Bacillus thuringiensis is a bacterium that parasitizes the caterpillars of some harmful moths and butterflies. Spraying or dusting plants with spores of this bacterium appear to be environmentally safe ways to attack such pests as the gypsy moth, the tent caterpillar, and the tobacco hornworm (which also attacks tomatoes). The bacteria kill by a toxin which they secrete. The gene for this toxin has been introduced into some crop plants in an effort to protect them from insect attack without the need for spraying.
Fig. 184.108.40.206 Bollworm. Image courtesy of the Monsanto Corp.
On the left is a cotton boll being attacked by a cotton bollworm. The cotton plant that produced the boll on the right contains and expresses the gene for the Bt toxin. The gene was introduced into the plant by genetic engineering. Transgenic cotton and corn (maize) containing the gene for Bt toxin were widely planted for the first time in 1996. By 2009, these crops had been planted on over 334 million acres (135x106 hectares) worldwide. In the United States over 70% of the cotton planted and 63% of the corn planted is now with transgenic varieties. Crops transgenic for Bt toxin have shown improved yields with greatly-reduced need for chemical insecticides. Not only does this reduce costs and possible health effects, but spares the natural enemies of other insect pests.
B. thuringiensis appears to be the same organism as Bacillus cereus, a usually harmless soil organism, and Bacillus anthracis, the cause of anthrax. What gives them their distinctive properties are the genes encoded by the plasmids they contain.