One of the many problems facing farmers is how to protect crops from insects while protecting the environment. Pesticides have a tendency to be harmful to a wider range of organisms and can runoff and contaminate the surrounding area. Obviously it would be a great help if we didn't have to use these chemicals, or could at least reduce the amount required. Wouldn't it be nice if something on the plants, such as a microbe, killed the insects. Bacillus thuringiensis, first registered as an insecticide in 1961, is the most widely used microbial pesticide. There are at least 412 commercially available preparations of the microbe and it's toxin.
Bacillus thuringiensis is a gram-positive, rod-shaped, spore-forming bacteria. During spore production, crystalline bodies are formed as a companion product.
The Bacillus thuringiensis toxin (B. t. toxin) is a crystal structure like the triangle at the left in this image. The crystals dissolve in the gut of the affected insect and paralyze the epithelial cells. This causes the insect to stop eating, and eventually die as the B. thuringiensis grows from the spores and reproduces in the insects blood. It affects over 150 insects, including lepidopterans, dipterans, and coleopterans. These include cabbage worms, tent caterpillars, mosquitoes and Colorado potato beetles. It has little or no toxic affect on humans or other animals unless directly injected into the gut.
Unfortunately, as with chemical insecticides, some insects will develop an immunity. An example is the diamondback moth which will select for resistant strains in the presence of B. t. toxin. This leads to a 62 fold increase in resistance after 6 generations. Also, to be effective, the B. t. toxin and spores must be ingested by an insect in a larval stage as the toxin has no effect on adult insect.
*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.
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