Bacillus thuringiensis commonly known as Bt is a naturally-occurring and soil-dwelling gram positive bacterium. It was first discovered in 1901 by the Japanese biologist Shigetane Ishiwatari while investigating the cause of death of large silkworm population. The name Bacillus thuringiensis is however, coined by the German biologist Ernest Berliner who rediscovered the bacterium in 1911, and demonstrated its insecticidal activity on the flour moth caterpillars, The bacterium was named after the German town of Thuringia, where he described it. He reported the existence of a parasporal crystal within the microorganism but its activity was not known until the 1950’s (Federici et al, 2010). Although the mode of action Bt was not known, farmers started to use it as pesticide as early as 1920, and the first commercialized Bt-based formulations called sporeine became available in France in 1938 (Federici et al, 2010).
Hannay, Fitz-James and Angus in 1956, demonstrated that the insecticidal activity of Bt was due to the parasporal crystal and is composed of proteins. Since then Bt became prospective for insect pest management in different countries of the world. In the United States, Bt was first manufactured commercially in 1958 and registered as a pesticide in 1961 by the Environmental Protection Agency (EPA). Here we review Bt toxin its mode of action and its application in pest management.
Use of Bt toxins in insect pest management
With the rising resistance to chemical insecticides and the growing concern for environmental protection Bt biopesticide has been implemented in many country. Currently more than 10 different Cry toxins (Cry1Aa, Cry1Ab, Cry1Ac, Cry1C, Cry1D,Cry1E, Cry1F Cry2aA, Cry2aB, Cry3a, Cry3b and Cry34/Cry35) are commercialized as biopesticides.
There are several varieties of Bt bacterium registered for specific insect pest, owing to their endotoxins specificity. For instance, the Bt varieties kurstaki and aizawai are used for Lepidoptera, while the variety israeliensis is used for the control of dipteran insects such as mosquitoes and blackflies.
The tenebrionis variety targets beetles such as Colorado potato beetle. The commercial Bt biopesticides are either used as powder spray containing dried spores and toxin crystals or as a liquid formulation. Another use of Bt in pest management is the production of genetically engineered crops that contain the Bt toxin gene from Bacillus thuringiensis. These Bt crops express the active Cry toxin in their tissues and kill the insect that feeds on them. More than 10 different Bt crops have been tested and successfully planted worldwide (Toenniessen et al., 2003).
Bacillus thuringiensis discovered over a century ago has quickly revealed its insecticidal activity and was used worldwide for insect pest control. Several genes of the toxin is found in different strains of the bacterium. Despite the tremendous advancement in science, the entire mode of action Bt Cry toxins is not known and is probably limiting its application. The biggest advantage of Bt toxin insecticide is its limited spectrum of activity, with no known harm to human, non-target species and the environment. However the report of insect developing resistance to Bt toxins is narrowing the hope of green agriculture.