The bacteria Bacillus thuringiensis is an example of a microbial biocontrol agent that can be used to control butterfly caterpillars. These are supplied in sachets as dried spores that are sprayed onto fruit trees and other sensitive plants, where the insect larvae eat the spores. The toxin is released in the larvae's gut, killing the larvae. The bacterial infection will only hurt the caterpillars; other insects will not be harmed. The scientists have introduced Bt toxin genes into plants. Such plants are resistant to attack by insect pests, e.g., Bt-cotton.

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