2000). Fact Sheet on the Bacillus thuringiensis subspecies kurstaki. Pest Management Regulatory Agency, Ottawa, Ont.

Health Canada. (2004). [electronic resource] Food Program - Novel Foods. [Accessed 2004 September 16] Available from: http://www.hc-sc.gc.ca/food-aliment/e_index.html

James, C. 2002. Global status of commercialized transgenic crops: 2000. ISAAA Briefs No. 27: preview, Ithaca, NY, International Service for the Acquisition of Agri-biotech Applications.

Pest Management Regulatory Agency (PMRA). (2004). [electronic resource] EDDENet PMRA's Pilot Registration Site. [Accessed 2004 September 15]
Available from: http://www.eddenet.pmra-arla.gc.ca/4.0/4.1.asp

Tabashnik, B.E. and Y. Carriere. (2004) Bt transgenic crops do not have favorable effects on resistant insects. Journal of Insect Science, 4:4. Available online: insectscience.org/4.4

Tabashnik, B.E. personal communication (2004). Insect resistance to field-evolved Bt crops. Date of communication: September 2004.

This article first appeared in Rural Delivery, Volume 29 #5 (11-13)